Toxicological Risk Assessment of Chemicals: A Practical Guide - Chapter 5 potx

Extrapolation of data from studies in experimental animals to the human situation involves twosteps: afirst step is to adjust the dose levels applied in the experimental animal studies to

5 Standard Setting:

Threshold Effects

Adve rse h ealth effect s can be consi dered to be of two types (see Secti on 4.2): those consi dered tohave a threshold, k nown as ‘‘thres hold effects ’’ (effec ts such as, e.g., organ-speci fic, neurologica l,immunol ogical, non-gen otoxic carci nogenicity , reprod ucti ve, develo pment al), and those for whi chthere is consi dered to be some risk at any exposur e level, know n as ‘‘non-t hreshold effect s’’ (effectssuch as, e.g., mut agenicity, genotoxici ty, genoto xic carcinogeni city) Tho ugh it is not possi ble todemon strate experiment ally the presen ce or absence of a thres hold, diffe rences in the approac h

to the hazard asses smen t of threshold versus non-thresho ld effects have been adopte d widely Thedistinct ion in approac he s is based primari ly on the prem ise that simple events such as in vitroactivati on an d covale nt bindi ng may be line ar over many orders of magnitud e, i.e., that these eventsoccur even at very low exposur e levels How ever, a simple pragm atic dist inction on this basis isincre asingly probl ematic as it is likely that there is a thres hold for a numbe r of genoto xic effect s; this

is addres sed in detai l in Cha pter 6

In the hazard asses sment proces s, descri bed in detail in Cha pter 4, all effect s observ ed areevalua ted in terms of the type and severi ty (adver se or non-adv erse) , their dose –response relation-ship, and the relevanc e for human s of the effects observ ed in experi mental animals For thresholdeffects, a No- or a Lowes t-Obser ved-Advers e-Effect Lev el (N=LOAE L), or alternati vely a Ben ch-mark Dos e (BMD), is deriv ed for every single effect in all the avail able studi es provi ded that dataare suffi cient for such an evalua tion In the last step of the hazard asses sment for thres hold effects,all this informat ion is asses sed in tota l in order to ident ify the critical effect(s) and to de rive aNOA EL, or LOAE L, for the critical effect(s)

The approac h of deriv ing a tole rable inta ke by divi ding the N=LOA EL, or alternati vely a BMDfor the critical effect(s) by an asses smen t facto r has been descri bed and discu ssed extens ively in thescien tific literat ure It is beyond the scope of this book to revie w all these refere nces This chapte rpresen ts an overvi ew of published extrapolat ion methods for the deriva tion of a tolerable inta kebased on the assessmen t factor approac h, i.e., limited to addres s effects with thres hold charact er-istics, and is not meant to be exhaust ive The mai n focus is on the rationale for and the use of theasses sment facto rs Pe rtinent guidan ce docum ents and revie ws for the issues add ressed inthis chapte r include WHO=IPCS (1994, 1996, 1999), US-EPA (2002, 2004), IGHRC (2003) ,ECETO C (2003) , KEMI (2003) , Kal berlah and Schneid er (1998) , Vermei re et al (1999) , andNielse n et al (2005)

The approac h of standard setting for non-t hresho ld effect s is addres sed in Cha pter 6

The development of regulatory standards derived from a standard such as, e.g., the TolerableDaily Intake or a Ref erence Dos e, is addres sed in Chapter 9

5.1 INTRODUCTION

Acco rding to the OECD=IPC S de finitions listed in Annexure 1 of Cha pter 1 (OECD 2003):Threshold is‘‘Dose or exposure concentration of a substance below that a stated effect is notobserved or expected to occur.’’

Tolerabl e Intak e is ‘‘Estimate d maxi mum amoun t of an agent, expres sed on a body mass basis,

to which each indi vidual in a (sub) p opulation may be exp osed over a speci fied perio d wi thoutappreci able risk ’’

A tolerable intake may have different units dependi ng on the route of adminis tration upon which

it is based, an d is general ly expres sed on a dail y or weekly basis For the oral and derm al route s, atole rable intake is general ly expres sed on a body weight basis , e.g., mg=kg body wei ght per day.Tho ugh not strictly an ‘‘ intake, ’’ tolerable intakes for inhalati on are generally expres sed as anairborne concentration, e.g., mg=m3

Acco rding to the OEC D=IPCS de finitions listed in Anne xure 1 of Chapter 1 (OECD 2003):Acceptable=Tolerable Daily Intake is ‘‘Estimated maximum amount of an agent, expressed on abody mass basis, to which an individual in a (sub) population may be exposed daily over its lifetimewithout appreciable health risk.’’

Reference Dose is ‘‘An estimate of the daily exposure dose that is likely to be withoutdeleterious effect even if continued exposure occurs over a lifetime.’’

Related terms: Acceptable=Tolerable Daily Intake

The term ‘‘acceptable’’ is used widely to describe ‘‘safe’’ levels of intake and is appliedfor chemicals to be used in food production such as, e.g., food additives, pesticides, and veterinarydrugs The term‘‘tolerable’’ is applied for chemicals unavoidably present in a media such as contamin-ants in, e.g., drinking water and food The term‘‘PTWI’’ (Provisional Tolerable Weekly Intake) isgenerally used for contaminants that may accumulate in the body, and the weekly designation is used tostress the importance of limiting intake over a period of time for such substances The tolerable intake issimilar in definition and intent to terms such as ‘‘Reference Dose’’ and ‘‘Reference Concentration’’(RfD=RfC), which are widely used by, e.g., the US-EPA For some substances, notably pesticides, the

‘‘ARfD’’ (Acute Reference Dose), is also established, often from shorter-term studies than those thatwould support the ADI The ARfD is defined as the amount of a substance in food that can be consumed

in the course of a day or at a single meal with no adverse effects

In inhalation studies, laboratory animals are generally exposed to an airborne chemical for alimited period of time, e.g., 6 h a day, 5 days per week Adjustment of such an intermittent exposure

to a continuous exposure scenario is regularly applied as a default procedure to inhalation studieswith repeated exposures but not to single-exposure inhalation toxicity studies Operationally, this isaccomplished by a correction for both the number of hours in a daily exposure period and thenumber of days per week that the exposures were performed In an inhalation study in whichanimals were exposed to an airborne concentration of a substance at 5 mg=m3

for 6 h a day, for

5 days per week, the adjustment of this intermittent exposure concentration to a continuous exposureconcentration would consider both hours per day and days per week: 5 mg=m33 6=24 h 3 5=7days=week ¼ 0.9 mg=m3

, with 0.9 mg=m3

being the concentration adjusted to continuous exposure.For systemic effects observed in inhalation studies, the determining factor for effects to occur atthe systemic target is generally the total dose rather than the concentration of the chemical in the air

In such cases, a tolerable intake (expressed as mg=kg body weight per day, or mg=m3depending onthe standard to be derived, i.e., a tolerable intake in its strict meaning, or a tolerable concentration) isestablished from the NOAEC, or LOAEC, derived in the inhalation study and adjusted forcontinuous exposure

For local effects, in contrast, the determining factor for effects to occur at the site offirst contact(mucous membrane of the respiratory tract, the eyes, or the skin) is generally the concentration ofthe chemical in the air rather than the total dose at the site offirst contact In such cases, a tolerableconcentration (expressed as mg=m3) is established from the NOAEC, or LOAEC, derived in theinhalation study without an adjustment to a continuous exposure

The overall principles for the derivation of a tolerable intake are equal irrespective of chemicalclass (e.g., food additives, pesticides, veterinary drugs, contaminants) although it should be recog-nized that the available database for chemicals deliberately added to, e.g., food is generally more

compr ehensive than for contam inants This is because there a re extens ive regul atory demands fortoxicit y data in relat ion to marketin g of che micals, which are intenti onally a pplied to food, etc Forthreshold effect s, a tole rable intake is generally deriv ed from the NOA EL, or LOA EL, for thecritical effect (s) by dividing the NOA EL=LOA EL, by an overall assessmen t facto r.

Acco rding to the OECD=IPC S de finitions listed in Annex ure 1 of Chapter 1 (OECD 20 03):Assessm ent Fa ctor is ‘‘ Numeri cal adjus tment used to extrap olate from experi mentall y deter -mined (dose –respon se) relationsh ips to e stimate the agent exposur e below whic h an advers e effect isnot like ly to occur ’’

Relate d term s: Safety Factor , Unce rtainty Factor, Extrapo lation Factor , Adjust ment Factor ,Conversi on Factor

The re is an enormous variability in the extent and natur e of different databa ses for chemicalsubstanc es Fo r examp le, in some cases, the evalua tion of a chemi cal must b e based on limit eddata in experi mental animals, whereas in other cases detai led infor mation on the vario us end-points, toxicokin etics, an d mode( s) of action may be available In some cases, the evalua tion can bebased on data on effect s in exposed human populatio ns Clearl y as the amount of infor mationavailable incre ases, the degree of und erstanding of the hazards expres sed also incre ases, and theuncert ainties due to lack of informat ion decreas e However , even with complex databa ses, uncer-tainties sti ll remain

The asses sment facto rs general ly applied in the estab lishmen t of a tolerable inta ke from theNOA EL, or LOA EL, for the crit ical effect (s) are appli ed in order to compe nsate for uncert aintiesinher ent to extrapolat ion of experiment al animals data to a g iven human situation, and for uncer-tainties in the toxicolog ical databa se, i.e., in cases wher e the substanc e-speci fic knowledge requi redfor risk asses smen t is not avail able As a co nsequen ce of the variabili ty in the extent and natur e ofdifferent databa ses for chemi cal substances, the range of assessmen t facto rs applied in the estab -lishmen t of a tole rable intake has been wide (1 –10,000), althoug h a value of 100 has been used mostoften An overview of diff erent approac hes in using assessmen t factors, historic ally and currently,

is provi ded in Secti on 5.2

The key areas of uncert ainty when using data from experi mental anim als include uncertaint yrelated to:

. Extrapo lation from anim al speci es to human s (Secti on 5.3)

. Variabi lity in the human popula tion (Secti on 5.4)

. Route-t o-route extra polation (S ection 5.5)

. Durati on of exposur e in experi mental studies (Section 5.6)

. Dose –respon se curve=NOA EL n ot estab lished (Section 5.7)

. Nature and severity of the effects (Section 5.8)

. Gaps or other de ficienci es in the databa se (Section 5.9)

5.2 ASSESSMENT FACTORS: GENERAL ASPECTS

In the context of assessment factors, it is important to distinguish between the two termsity’’ and ‘‘uncertainty.’’ Variability refers to observed differences attributable to true heterogeneity

‘‘variabil-or diversity, i.e., inherent biological differences between species, strains, and individuals Variability

is the result of natural random processes and is usually not reducible by further measurement orstudy although it can be better characterized Uncertainty relates to lack of knowledge about, e.g.,models, parameters, constants, data, etc., and can sometimes be minimized, reduced, or eliminated

if additional information is obtained (US-EPA 2003)

It should be recognized that a lack of knowledge of variability is a source of uncertainty.The terminology within this area is not standardized Other terms include ‘‘safety factor,’’

‘‘uncertainty factor,’’ ‘‘extrapolation factor,’’ ‘‘adjustment factor,’’ and ‘‘conversion factor.’’ None

of these terms are ideal For examp le, the term safety facto r has implic ations of absol ute safety,wher eas the term uncert ainty facto r, alth ough being broader, may be interpret ed different ly inrelation to varia bility a nd uncert ainty For the sake of clarity in this book, the term asses smentfacto r is used and is meant as a g eneral term to cover all facto rs desig nated in the literatu re as safetyfacto r, unce rtainty facto r, extrapolat ion facto r, adjus tment facto r, convers ion facto r, etc The otherment ioned term s are not used unless refere nce is made to a speci fic term or met hod The asses smentfacto r can cover both varia bility and uncert ainty.

The follow ing secti on gives an overview of different approac hes in using assessmen t factors,histori cally and curren tly, beginn ing with the introduct ion of the so-cal led ‘‘safet y factor approac h’’

in the mid-1950s and re flecting the develo pment up to the regul atory approac hes c urrently used byinte rnational and federal b odies The overview does not attempt to cover all publi cations in thisfield, but includes the approac hes sugges ted by diff erent scien tifi c groups and inte rnational andfederal bodies, which are considered as being the most central ones in the development of theapproaches currently used regulatory Default assessment factors used or suggested in the variousapproac hes are summ arized in Table 5 1

5.2.1 ASSESSMENTFACTORS: VARIOUSAPPROACHES

Historically, the so-called safety factor approach was introduced in the United States in the 1950s in response to the legislative needs in the area of the safety of chemical food additives(Lehman and Fitzhugh 1954) This approach proposed that a ‘‘safe level’’ of chemical foodadditives could be derived from a chronic NOAEL from animal studies divided by a 100-foldsafety factor The 100-fold safety factor as proposed by Lehman and Fitzhugh was based on alimited analysis of subchronic=chronic data on fluorine and arsenic in rats, dogs, and humans, andalso on the assumption that the human population as a whole is heterogeneous Initially, Lehmanand Fitzhugh reasoned that the safety factor of 100 accounted for several areas of uncertainty:

mid-. Intraspecies (human-to-human) variability

. Interspecies (animal-to-human) variability

. Allowance for sensitive human populations due to illness when compared with healthyexperimental animals

. Possible synergistic action of the many intentional and unintentional food additives orcontaminants

In 1961, the Joint FAO=WHO Expert Committee on Food Additives (JECFA) and the Joint Meeting

of Experts on Pesticides Residues (JMPR) adopted this approach in a slightly modified form: Thesafe level was called the Acceptable Daily Intake (ADI) and expressed in mg=kg body weight perday (Vermeire et al 1999, ECETOC 2003) Usually, a safety factor of 100 is used by JECFA andJMPR for establishing ADIs by this ADI approach; however, the procedures adopted by JECFA andJMPR do not generate a clear justification for deviation from the factor of 100, but in someindividual cases, an expert explanation is given for the use of factors other than 100 (Vermeire

et al 1999)

It is apparent that the factor of 100 has no quantitative bases, and the choice of the value 100 ismore or less arbitrary (Vermeire et al 1999) Retrospectively, some attempts have been made tosupport a 100-fold factor (Bigwood 1973, Lu 1979, Vettorazzi 1977 as reviewed in Vermeire et al

1999 and KEMI 2003), and the 100-fold factor was found to be justified

The 100-fold safety factor has traditionally been interpreted as the product of two factors withdefault values of 10 For example, according to WHO=IPCS (1987), the safety factor is intended toprovide an adequate Margin of Safety (MOS) by assuming that the human being is 10 times moresensitive than the test animal and that the difference of sensitivity within the human population is in

a 10-fold range

US-EPA (2002)

Renwick (1993) WHO

LLN (1990)

ECETOC (2003)

TNO (1996)

Kalberlah and Schneider (1998)

KEMI (2003) a

D-EPA (2006) Interspecies 10 10 10 10 Toxicokinetic Ab 4 4 Ab 4 (rat) Toxicodynamic 100.5 2.5 2.5 1 2.5

Inhalation 3 Interindividual 10 10 10 5 10 25 10

a

See also Table 5.2.

b A is a calculated adjustment factor allowing for the differences in caloric requirement.

5.2.1 1 US-E PA Appr oach

In 1988, the US-EPA adopte d the ADI approac h in its regul atory meas ures against envir onmen talpoll ution; with a numbe r of modi fi cations (US-E PA 1988, 1993) Inst ead of the terms ADI andsafet y facto r, the term s Reference Dose (RfD) and uncert ainty facto r (UF), respec tively, wer eselec ted The RfD is deriv ed from the NOA EL by divi ding by the overall UF The o verall UForigina lly sugges ted and recon firmed in 2002 (US-E PA 2002) general ly consi sts of a 10-fol d factorfor each of the following:

. Huma n varia tion in sensi tivity (UFH )

. Inter species extra polation (UFA )

. Use of the NO AEL obtained from a less than lif etime study (UFS )

. Use of a LOAE L in the absence of a NOAEL (UFL )

. Adeq uacy of the total database (UFD)

Acco rding to US-EPA (1993), the fi rst four of the above-ment ioned factors a re adapte d fromDour son and Stara (1983)

The exact value of the UFs chosen shoul d depend on the quali ty of the studie s av ailable, the extent

of the database, and scien tific judgment (US- EPA 2002 ) The default facto rs typic ally used cover asingle order of magni tude (i.e., 10 1) By convent ion, in the US-EPA , a value of 3 is used in place ofone-hal f powe r (i.e., 10 0.5 ) when appropriat e The se half- power values should be facto red as wholenumbe rs when they occur singly but as powers or logs when they occur in tandem A composit e UF

of 3 and 10 would thus be expres sed as 30 (3 3 10 1), whereas a compo site UF of 3 and 3 would

be expressed as 10 (10 0.5 3 10 0.5 ¼ 10 1) It shoul d be n oted, in addition, that rigid applicati on of log(i.e., 10 1) or ½ log (i.e., 10 0.5 ) un its for UFs could lead to an illogic al set of refere nce values ; therefore,

it ha s been empha sized that appli cation of scien tific judgm ent is critical to the overall proces s

It is also noted that there is overl ap in the individua l UFs and that the appli cation of five UFs often for the chronic reference value (yielding a total UF of 100,000) is inappr opria te In fact, in caseswher e maxi mum uncertaint y ex ists in a ll fi ve areas, it is unlikely that the databa se is suf ficient toderiv e a refere nce value Uncertainty in four areas may also indi cate that the database is insuf fi cient

to deriv e a reference value In the c ase of the RfC, the maxi mum UF would be 3,000, whereas themaxi mum would be 10,000 for the RfD Thi s is because the deriv ation of RfCs and RfDs hasevolve d some what different ly The RfC methodol ogy (US- EPA 1994) recommend s divi ding theinte rspecies UF in half, one-half (10 0.5 ) each for toxicokin etic and toxi codynam ic consi derations,and it includes a Dosimetric Adjustment Factor (DAF, represents a multiplicative factor used to adjust

an observed exposure concentration in a particular laboratory species to an exposure concentrationfor humans that would be associated with the same delivered dose) to account for toxicokineticdifferences in calculating the Human Equivalent Concentration (HEC), thus reducing the interspecies

UF to 3 for toxicodynamic issues RfDs, however, do not incorporate a DAF for deriving a HumanEqu ivalent Dos e (HED), and the interspeci es UF of 10 is typi cally applied, see also Secti on 5.3 4 It isrecommended to limit the total UF applied for any particular chemical to no more than 3000, for bothRfDs and RfCs, and avoiding the derivation of a reference value that involves application of the full10-fold UF in four or more areas of extrapolation

In addition, a modifying factor (MF) could be applied (US-EPA 1993) The MF is in reality anadditional UF that is greater than 0 and less than or equal to 10; the default value is 1 The MF shouldaccount for uncertainties of the study and database not explicitly handled by the use of the general UFs;e.g., the completeness of the overall database and the number of species tested In the 2002 review of theRfD and RfC processes (US-EPA 2002), it was recommended that use of the MF be discontinued as itwas considered that the uncertainties accounted for by the MF is sufficiently accounted for by thegeneral UF

The US-EPA staff paper from 2004 titled‘‘An Examination of EPA Risk Assessment Principlesand Practices’’ (US-EPA 2004) provides comprehensive and detailed information on the

pract ices empl oyed in risk asses smen t, incl uding use of UFs and use of default and extra polationassumpti ons.

5.2.1 2 Calabres e and Gilbert Appro ach

Calabres e and Gilber t (1993) hav e demonstra ted the lack of indepe ndence of the interspeci es andintraspeci es UFs, as well as of the intraspeci es and the less -than-lifeti me UFs Based on thei ranalys es, the author s conclu ded that most of the recommend ed US-EPA stand ards based on animalmodels needed to have some of their UFs modi fied They recom mended the following modi fi cations

of the intraspeci es UF, see also Secti on 5.4 2:

. Signi ficantly less-than -lifeti me anim al study : 5

. When the anim al study is for a norm al experi mental lifetime (2 years in rodent s): 4

. Occu pational epidemiolo gical study: 10

. Environ menta l epidemiol ogical study , if study was for a no rmal human life span: 5

5.2.1 3 Renwic k Appro ach

The approac h propos ed by Renwick (1991, 1993) is also b ased on the 100-fo ld facto r It attempt s togive a scien tifi c basis to the default values of 10 for the interspeci es and 10 for the intraspeci es(interin dividual human ) diff erences Ren wick also propos ed a divi sion of each of these UFs intosub-f actors to allo w for separa te evalua tions of diff erences in toxicokin etics and toxicodyna mics.The advanta ge of such a subdi vision is that compo nents of these UFs can be addres sed wher e dataare avail able; for example, if ava ilable data show simil ar toxicokin etic s of a given chemical inexperi mental anim als and humans, then only an inte rspecies e xtrapolat ion facto r would be nee ded toaccount for diff erences in toxi codynam ics Renwi ck examined the relative magnitud e of toxicoki-netic and toxicodynam ic varia tions between and wi thin species in detail He found that toxi cokineticdifferences were generally greater than toxicodyn amic d ifferences resul ting in the propos al that the10-fold facto rs (for inter- and intraspeci es variation ) shoul d, by defaul t, be subdi vided into factors of

4 for toxicokin etics and 2.5 for toxi codynam ics It should be no ted that the propos ed defaul t valueswere deriv ed from limited data

The WHO=IPCS (1994) ha s adopte d the approac h set forth by Ren wick (1993) with onedeviation, see Figure 5.1 Whil e the UF for inte rspecies (animal- to-human) extra polation shoul d besplit into defaul t values of 4 for toxi cokinetics and 2.5 for toxicodynam ics, the UF for intraspeci es(hum an-to-hum an) extra polation shoul d be split evenly be tween both aspect s, i.e., a sub-f actor of 3.16for both toxicokin etics and toxicodynam ics The reason for this deviation from Renwi ck ’s initialsugges tion was that the WHO=IPCS consi dered that the slightly greater varia bility in the kinetics inhuman s compa red with dy namics was not suf ficient to warrant an unequal subdivisi on of the 10-fol dfacto r into a toxic okinetic facto r of 4 and a toxicodynam ic facto r of 2.5 Actual data shoul d be used torepla ce the defaul t values if availab le It was furt hermore noted that precise defaul t values for kineticsand dy namics cannot be expect ed on the basis of a subdivisio n of the imp recise 10-fold compo sitefacto r for interspeci es as wel l as for the inte rindivid ual varia tion Acco rding to WHO=IPCS, thedefaul t v alues sugges ted above wer e consi dered as being reason able since they provide a positivevalue greater than 2 for both aspects and are compatible with the species differences in physiologicalparameters such as renal and hepatic bloodflow It was also noted that since the database examinedwas limited, the default values suggested for subdivision of interspecies and interindividual variationshould be adopted on an interim basis

5.2.1 4 Lewis –Lyn ch–Nikiforov Appr oach

In 1990, Lewis et al published a new approach introducing flexibility such that both newinformation and expert judgment could be readily incorporated The Lewis–Lynch–Nikiforov(LLN) method, and its refinements, are extensions of established principles and procedures, and

guides the data evalua tor to adjus t experi mentall y determin ed ‘‘no-eff ect ’’ (or ‘‘ minim um effect ’’)level s from expe rimental animal studies taking the foll owing aspects into account :

. Known diff erences between labor atory anim als and human s and between experi mentalcondit ions and the real world

. Sensit ivity of the exposed human popula tions

. Streng th of eviden ce that the chemi cal presen ts a real haz ard to human healt h

. Gene ral quali ty of the experi mental databa se

. Unce rtaintie s in extrapolat ing from labor atory anim als to h umans

. Potency of the toxic agent

. Typ e and severi ty of the putat ive advers e effect

Acco rding to Lewis et al (1990) , a step- by-step sequenc e is used Initially, a quali tative deter atio n is made as to the strength o f eviden ce that the putative toxi c agent presen ts an a ctual healthhazard to human s, i.e., how like ly is this agent to p roduce the suspec ted adverse effect in human s?

min-In contrast to the ADI and RfD method wher e no speci fic c onsideration is given to judgi ng thelike lihood that a chemical presents a real health hazard, the ‘‘stre ngth of the quali tative evidence ’’ isscored expli citly and separa tely in the LLN approac h The NA ELhuman is estimat ed from laboratoryresear ch results, using the follow ing a lgorithm:

NAE Lhuman ¼[I][R][ QNOA ELanimal [S]

1 ][Q 2 ][Q 3 ][U][ C]

[S] is the aggrega te ‘‘scali ng facto r ’’ to account for known quanti tative diff erences betw een speciesand between labor atory experi mental condition s and the real world The default value is 1,indi cating that animals and humans are equivalent in these dim ension s

[I] is the adjus tme nt facto r to account for anticipat ed g reater suscep tibility among mem bers ofthe test animal populatio n than was ob served in the experi ment, i.e., to account for intraspeci esvaria bility The default value is 10, indi cating that extre mely high variability was observ ed (orwould be expect ed) a mong anim als

Uncertainty factor 100

Interspecies differences (differences between humans and common laboratory animals)

10

dynamics

Toxico-10 0.4 (2.5)

Intraspecies differences (differences among humans; interindividual differences) 10

kinetics

Toxico-10 0.6 (4.0)

dynamics

Toxico-10 0.5 (3.2)

kinetics

Toxico-10 0.5 (3.2)

FIGURE 5.1 Subdivision of the 100-fold UF showing the relationship between the use of UFs (above thedashed line), and the proposed subdivisions (below the dashed line) based on toxicokinetics and toxicody-namics (From Renwick, A.G., Food Addit Contam., 10, 275, 1993; WHO=IPCS Assessing human health risks

of chemicals: Derivation of guidance values for health-based exposure limits Environmental Health Criteria

170 Geneva, 1994 Available at http:== www.inchem.org=documents=ehc=ehc=ehc170.htm)

[R] is the adjus tment facto r to account for anti cipated differences in suscep tibility be tweenhuman s and the laboratory animals, i.e., to account for inte rspecies varia bility The defaul t value is

10, indicati ng that humans are much more susceptible

[Q1–3] and [U] are adjustm ent factors to account for varia tions in the reliabilit y of the databa se(data quali ty) and other source s of uncert ainty in the data e valuation p rocess

[Q1] reflects the data evalua tor’ s certa inty that the agent actually causes the speci fic ‘‘criticaleffect ’’ in human s The default value is 1, indi cating that the agent causes similar toxic effect s inanimals and humans

[Q2] is employed when e xtrapolat ing data from subchro nic studies to estimat e risk from lifelongexposur es The defaul t value 10, indi cating great uncert ainty in esti mating the NOAELchronic fromthe NO AELsubchronic

[Q3] is empl oyed when e xtrapolat ing LOAE Ls to NOA ELs The defaul t value 10, indicati ngextre mely great uncert ainty associated with using a LOAE Lanimal to estimate a NAEL human .[U] is used to account for resi dual uncertaint y in estimates of [S], [I], and [R] The default value

is 10 indicati ng very great overal l uncertaint y, which has not alrea dy been account ed for in [Q1–3].[C] is a nonsci enti fi c, judgm ental safet y factor, i.e., a socia l or poli tical value judgm ent Thedefaul t value is 1, indicating that no addit ional MOS is needed over that provi ded by the inheren tlyconser vative procedu re above

An aggrega te adjus tment factor of about 2 50 is typical; the theor etical maximum v alue is100,000

By appli cation of factors [Q1 –3] and [U], this approac h attempted to separa te scien tific ments from poli cy=value judgmen ts Acco rding to the author s, there are three distingu ishingfeatu res of the LLN approac h The first is the empha sis on careful discr iminat ion among theadjus tments The second is on discr iminat ion betw een ‘‘best esti mates ’’ of the correct adjus tmentsfor [S], [I], and [R] and the compl etely separa te adjus tment for overal l un certainty The thir d is onsecuring scien tific consens us on the adjus tment values It should be recognized, howe ver, that inpract ice, it will not be possible to dist inguish all these facto rs, and that some factors may not beindepe ndent of each other It could also be que stioned whether a nonsci enti fic factor [C] shoul d

judg-be discussed in a scien tific risk asses sment

of the NO AEL, or LOAE L, and the exposur e estimate, see Secti on 8.3.3 The ratio resul ting fromthis comparison is called the Margin of Safety (MOS) The TGD recommends the followingparameters to be considered in assessing the MOS:

. Uncertainty arising, among other factors, from the variability in the experimental data andintra- and interspecies variation

. Nature and severity of the effect

. Human population to which the quantitative and=or qualitative information on exposureapplies

. Differences in exposure (route, duration, frequency, and pattern)

. Dose–response relationship observed

. Overall confidence in the database

These parameters are parallel to those being considered in the evaluation of the assessment factors to

be applied in the establishment of a tolerable intake

The TGD has been revised and the second edition was published in 2003 (EC 2003) However,the human health risk characterization part was not included in this second edition Afinal draftversion of the human health risk characterization part was released in 2005 with a detailed guidance

on, among others, the main issues to be included in derivation of the‘‘reference MOS’’ (MOSref),which is analogous to an overall assessment factor The individual factors contributing to theMOSref are described separately and guidance is given on how to combine these into the MOSref.The guidance provided in this draft version has been extensively used in relation to the riskassessment of prioritized substances carried out since the draft version was released; however,this version is not publicly available

In the new EU chemicals regulation REACH, which entered into force on 1 June 2007, detailedguidance documents on different REACH elements, including risk characterization and the use ofassessment factors, are currently in preparation (spring 2007) These documents will probably beavailable on the EU DG Environment REACH Web site (EU 2006) when published

5.2.1.6 ECETOC Approach

The approach recommended by the ECETOC (1995) is to derive the best scientific estimate of aHuman No-Adverse-Effect Level, referred to in the report as the Predicted No-Adverse-Effect Level(PNAEL) The approach distinguishes three stages:

. Application of a scientifically derived adjustment factor to the NOAEL, or LOAEL, of thecritical effect established in the pivotal study It is stated that if the database is inadequate,then human PNAELs cannot be derived scientifically

. Application of a UF to the PNAEL to take into account the degree of scientific uncertaintyinvolved The following degrees of confidence in the human PNAEL are suggested:high¼ 1, medium ¼ 1–2, low ¼ larger UF

. Application of a nonscientifically based safety factor to take into account political aspects,socioeconomic aspects (cost–benefit considerations), or risk perception factors (the nature

of the effect may justify the use of an additional factor)

The scientifically derived adjustment factors include the following elements:

. Experimental exposure in relation to the expected human exposure: a default value of 3 forextrapolation from short-term to subchronic exposure; a default value of 2–3 for extrapol-ation from subchronic to chronic exposure

. Extrapolation from LOAEL to NOAEL: a default value of 3

. Route-to-route extrapolation: no default value

. Interspecies extrapolation (animal-to-human): a default value of 4 for oral exposure (for therat with a body weight of 250 g and based on caloric demands); a default value of 1 forinhalation

. Intraspecies extrapolation (human-to-human): a default value of 3 for the general tion; a default value of 2 for workers

popula-This approach discriminates factors to a large extent in order to distinguish between the singleadjustments and to separate best estimates from uncertainty It should be noted that the ECETOCapproach does not mention the establishment of an overall factor and although they mention that alldiscriminated aspects introduce uncertainties, they do not give guidance on how to account for this

It could also be questioned here whether a nonscientific factor should be discussed in a scientific riskassessment

In a more recent report, ECETOC (2003) has further developed many of the principlesestablished in the previous report (ECETOC 1995) and replaced the guidance provided therein on

the use of asses sment facto rs in human health risk assessmen t The report provi des a step-by-st epguidan ce for deriv ing an approximat ion of a safe exposur e level for human s from the appropriat eNOA EL, or LOAE L, observ ed in anim al studies, inclu ding guidance on asses sment facto rs.The most scienti fically supportabl e values for de fault asses sment facto rs recom mende d to be used

in the absence of subst ance-s pecifi c infor mation include:

. LOAE L-to-NO AEL extrapolat ion: a default value of 3

. Durati on extrapolat ion: subacute to c hronic, a default value of 6; subchr onic to chronic, adefaul t value of 2; local effects by inhalati on, a defaul t value of 1

. Route-t o-route extrapolat ion: oral to inhal ation, no defaul t value; oral to derm al, no defaultvalue

. Interspeci es extrapolat ion (animal- to-hu man): systemi c effect s (scal ing): mouse, a defaultvalue of 7; rat, a default value of 4; monke y, a defaul t value of 2; dog, a defaul t value o f 2.Local effects by inhalati on: a default value of 1

. Intraspeci es extra polation (huma n-to-huma n): syst emic and local effects: a defaul t value of

5 for the general popula tion; a d efault value of 3 for workers

Similar ly to the previous ECET OC app roach, this revised approac h does not ment ion the estab ment of an overall factor

lish-5.2.1 7 Dutch Approa ches

TNO (the Netherl ands Organis ation for Applied Scient i fic Research) has set up a method forsetting Health-based Occu pational Ref erence Valu es (HBOR Vs) (Hakkert et al 1996) TheHBOR V is derived from the selec ted NOA EL by app lication of asses sment factors compe nsating foruncert ainties inher ent to extrapolat ion of experi mental data to a given h uman situati on an dfor uncertaint ies in the toxi cological databa se The asses smen t facto rs should be derived considerin gthe toxi city pro file of the substance; if the availab le data are insuf ficient , an overal l assessmen t facto r

is used compr ising vario us sub-factor s related to:

. Interspecies differences (animal-to-human): mouse, a default value of 73 3; rat, a defaultvalue of 43 3; rabbit, a default value of 2.4 3 3; dog, a default value of 1.4 3 3 The firstfactor for each species is a calculated adjustment factor, allowing for differences in basalmetabolic rate (proportional to the 0.75th power of body weight) The second factor of 3 isthe assessmen t factor appli ed for remainin g uncertaint ies (Se ction 5.3.3), for which thedefault value is 3 For local skin and respiratory tract effects, the assessment factor is 3, asadjustment for differences in body size is inappropriate

. Intraspecies differences (human-to-human): a default value of 3 (workers), a default value

. Type of critical effect: a default value of 1

. Dose–response curve: a default value of 1

. Confidence of the database: a default value of 1

. Route-to-route: no default value

Principally, the overall assessment factor is established by multiplication of the separate factors.The authors note that in practice it is not possible to distinguish all above-mentioned factors, andsome factors are not independent of each other Therefore, straightforward multiplication may lead

to an unreas onably high ov erall factor Di scussion and weighin g of the individua l factors arethere fore essent ial to estab lish a reliable an d just ifiable overal l asses sment factor.

Ver meire et al (1999) have p ublished a discu ssion paper with focus on asses smen t facto rsfor human h ealth risk asses sment The stat us quo with regard to asses smen t facto rs is reviewed andthe paper discusses the develo pment of a formal, harm onized set of asses sment facto rs Optionsare presen ted for a set of d efault values and probabi listic distrib utions for asses smen t facto rs based onthe state of the art Methods of combining default values or probabilistic distributions of assessmentfacto rs (Secti on 5.11) are also descri bed In relation to asses smen t facto rs, the authors recommend ed:

. For interspecies (animal-to-human) extrapolation, allometric scaling on the basis of caloricdemands (the 0.75th power of body weight) is considered preferable above scaling on bodyweight

. Traditional extrapolation approach, based on more or less arbitrarily chosen factors of 10,

is considered simple to apply but obscures the relative contributions of scientific argumentsand policy judgments The other default approaches, including the application of a toxicityprofile-derived factor as suggested by TNO, make better use of the data available

. Worst-case character of the traditional default assessment factors is considered doubtful asthe 95th percentile for the proposed distributions for the interspecies (animal-to-human)factor and the subchronic-to-chronic duration factor are considerably higher than 10 Inaddition, the limited data on intraspecies (human-to-human) variation is also considered toindicate that a default factor of 10 may not be sufficient

. Derivation of approximations of the distribution of assessment factors from historical data(based on NOAEL ratios) has limitations as the use of the NOAEL instead of the True No-Adverse-Effect Level brings along the variation (error) in the NOAELs

. Application of assessment factors derived from currently estimated distributions of ment factors may lead to very wide distributions of the overall assessment factor

assess-. Probabilistic multiplication of distributions of assessment factors is preferred above thesimple multiplication of percentiles to avoid extreme conservatism

A more recent Dutch report (Vermeire et al 2001) provides a practical guide for the application ofprobabilistic distributions of default assessment factors in human health risk assessments, and it isstated that the proposed distributions will be applied in risk assessments of new and existingsubstances and biocides prepared at RIVM (the National Institute of Public Health and theEnvironment) and TNO The report concentrated on the quantification of default distributions ofthe assessment factors related to interspecies extrapolation (animal-to-human), intraspecies extrapol-ation (human-to-human), and exposure duration extrapolation

5.2.1.8 Kalberlah and Schneider Approach

In a report on a research project‘‘quantification of extrapolation factors’’ (Kalberlah and Schneider1998), it is noted that extrapolation factors are intended to replace lack of knowledge by a plausibleassumption, and that institutions with responsibility for establishing the rules must decide whichlevel of statistical certainty, e.g., applicable for 50% or for 90% of a representative selection ofsubstances, is desired for the selection of a standard value It is furthermore noted that extrapolationfactors are required for: (1) time extrapolation, e.g., from a subchronic to a chronic duration ofexposure; (2) extrapolation from the LOAEL to the NAEL; (3) interspecies extrapolation, i.e., fromexperimental animals to humans; and (4) intraspecies extrapolation, i.e., from groups of personswith average sensitivity to groups of persons characterized by special sensitivity In addition to theseextrapolations, route-to-route extrapolation, e.g., oral-to-inhalation or dermal-to-oral must also bediscussed

If no substance-s peci fic know ledge at all is avail able for one of the extrapolat ion steps,the extra polation facto r in each case is used in unalt ered form; this facto r is described as thestand ard value.

On average, a factor of 2–3 was consid ered suf ficient for time extra polation from a subchr onic

to a ch ronic duration of exposure A higher facto r is required in order to cover the 90th percentil e.Extrapo lation from the LOA EL to the NAE L using standard facto rs shoul d not be und ertaken;the ben chmark method shoul d be used instead

If physiologi cally based pharm acokin etic (PBP K) models cannot be used, interspeci es ation is best undertak en by means of scali ng accordi ng to basal met abolic rate, see Secti on 5.3.2.3

extrapol-A second aspect, interspeci es varia bility, shoul d be consi dered in cases where a higher than averagelevel of safety (achi eved by consi deration of a higher percentil e of the substances) is desired

In general, an intraspeci es facto r of 10 should be suffi cient to re flect the toxi cokinetic va riabilitybetween healt hy adults; ho wever, it is not suf ficient wi th regard to toxi codynam ic varia bility andpossibly only consi ders risk groups to a lim ited extent

If subst ance-s pecifi c know ledge regard ing indi vidual extrapolat ion steps is avail able but notsuf ficient to be able to d ispense wi th the extrap olation enti rely, the substance-s peci fic infor mation isused on a priority basis for the purpos e of modi fying the stand ard value, reduct ion or possi bly also

an increase

A furt her probl em lies in the combinati on o f the indi vidual extra polation facto rs to form atotal extra polation factor The type of combi nation results from the dependen ce or indepe ndence ofthe individua l sub-factor s According to current know ledge, multiplica tive combi nation of theindividua l facto rs is assum ed Su bstance-spe ci fic knowledge about the inte rdepend encies amongthe sub-f actor s may lead to modi ficati on, i.e., a reduct ion of the total extra polation facto r

Cha pter 5 of the docum ent reviews the UFs used by UK Governm ent department s, agenci es,and their advisory committees in human health risk assessment Default values for UFs are provided

in Table 3 in the UK document with the factors separated into four classes: (1) animal-to-humanfactor, (2) human variability factor, (3) quality or quantity of data factor, and (4) severity of effectfactor The following chemical sectors are addressed: food additives and contaminants, pesticidesand biocides, air pollutants, drinking water contaminants, soil contaminants, consumer products andcosmetics, veterinary products, human medicines, medical devices, and industrial chemicals

5.2.1.10 Swedish National Chemicals Inspectorate’s Approach

The Swedish National Chemicals Inspectorate (KEMI) has published an extensive review on humanhealth risk assessment with focus on the application of assessment factors in risk assessments forplant protection products, industrial chemicals, and biocidal products within the European Union(KEMI 2003)

One of the main conclusions drawn from the evaluation of the available data on defaultassessment factors was that the conventionally used factor of 100 (10 for animal-to-human and

10 for human-to-human variations) is probably an underestimate It is stated that it is likely that theanimal-to-human extrapolation is greatly underestimated, and in the case of human-to-humanvariability, an assessment factor of 10–16 is considered as a minimum

Attention is also drawn to the fact that there are some other elements not included in thetraditional assessment factor of 10 including adequacy of the database, nature of the effect, duration

of exposure, route-to-route extrapolation, and considerations of extra-sensitive subpopulations such

as children, the elderly, and patients under medical treatment

The use of default assessment factors is recommended in risk assessments, when justifiable,although the scientific background for such factors in general was considered unsatisfactory Thedefault assessment factors suggested are summarized in Table 5.2 It is recommended to useassessment factors derived from probabilistic distributions in favor of deterministic assessmentfactors, see Table 5.2

TABLE 5.2

Deterministic and Probabilistic Assessment Factors Suggested for Use

in Human Health Risk Assessment

Area to be Extrapolated

Assessment Factor Deterministic Approach Probabilistic Approach Adequacy of the toxicological database

Nature of the effect

Duration of exposure

subchronic (3 months) to chronic (24 months) 8 10 (90th); 16 (95th); 37 (99th)

Route-to-route extrapolation

Dose –response curve

TK: toxicokinetics.

TD: toxicodynamics.

BMDL 5 : 5% lower con fidence limit of the benchmark dose.

5.2.1 11 Dani sh EPA’ s Appr oach

In Denm ark, health-bas ed quality criteria are set for chemical subst ances in soil, drink ing water,and ambi ent air accordi ng to princ iples laid do wn in a guidan ce docum ent from the Dani shEnviron mental Protec tion Agency (D-E PA 2006) The princ iples laid down in the guidan cedocum ent are based on an extensive revie w addressing the hazard asses smen t o f chemicals ,including applicati on of asses sment facto rs (Niels en et al 2005)

For threshold effects, a Tolerabl e Daily Intake (TDI ) is calcula ted by divi ding the NOA EL(or LOA EL) for the crit ical effect(s) with an overal l UF The current practice according to theD-EPA in relation to the setting of quali ty crit eria for chemical substances in soil , drink ing water,and ambient air is to divide the overal l UF into three catego ries (D- EPA 2 006):

. UFI account s for the inte rspecies variation in suscep tibili ty The default value is 10 whencorrection for diff erences in body size between human s and experi mental anim als is based

on the body weight

. UFII accou nts for the differences in interindi vidual suscep tibili ty The defaul t value is 10.. UFIII accoun ts for the quality and relev ance of the databa se, i.e., account s for theuncert ainties in the estab lishmen t of a NO AEL for the crit ical effect The UFIII includeselements such as (1) the quali ty of the databa se, e.g., data on speci fic toxic endpoi nts arelacking or inadeq uate, default value of 1–10; (2) route-to- route extra polation, e.g., nostudies using the appropr iate exposur e route are av ailable, no defaul t value; (3) LOA EL-to-NOA EL extra polation, e g., a NOAEL cannot be estab lished for the critical effect, defaultvalue of 1 0; (4) subchr onic-to-ch ronic extra polation, e.g., no chroni c studi es on whi ch toestab lish the NOAEL are availab le, default value of 10; and (5) nature and severi ty oftoxicit y, e.g., the crit ical effect is toxicity to reprod uction, carci nogeni city or sensitiz ation,defaul t value of up to 10 A de fault value for UFIII has no t been recommend ed; however, avalue from 1 to 100 is generally used The value is ev aluated case-b y-case based on expertjudgme nt

The overal l UF is derived by mul tiplication of the single UFs Recogniz ing that the overall UFmight be unrealist ically high, a final revie w of the overall UF is pe rformed in relat ion to theavailable data If the magni tude of the overal l UF is very high (e.g., a bove 10,000), the databa se

is con sidered as being too limit ed in order to set a healt h-base d quality criteria in soil, drink ingwater, and ambient air for the speci fi c chemi cal subst ance

5.2.1 12 Chem ical-Speci fic Assess ment Factors

A WHO=IPC S (2005) Har monizati on Projec t Docu ment has propos ed using chemical- speci fi ctoxicological data instead of default assessment factors, when possible The concept of Chemical-Specific Adjustment Factors (CSAFs) has been introduced to provide a method for the incorporation

of quantitative data on interspecies differences or human variability in either toxicokinetics ortoxicodynamics into the risk assessment procedure, by modifying the relevant default UF of 10.Incorporation of toxicokinetic or toxicodynamic data becomes possible if each factor of 10 isdivided into appropriately weighted sub-factors as suggested by Renwick (1991, 1993) and adopted

by WHO=IPC S (1994) , see Secti on 5.2.1 3

When appropriate chemical-specific data are available, a CSAF can be used to replace the relevantdefault sub-factor; for example, suitable data defining the difference in target organ exposure in animalsand humans could be used to derive a CSAF to replace the uncertainty sub-factor for animal to humandifferences in toxicokinetics (a factor of 4) The overall UF would then be the value obtained onmultiplying the CSAF(s), used to replace default sub-factor(s), by the remaining default sub-factor(s) forwhich suitable data were not available In this way, chemical-specific data in one area could beintroduced quantitatively into the derivation of a tolerable intake, and data would replace uncertainty

The WHO=IPCS (2005) guidan ce docum ent describ es the types and quali ty of data that could beused to deriv e a CSAF The guidance is separa ted into four main sections coveri ng each of the fourdiff erent areas wher e CSAFs can be introd uced to replace a defaul t sub-f actor:

. Data relate d to interspeci es diff erences in toxicokin etics

. Data relate d to interspeci es diff erences in toxicodynam ics

. Data relate d to human varia bility in toxicokin etic s

. Data relate d to human varia bility in toxicodynam ics

The combi nation of adjustm ent facto rs and defaul t UFs to deriv e an overal l UF is also addres sed

In the 2002 revie w of the RfD and RfC proces ses (US- EPA 2002), the growing suppor t for theuse of CSAFs in place of DAFs was noted, and this will provide an incent ive to fill exist ing datagaps The US-EPA has not yet estab lished a guidan ce for the use of chemi cal-speci fic data forderiv ing UFs, but the divi sion of UFs into toxicodynam ic an d toxi cokine tic compo nents is in theRfC methodol ogy (US-E PA 1994) It was pointed out that, for many substances, there are relat ivelyfew data available to serve a s an adequat e basis to repla ce defaul ts for inte rspecies diff erences andhuman varia bility with more informat ive CSAFs Current ly, relevant data for consi deration are oftenrest ricted to the component of u ncertaint y relat ed to interspeci es diff erences in toxicokin etic s.5.2.1 13 Chi ldren-Sp ecifi c Assessmen t Factor

Con cern has been rais ed that infan ts and children are at higher risk than adults from exposur e toenvir onmen tal c hemicals The quest ion of an extra assessmen t facto r in the hazard and riskasses sment for chemi cals of concern for chil dren has there fore been raised and the rationale forsuch a children-s pecifi c asses smen t facto r has been discussed

Ren wick et al (2000) have performed an analys is of the need for an addit ional UF for infants andchil dren The y consi dered that the propos al to introduce an additional 10-fold facto r when exposur e

of infan ts and chil dren is anticipat ed implie s either age -related differences between speci es ordiff erences wi thin human s, which exceed those presen t in adults Altern ativ ely, the extra factorcould be relat ed to de ficienci es of curren t testing methods or concern s over irreversibil ity in devel-oping organ syst ems The y conclu ded that the available data did not provide a scienti fic rationale for

an extra facto r due to inadequacy of inter- and intraspeci es UFs Justi fication for the facto r thereforemust relate to the adequacy and sensitivity of curren t met hods or concern about irreversi ble effects inthe develo ping organi sm They also pointed out that when adequate reprod uction, multigenerat ion, ordevelo pment al studies are cond ucted, there wi ll be no need for an addit ional 10-fold facto r

In setting pesticide tole rances , the U.S Fo od Qual ity Protec tion Act (FQPA) adopte d in 1996directed the US-EPA to apply an extra safety factor of 10 in assessing the risks to infants and children(US-EPA 1996) This additional 10-fold MOS should take into account the potential for pre- andpostnatal toxicity, and the completeness of the toxicology and exposure databases recognizing thatmaturing organ systems of infants and children may be susceptible to injury by chemicals There may

be developmental periods, i.e., windows of vulnerability, when endocrine, reproductive, immune, andnervous systems are particula rly sensiti ve to certa in chemi cals, see Secti on 5.4 1.1 When data aremissing or inadequate for an evaluation of the age group or of a window of vulnerability duringdevelopment, the application of the extra factor, in addition to the general default factor of 10 forintraspecies variation (human-to-human), was considered appropriate

The FQPA authorizes the US-EPA to replace this additional 10-fold factor with a factor of adifferent value (higher or lower, including 1) only if, on the basis of reliable data, the resulting level

of exposure would be safe for infants and children In practice, factors of 3 and 10 have been used,and the factor has also been used in cases when data have been sufficient but there were reasons forconcern (US-EPA 2002)

In addition to considering the FQPA-relevant areas of uncertainty, assessments of pesticide risk

to children also consider applying part or all of the FQPA factors in certain situations to account for

areas of resi dual uncertaint y that the traditional UFs do not addres s or for which they are believed to

be insuf ficient These areas of residual uncert ainty incl ude exposur e unc ertainties and high concernfor an observ ed suscep tibility (US-EPA 2002)

The US-EPA has conclu ded that in many cases, concern s regard ing pre- and postnatal toxicit ycan be addres sed by calculating an Rf D by using pre- or postn atal develo pmental endpoi ntsand applying the UFs (interspe cies (Secti on 5.3), intr aspecies (Section 5.4), LOAE L-to-NO AEL(Section 5.7), subchronic- to-ch ronic (Secti on 5.6), and databa se-de ficiency (Section 5.9)) to accountfor de ficienci es in the toxi city data when there are gaps consi dered essential for setting a refere ncevalue, incl uding lack of data on children (US-E PA 2002)

The overlap of areas covered by the FQPA factor and those a ddressed by the traditional UFswas recogni zed, a nd it was conclu ded that the curren t UFs, if appropr iately applied using theapproac hes recom mende d in the review (i.e., US-EPA 2002), will be adequat e in most cases tocover concern s and uncert ainties regard ing the potential for pre- and postn atal toxi city and thecompleten ess o f the toxi cology databa se In other words, an addit ional UF is not needed in theRfC=RfD met hodolo gy because the curren tly available facto rs are considered suf ficient to accountfor uncert ainties in the databa se from whi ch the refere nce values are deriv ed (and it does not exclud ethe possi bility that these UFs may be decreased or increased from the default value of 10)

In a report prepared for the Danish Environ mental Protec tion Agency (Niels en et al 2001) withthe purpose of revie wing the knowledge on the ex posure and vulner ability of human s to chemicalsubstanc es durin g the embryonic, fetal, and postn atal perio ds, it was stro ngly recommend ed toperfor m child-spec ifi c risk a ssessments for che mical substances in product s and foods inte nded forchildren (e.g., in cosm etics, toys, child care products, food addit ives in prefer red foods, andpesticide resid ues in proces sed baby foods and infan t formulae) In addition, in the risk asses sment

of chemical subst ances in other use catego ries than the above-m entioned, it was recom mende dspeci fically to focus on children, including the unborn child, if a potential e xposure to agiven subst ance may occur to these a ge groups Fu rthermor e, it was recom mende d that the riskasses sment should be perfor med by expert s on a case-b y-case basis for each substa nce and for eachexposur e scenar io In cases where the available data are insuf ficient to evalua te the suscep tibility ofchildren, incl uding the unborn chil d, it was stro ngly recom mende d that addit ional safety meas ures(choi ce of safety facto rs) should be considered when tolerable inta kes are establis hed for chemicalsubstanc es in products and foods inte nded for children

In conclu sion, the traditi onal a ssessment facto rs (interspe cies, intr aspecies, subchr to-ch ronic, LOAE L-to-NO AEL, and database-de ficiency ) are considered to cover the concern sand uncertaint ies for children adequat ely, i.e., no children-s pecifi c asses smen t facto r is neededwhen setting tole rable intakes However , it is recom mended to perfor m children-s peci fic riskasses sments for chemical subst ances in products and foods intended for chil dren, based on speci ficexposur e assessmen ts for children

onic-5.3 INTERSPECIES EXTRAPOLATION (ANIMAL-TO-HUMAN)

Data from studies in expe rimental animals are the typical starting point s for hazard and riskassessments of chemical substances and thus differences in sensitivity between experimental animalsand humans need to be addressed, with the default assumption that humans are more sensitivethan experimental animals The rationale for extrapolation of toxicity data across species is founded

in the commonality of anatomic characteristics and the universality of physiological functions andbiochemical reactions, despite the great diversity of sizes, shapes, and forms of mammalian species.This section gives a short introduction regarding the biological variation between mammalianspeci es (Secti on 5.3.1 ) as a basis for the subseq uent section on allo metric scaling (Section 5.3.2).Then a number of analyses performed regarding the validity of the default assessment factor of 10are revie wed (Secti ons 5.3.3 and 5.3.4) Finally, the key issues are summari zed and our recom -menda tions are presen ted (Section 5.3.5)

of new pharmaceuticals, determining whether animal toxicity studies identified concordant targetorgan toxicities in humans Data were compiled for 150 compounds with 221 human toxicity eventsreported; multiple human toxicity was reported in 47 cases The results showed a positive humantoxicity concordance rate of 71% for rodent and non-rodent species, with non-rodents alone beingpredictive for 63% of human toxicity and rodents alone for 43% The highest incidence of overallconcordance was seen in hematological, gastrointestinal, and cardiovascular effects, and the leastwas seen in cutaneous effects.

Although testing of chemicals in experimental animals to a great extent is predictive for humantoxicity, humans might be more or less susceptible to the effect(s) exerted by a toxic chemicalcompared with other mammals, as also is the case between animal species Absorption, distributionand storage, excretion, metabolism, site or target organ, and mechanism(s) of action are all involved

in the toxicological response to a chemical Thus, interspecies differences result from variation inthe sensitivity of species due to differences in toxicokinetics as well as in toxicodynamics Some ofthe toxicokinetic differences can be explained by differences in body size and related differences inbasal metabolic rate (caloric requirement)

In general, absorption of chemicals is comparable among vertebrate species for the oral andinhalation route, whereas differences in dermal absorption are much more pronounced because ofdifferences in skin morphology between vertebrates The distribution and storage of chemicals, oncethey have been absorbed, also tend to be comparable across vertebrates, although there are differencesrelated to, e.g., protein binding In terms of renal and pulmonary excretion, the differences betweenthe common laboratory animals and humans are minimal The metabolism of chemicals is, however,generally far from comparable from species to species Not only are different metabolites some-times formed, but the rate of formation of identical metabolites may also be species specific Themechanism(s) and sites of action may also differ across species, both qualitatively and quantitatively

A classical example is the thalidomide-induced teratogenicity in humans where the variabilitybetween species in susceptibility is thought to be largely explained by different metabolic pathways

in humans and laboratory animals, since the ultimate human teratogen is a metabolite of thalidomide.This highlights the importance of uncertainties in interspecies extrapolation in cases where theexpression of chemical toxicity is related to its metabolism, and it is widely believed that interspeciesdifferences in metabolism of xenobiotics is usually the most significant explanatory factor forobserved interspecies differences (Davidson et al 1986, Voisin et al 1990, Calabrese et al 1992).RIVM, the Dutch National Institute for Public Health and the Environment, has launched a Website in January 2006 with information of physiological and anatomical parameter values in variousspecies frequently used in toxicity testing The parameters are focused on organs and tissuesrelevant for pharmacokinetics following oral exposure The aim of the Web site is to gain insightinto the impact of anatomical and physiological differences between species on the pharmacoki-netics This insight may lead to improved species selection and subsequently to improved animal-to-human extrapolation (RIVM 2007)

In addition to the toxicokinetic and toxicodynamic differences mentioned above, other aspects

of differences between experimental animals and humans include different types of organs andtissues, differences in digestion, and differences in the structure of the upper respiratory tract.Furthermore, animal studies are performed in homogenous groups of animals, but the results have to

be applied for the protection of all individuals in a heterogeneous population of humans Inconsequence of this, interspecies variation must also be expected

Extrapolation of data from studies in experimental animals to the human situation involves twosteps: afirst step is to adjust the dose levels applied in the experimental animal studies to humanequivalent dose levels, i.e., a correction for differences in body size between laboratory animals andhumans A second step involves the application of an assessment factor to compensate foruncertainties inherent in toxicity data as well as the interspecies variation in biological susceptibil-ity These two steps are addressed in the following sections.

5.3.2 ADJUSTMENT FORDIFFERENCES INBODYSIZE: ALLOMETRY=SCALING

One aspect in the extrapolation of data from studies in experimental animals to the human situation

is, as mentioned above, a correction of the dose levels in experimental animal studies to equivalenthuman dose levels, e.g., a NOAEL derived from an animal study to the equivalent human NOAEL.Adolph (1949, as cited in Davidson et al 1986, Voisin et al 1990, ECETOC 2003) compiled alist of 34 morphological, physiological, and biochemical parameters, which correlated with inter-species body weight in accordance with the following general allometric equation:

Y¼ aWn

where

Y is a biological function

W is body weight

a, n are species-independent constants for the biological function Y

Values obtained for the exponent n ranged from 0.08 to 1.31 The geometric mean of all

n values was 0.82 and a frequency distribution indicated that values from about 0.67 to 0.75 weremost prominent (Adolph 1949, as cited in ECETOC 2003)

Today, well over 100 biological parameters of mammals are known to be linearly related tobody weight and highly predictable on an interspecies basis (Davidson et al 1986, Voisin et al

1990, Calabrese et al 1992) The allometric equation has traditionally been used for extrapolation ofexperimental data concerning physiological and biochemical functions from one mammalianspecies to another In addition, the allometric equation has also been used extensively as the basisfor extrapolation, or scaling, of e.g., a NOAEL derived for a chemical from studies in experimentalanimals to an equivalent human NOAEL, i.e., a correction for differences in body size betweenhumans and experimental animals

Where an interspecies correlation is assumed to exist between Y (biological effect) and bodyweight, such that if Y¼ aWnand the dose (mg) associated with Y in an experimental animal equals

X, then

Y¼ f (X) ¼ f (aWn)For the observed dose in an experimental animal study, Xanimal, and the equivalent dose in man,

Xhuman, the scaling factor for man from the experimental animal is

Scaling ¼ Xhuman=Xanimal¼ a(Whuman)n=a(Wanimal)n

and

Xhuman¼ Xanimal [Whuman=Wanimal]n

To correct for differences in body size between humans and experimental animals, three measures ofbody size are used in practice as the basis for the extrapolation: body weight, body surface area, andcaloric requirement (Feron et al 1990, Vermeire et al 1999, KEMI 2003)

The reasons for using these three measures and the advantages and disadvantages of theiruse have been described by Davidson et al (1986) and Vocci and Farber (1988) In thesepapers, it is also explained why the body weight can be used in all three cases However, thebody weight should be taken to the power of 1, 0.67, and 0.75 for the body weight approach,the body surface area approach, and the caloric requirement approach, respectively Thesefiguresindicate that the approach used to correct for differences in body size will clearly affect the value ofthe NOAEL adjusted to the body size of humans.

5.3.2.1 Adjustment for Differences in Body Size: Body Weight Approach

Body weight is considered as being the most easily and accurately measurable of the three measures

of body size used in practice as the basis for the extrapolation, and most often provides thequantitative basis for the correction of doses for differences in body size between experimentalanimals and humans

When correction for differences in body size is based on body weight, the exponent n in theallometric equation is 1 and the human dose Xhuman(expressed in mg) can be calculated as follows:

Xhuman¼ Xanimal [Whuman=Wanimal]1

The scaling factor [Whuman=Wanimal]1between a man weighing 70 kg and a rat weighing 250 g is 280when correction for differences in body size is based on body weight Similarly, the scaling factorbetween a man weighing 70 kg and a mouse weighing 35 g is 2000 It should be recognized that thescaling factor and thus the uncertainty in extrapolating doses from experimental animals toequivalent human doses is heavily dependent on the choice of body weight for man as well as forexperimental animals

When the observed dose in an experimental animal study is expressed in mg=kg body weight,then the equivalent human dose (in mg=kg body weight) is equal to the dose in the experimentalanimal study as the scaling factor is 1 This is illustrated by the following example: a NOAEL of

1 mg has been derived from an experimental study with rats By assuming a body weight of 250 gfor the rat, the NOAEL is 4 mg=kg body weight The equivalent human NOAEL can be calculated

to 280 mg based on a human body weight of 70 kg (1 mg3 [70 kg=0.25 kg]), or 4 mg=kg bodyweight (280 mg=70 kg), see also Table 5.3

TABLE 5.3

Adjustment of Dose Levels for Differences in Body Size between Humans

and Experimental Animals Examples of Deriving a Human NOAEL by the Various

Approaches for a Chemical with a NOAEL of 1 mg in a Rat Study

Rat NOAEL (mg)

Rat NOAEL

Human NOAEL (mg =kg bw) Body weight

approach a

(1 mg =0.25 kg)

280 mg (1 mg 3 [70 kg=0.25 kg] 1 )

4 mg =kg bw (280 mg =70 kg) Body surface area

approach b

(1 mg =0.25 kg)

43.6 mg (1 mg 3 [70 kg=0.25 kg] 0.67 )

0.62 mg =kg bw (43.6 mg =70 kg) Caloric requirement

approachc

(1 mg =0.25 kg)

68.4 mg (1 mg 3 [70 kg=0.25 kg] 0.75

)

0.98 mg=kg bw (68.4 mg =70 kg) Note: Assumed body weight (bw) of rat 0.25 kg, of human 70 kg.

According to Voisin et al (1990), physiological and metabolic processes such as renal function,metabolic rate, and cardiac function are not directly proportional to body weight and thus the toxiceffects influenced by these physiological processes are not proportional to body weight, especiallywhen extrapolating from smaller to larger animals Consequently, interspecies comparisons baseddirectly on body weight are likely to be very inaccurate in predicting chemical-induced toxicityacross species.

5.3.2.2 Adjustment for Differences in Body Size: Body Surface Area Approach

The surface area approach has been proposed as an alternative to correction for differences in bodysize based on body weight This approach is founded on the notion that the basal metabolic rate ofvertebrates is a fundamental biological parameter, i.e., afinal common expression of physiologicaland biochemical functions, which is remarkably well related to the body surface area across speciesand within species (Davidson et al 1986)

The most comprehensive attempt to assess interspecies differences in susceptibility to toxicresponses, based on two different dose correction approaches (body weight versus body surfacearea), was published in the classic paper by Freireich et al (1966, as reviewed in Davidson et al

1986, Calabrese et al 1992, Grönlund 1992) The authors attempted to standardize varioustoxicological studies for 18 anticancer drugs performed in adult mice, rats, hamsters, dogs, mon-keys, and humans Thefindings of this study led to the conclusion that the toxic effects of an agentwere similar across species when the dose was measured on the basis of the body surface area.Dourson and Stara (1983) have noted that dose conversions based on body surface in generalmore accurately reflect differences among species in several biological parameters when compared

to conversions based on the body weight

Calabrese et al (1992) have also noted that the use of the body surface area approach for correctionfor differences in body size between experimental animals and humans is a more conservative approachthan use of the body weight approach The authors also noted that as the animal model approacheshuman dimensions of weight and surface area, the differential in dose correction between body weightand body surface area is minimized Furthermore, the body surface area approach was considered likely

to account for certain toxicokinetic differences, especially those associated with interspecies differences

in bloodflow to organs or enzymatic parameters of importance for the metabolism of substances, whichtypically scale according to surface area However, the body surface area approach did not appear toaddress issues of interspecies variation due to, e.g., differences in absorption efficiencies, thickness ofepidermal tissue, number of hairs per square centimeter of skin, the presence and quantity of the gutmicroflora, the relative dominance of oxidative and conjugative metabolic pathways, and the rate ofbiliary excretion It was also stated that the relative importance of these factors will differ fromcompound to compound and from species to species, ranging from unimportant to critical and thusstandard dose correction practice does not eliminate the reality of variability with respect to how thedifferent species handle and respond to agents over a wide range of doses

Renwick (1999) has noted that most physiological and many biochemical processes correlatebetter with body surface area than with body weight For compounds, which are metabolized byprocesses of intermediary metabolism, or for which the clearance is determined largely by bloodflow to the organ(s) of elimination, there is a significant discrepancy between doses expressed on thebasis of body surface area and those based on body weight, when comparing rodents with humans.Interspecies factors of about 3–4 and 8–10 would be necessary for rats and mice, respectively, toconvert the external dose expressed in mg=kg body weight into a dose based on body surface area,and therefore, more closely related to the species differences in basal metabolic rate and organ bloodflows between humans and these species

The above-mentioned references thus indicate that the body surface area approach apparently is

a more feasible approach than the body weight approach in terms of dose correction for differences

in body size between experimental animals and humans

The allomet ric equation relating the body surfa ce area (BSA) to the body weight ( W ) is asfollow s:

to W 0.67 The refore, scaling by the body weight approac h provides equiva lent human doses ofroughl y an order of magnitud e great er than scali ng by the bo dy surface area approac h Henc e,scaling by the body weight approac h without a biol ogically justi fiable reason may overes timate theequiva lent human dose

When the observ ed d ose in an experiment al anim al study is expres sed in mg=kg body weight,then the equiva lent human dose (in mg=kg body wei ght) is equal to the dose in the experi mentalanim al study divi ded by a scalin g facto r a ccording to the follow ing equati on:

Acco rding to this e quation, the scaling facto r [ Whuman=W animal ] 0.33 is 6.4 ‘‘ to man (70 kg) from rat(250 g) ’’ a nd 12.3 ‘‘to man (70 kg) from mous e (35 g) ’’ This is illust rated by the follow ing examp le:

a NO AEL of 1 mg has been deriv ed from an experi mental study wi th rats By assum ing a bodyweight of 250 g for the rat, the NO AEL is 4 mg=kg b ody weight According to Equ ation 5.1, theequiva lent human NOA EL can be calcul ated to 43.6 mg based on a human body weight of 70 kg(1 mg 3 [70 kg=0.25 kg] 0.67 ), or 0.62 mg=kg body weight (43.6 mg=70 kg) Acco rding toEqu ation 5.2, the eq uivalent human NOAEL can be calculated to 0.62 mg=kg body weight(4 mg=kg 3 [70 kg=0.25 kg] 0.33 ), i.e., the anim al d ose (in mg=kg bo dy weight) divided by thescaling facto r, in this case 6.4 Se e also Table 5.3

The surfa ce area for different speci es can be calcul ated by empi rically derived equations using aspeci es-speci fic ‘‘shape factor, ’’ which depends on the ration of weight to height (Vo isin et al.1990) Acco rding to Voi sin et al (1990) , there are severa l limitati ons in the accuracy of conversionsbased on body surface area: (1) the surface area appears to be dif ficult to esti mate; (2) some analyseshave indicated that the exponent of 2=3 may be inaccu rate; (3) some physiologi cal parameter s arenot so well related to body surface area; (4) body surface area convers ions are inaccu rate when themode of ad ministration is different across speci es; and (5) n ot all types of toxi city correl ate withbody surfa ce area, e.g., skin toxi city

5.3.2.3 Adjustment for Differences in Body Size: Caloric Requirement Approach

The caloric requirement, or metabolic rate approach, has also been proposed as an alternative tocorrection for differences in body size based on body weight

A number of parameters such as, e.g., renal clearance, basal oxygen consumption (metabolicrate), area under the curve (AUC), maximum metabolic velocity, or cardiac output correlate to thebody weight to the power of 0.75 (W0.75) Further support for the power of 0.75 comes from a moretheoretical approach based on fractal geometric and energy conservation rules for mammalianspecies (West et al 1997, 1999, as cited in ECETOC 2003).

It is important to note that extrapolation using allometric scaling based on metabolic rateassumes that the parent compound is the toxic agent and that the detoxification is related tothe metabolic rate and thus controls the tissue level This is relevant for oral exposure only(ECETOC 2003)

Feron et al (1990) have concluded that, in general, adjustment for differences in body sizebetween experimental animals and humans should be based on caloric requirement (energy metabo-lism) as this was considered to be both scientifically sound and of practical significance

In 1992, the US-EPA has adopted the caloric requirement approach for oral exposures (US-EPA

1992, as cited in US-EPA 2005), and it is stated that doses should be scaled from animals to humans

on the basis of equivalence of milligrams of the agent normalized by the 3=4 power of body weight(W0.75) per day (US-EPA 2005) The 3=4 power is considered as being consistent with currentscience, including empirical data that allow comparison of potencies in humans and animals, and it

is also supported by analysis of the allometric variation of key physiological parameters acrossmammalian species It is generally more appropriate at low doses, where sources of nonlinearitysuch as saturation of enzyme activity are less likely to occur This scaling is intended as an unbiasedestimate rather than a conservative one It is furthermore noted that equating exposure concentra-tions in food or water is an alternative version of the same approach, because daily intakes of food orwater are approximately proportional to W0.75

Vermeire et al (1999) have noted that scaling on the basis of surface area or caloric demand can

be considered more appropriate compared to extrapolation based on body weight; however, theyalso noted that experimental work did not answer the question regarding which of these twomethods is the most correct Based on theoretical grounds, and supported by their own analyses,Vermeire et al (1999) concluded that scaling on the basis of caloric demand to adjust oral NOAELsfor metabolic size can be considered more appropriate compared with extrapolation based on bodyweight It was also noted that an allometric exponent of 0.67, i.e., the body surface area approach,seems to better describe intraspecies relations

Based on theoretical grounds, the TNO (Hakkert et al 1996) and Kalberlah and Schneider(1998) consider the interspecies extrapolation based on caloric demands (W0.75) as preferable abovescaling on body weight

The allometric equation relating the caloric requirement (CR) to the body weight (W) is asfollows:

The scaling factor [Whuman=Wanimal]0.75between a man weighing 70 kg and a rat weighing 250 g is68.4 when correction for differences in body size is based on caloric requirement Similarly, thescaling factor between a man weighing 70 kg and a mouse weighing 35 g is 299 The correspondingscaling factors obtained based on the body weight approach are 280 and 2000, respectively

Thu s, the diff erence be tween the two extrapolat ion bases, W1 and W 0.75, for ‘‘ to man from rat ’’ andfor ‘‘to man from mous e ’’ is 4.1-fol d and 6.7-fold, respec tive ly, g reater for W 1

compa red to W 0.75 .The refore, scaling by the body wei ght approac h p rovides equivalent human doses great er thanscaling by the caloric requi rement approac h Henc e, scali ng by the body weight approac h wi thout abiol ogically just ifi able reason may overes timate the equ ivalent human dose As can be seen from thetwo scaling facto rs d erived for rat and mous e, respec tively, the great er the diff erence in body weightbetwee n anim al and man, the great er the scaling factor; this is illust rated in Tab le 5.4

When the observ ed d ose in an experiment al anim al study is expres sed in mg=kg body weight,then the equiva lent human dose (in mg=kg body wei ght) is equal to the dose in the experi mentalanim al study divi ded by the scaling facto r accordi ng to the following equation:

Acco rding to this e quation, the scaling facto r [ Whuman=W animal ] 0.25 is 4.1 ‘‘ to man (70 kg) from rat(250 g) ’’ and 6.7 ‘‘ to man (70 kg) from mous e (35 g) ’’ This is illust rated by the foll owing examp le:

a NO AEL of 1 mg has been deriv ed from an experi mental study wi th rats By assum ing a bodyweight of 250 g for the rat, the NO AEL is 4 mg=kg b ody weight According to Equ ation 5.3, theequiva lent human NOA EL can be calcul ated to 68.4 mg based on a human body weight of 70 kg(1 mg 3 [70 kg=0.25 kg] 0.75 ), or 0.98 mg=kg body weight (68.4 mg=70 kg) Acco rding toEqu ation 5.4, the eq uivalent human NOAEL can be calculated to 0.98 mg=kg body weight(4 mg=kg 3 [70 kg=0.25 kg] 0.25 ), i.e., the anim al d ose (in mg=kg bo dy weight) divided by thescaling facto r, in this case 4.1 Se e also Table 5.3

5.3.2 4 Adju stment for Differences in Body Size: Exp osure Route

Acco rding to Feron et al (1990) , simpli city is p robably the main reason for applying the caloricrequi rement approac h in extra polating inhalati on toxicit y data from anim als to human s This method

is based on the assumpti on that (small) animals and human s breat he at a rate related to thei r need foroxygen, thus automati cally at a rate dep ending on thei r caloric requi rement (energ y metaboli sm),and thus, most imp ortantly, they are automati cally being exp osed to chemicals occurring in thebreathing atmosphere at a rate similar to that of the caloric requirement

TABLE 5.4

Scalin g Facto rs for Adjusti ng the Dose Express ed Per Unit Body

Weight to the Dose Express ed Per Unit Caloric Requirement Taking

70 kg as the Body Weight for an Adul t Human

Animal Species Body Weight (kg) Scaling Factor

Note: When the dose for a given species is expressed in mg=kg body weight, the equivalent

human dose (in mg =kg body weight) is obtained by dividing the animal dose by the scaling factor.

In p ractice, this means that no adjustm ent for diff erence in body size is needed for a NO AECobtained for systemi c effect s in an inhalation toxicit y study (van Gend eren 19 88, Feron et al 1990,Vermei re et al 1999, KEMI 2003) For example, a NOA EC of 50 mg=m 3 observ ed for laboratoryanimals is also the equiva lent human NOA EC (note that so far species-sp eci fic sensi tivity has notbeen taken into a ccount).

As ment ioned in Secti on 5.3 2.3, extra polation using allomet ric scaling based on metaboli c rateassumes that the parent c ompound is the toxi c agent and that the detoxi fi cation is relat ed to themetaboli c rate and thus contr ols the tissue level This is relev ant for oral exposur e only With regard

to inhalation of subst ances, whi ch act syst emicall y, the lower detoxi fication (metabo lic) rate inlarge r anim als is balanc ed b y a low er uptake (lower respirato ry rate) and thus no scaling facto r isneeded (ECETO C 2003)

For subst ances with local effect s on the respi ratory tract, no g eneral approac h for interspeci esscaling c an be given Anatom ical and physi ological differences in the airways be tween experi mentalanimals and humans contr ibute to inte rspecies diff erences in local effects observ ed between animalsand hu mans, see Secti on 4.7.8 It shoul d be noted, howe ver, that for local effect s the deter miningfacto r for effect s to occur in the respirato ry tract is general ly the c oncentrati on of the chemi cal in theair rathe r than the tota l dose and thus allomet ric scalin g is not relev ant

5.3.2 5 Adjustm ent for Differences in Body Size: PBPK Models

Extrapo lation betw een species shoul d ideal ly take into account metaboli c routes, i.e., the absence orpresen ce of metaboli tes, as wel l as the relative rate of formati on of the indi vidual metaboli tes InPBPK model s (Sec tion 4.3.6), both aspect s (nonlinear ity, form ation of active met abolites) areincor porated This modeling techni que uses compa rtments that correspond to actual tis sues or tissuegroups of the body Size, blood flow, air flow, etc are taken into ac count, in addition to speci ficcompo und-re lated parameter s such as partition coef ficients and met abolic rate data Based o n suchstudies, target-or gan concent rations of active metaboli tes can be predicted in experiment al animalsand human s, thus providing the best p ossible basis for extra polation (Feron et al 1990)

Acco rding to Cl ewell et al (2002a) , PBPK model ing provi des imp ortant capabi lities forimprovi ng the reliabilit y of the extrapolat ions across dose, speci es, and exposur e route that aregeneral ly required in che mical risk assessmen t regard less of the toxic endpoi nt being consi dered.The author s have described an approac h, which provides a comm on tem plate for incorporat ingpharm acokinetic modeling to estimat e tissue dosim etry (e.g., tis sue concentrati ons, body burdens ,area under the curve, see Sec tion 4.3.5) into chemical risk assessmen t They noted that chemi cal riskasses sments typically depe nd up on co mparisons a cross speci es that often simpli fy to ratios re flect-ing the diff erences , and have descri bed the uses of this ratio concept and discussed the advant ages of

a pharm acokine tic-based approac h as compa red to the use of default dosime try Based on theiranalys es, they con cluded that the correct relat ionship for cross-speci es dosim etry depends o nwhether the toxicity is due to the parent chemi cal or a met abolite, and in the case of toxi city from

a metaboli te, whet her the metab olite is highly react ive o r suf ficient ly stabl e to enter the circu lation.Moreo ver, the nature of the cross -speci es relationsh ip for each of these possi bilities is different fororal e xposure than for inhalati on Therefor e, PBPK model ing is required to improve the reliabil ity ofcross -species extra polation that considers the natur e of the toxic entity Thus, the avail ability

of infor mation on the parent compo und and its met abolism may allo w modi ficati on of the de faultassessment factor

5.3.3 REMAININGSPECIES-SPECIFICDIFFERENCES

As mentioned earlier, the interspecies differences can be divided into differences in metabolic size(Section 5 3.2) an d remainin g species-sp eci fic differences The average sensitivity of human s to theadverse effects of chemicals (after scaling for caloric requirement) is comparable to that of otherspecies (KEMI 2003) However, an extra assessment factor is needed to account for the remaining

inte rspecies differences , whi ch incl ude diff erences in toxicodynam ics as well as in specie s-speci fictoxi cokinetics such as different e xpressions of met abolizing enzymes.

To account for the rema ining inte rspecies uncert ainties, a defaul t factor is usually used Intheor y, the remainin g uncert ainty could be asses sed by compa ring NOAELs in test a nimals withestimat es of human NOA ELs However , in practice, such an asses smen t must rely on data fromstudies de rived experi mentall y for the same subst ance in different anim al speci es because humandata are lacki ng The degree of remainin g inte rspecies uncert ainty may be obtained by examiningthe differences (ratios) of the NOA ELs estab lished for the same subst ance in diff erent speci es Theuncert ainty in e xtrapolat ing from anim als to humans is like ly to be at least as large as the uncert ainty

in extrapolat ing among mice, rats, and dogs (Vermeire et al 1999)

Fo r the purpos e of asses sing the rema inin g inte rspecies uncertaint y, Ver meire et al (1999)coll ected and analyz ed data for 184 chemicals tested in diff erent species and via different exposur eroute s NOA ELs wer e selec ted from studi es with mice, rats, and dogs exposed to the same chemicalvia the same exposure route and with the same durat ion of exposur e Two catego ries of exposur edurat ion were de fined, subacut e and (sub) chronic, in order to increase the compa rability of thediff erent studies The de finition of these exposur e catego ries is specie s speci fi c, partly dependi ng onthei r maxi mum lif etime Subacut e exposur e was d e fined as 21 –50 days for the mouse and rat, and as

28–90 days for the dog; (sub)chronic exposure was defined as 90–730 days for the mouse and rat,and as 365–730 days for the dog The oral NOAELs were adjusted to account for differences inmet abolic size, i.e., by the calor ic requi rement approac h (S ection 5.3.2.3)

In order to increase the comparability of the derived factors to the actual uncertainty to-human), the ratios were calculated by dividing the NOAELs derived in the smaller animal by theNOAEL derived in the larger animal The following ratios for oral exposure were calculated:NOAELmouse=NOAELrat, NOAELmouse=NOAELdog, and NOAELrat=NOAELdog For respiratorytoxicity data, only the ratios NOAECmouse=NOAECrat were analyzed, as insufficient data forstatistical analyses were available with respect to the other ratios For dermal toxicity, insufficientdata were available for further analyses The ratios, both adjusted and unadjusted for metabolic size,were evaluated by examining their distributions, see Table 5.5

(animal-The results suggest that the distribution of the ratios can be described sufficiently by a lognormaldistribution If the interspecies differences would depend only on the differences in metabolic size,

TABLE 5.5

Distribution of Parameters Derived from the NOAEL Ratios

Source: Modified from Vermeire, T., Stevenson, H., Pieters, M.N., Rennen, M., Slob, W., and

Hakkert, B.C., Crit Rev Toxicol., 29, 439, 1999.

and if NO AELs were perfect e stimates of the true no-eff ect level s (which they clear ly are not), thegeome tric mean (GM ) and the geome tric standard d eviation (GSD) of the ratio distrib utions would

be unity The GMs of the ratios of a djusted NOAELs for mouse=rat and mous e=dog, but not forrat=dog, were close r to 1 than the means of the unadju sted NOA ELs givi ng some suppor t to the idea

of a ccounting for the differences in metaboli c size (scaling based on calor ic requi rement) In theabsence of equivalent human NOA ELs, it was sugges ted that this lognormal distrib ution (GM 1;GSD 6) woul d also charact erize the remainin g interspeci es differences between a nimals andhuman s If the 90th, 95th, and 99th percent iles are calcul ated from this distrib ution of remainin ginterspeci es diff erences, default values for the a ssessment factor adjusted for met abolic size would

be 10, 1 9, and 65, respec tive ly; the often applie d default facto r of 12 (adju stment for met abolic size

4, remainin g uncertaint y 3) coincid ed wi th the 73 rd percent ile

A reanal ysis and extens ion of this databa se (now 198 substanc es) led to a lower stand arddeviation (GSD 4.5) (R ennen et al 2001, Ver meire et al 2001) If the 90th, 95th, and 99thpercent iles are calcul ated from this distrib ution of remainin g interspeci es diff erences, default valuesfor the assessmen t facto r adjus ted for metaboli c size woul d be 7, 12, and 33, respec tively Thepercent iles for a defaul t factor o f 10 (not adjus ted for allo metric scaling) were 77 for extra polatingoral data for mous e-to-rat, 81 for rat-to-do g, and 66 for mous e-to- dog; for a defaul t facto r of

3 (adjusted for allo metric scali ng), the percent iles were 50 for mous e-to-rat, 52 for rat-to-do g, and

59 for mouse-to- dog For extrapolat ion from the rat to human , the traditio nal facto r of 10 coinci dedwith the 73rd percent ile when an allo metric scaling facto r for differences in met abolic size isincluded , in this case 4 for extrapolat ion from a rat study (KEMI 2003)

The se analys es thus indicate that the default facto r of 3 for rema ining uncert ainty, as well as thetraditi onal factor of 10 for interspeci es diff erences in general , in many cases does no t suffi cientlyaccount for the rema inin g inte rspecies differences

5.3.4 A SSESSMENT FACTOR FOR INTERSPECIES V ARIATION (ANIMAL -TO-HUMAN ):

DEFAULT V ALUE

The interspeci es asses smen t facto r is general ly recogni zed as provi ding an extrapolat ion from theaverage animal studied to an average human being, assum ing that human s are 10 time s moresensiti ve to a chemi cal’ s toxic effects than experiment al anim als When dose correction fordifferences in body size betwe en experi mental animals and human s is performed by the bodyweight approac h (Section 5.3.2.1), the traditionally used defaul t inte rspecies assessmen t factor is 10;howe ver, the rationale for this value is not know n

In the following text, various studies will be described, which attempt to establish a scientificrationale for the selection of the interspecies assessment factor Based on these studies, it can beconclu ded that a speci es-speci fic default facto r based on differences in calor ic requireme nt (see

interspecies differences should preferentially be described probabilistically, or a deterministicdefault factor of 2.5 could be used for extrapolation of data from rat studies to the human situation.Dourson and Stara (1983) plotted experimental animal weights versus an interspecies adjust-ment factor, calculated as the cube root of the assumed average human body weight (70 kg) divided

by the experimental animal body weight These interspecies adjustment factors were stated toaccount for differences in doses expressed as mg=kg body weight due to different body surfaceareas between experimental animals and humans, based on the assumption that different speciesare equally sensitive to the effects of a toxic substance on a dose per unit body surface area Whenthis surface area dose is converted to corresponding units of mg=kg body weight, species withgreater body weight (e.g., humans) appeared to be more sensitive to the toxicity of a compound thanspecies of smaller body weight (e.g., rodents); the factors varied from 1 (humans) up to 15 (mice).The factors were thought of as reductions in experimental animal dose (in mg=kg body weight)needed to estimate a comparable human dose (in mg=kg body weight) The factors were also viewed

as support of a 10-fold UF to account for interspeci es varia bility to the toxi city of a chemical whenestimat ing an Acce ptable Daily Intak e (ADI) from animal do ses expressed as mg=kg body weight.The author s found that the 10-fold facto r in that way appeared to incor porat e a MO S if theunderl ying assum ption of dose equiva lence among speci es per unit of surfa ce area is correct , i.e.,

if dose co rrection for diff erences in body size between experi mental animals and humans isperfor med by the body surfa ce area a pproach (Secti on 5.3.2.2)

Fe ron et al (1990) con cluded that the sensi tivity o f humans to chemi cals is probabl y not verydiff erent from that of other mam mals, and that a syst ematic error is made by carryi ng outextra polation by using the body wei ght approac h For met abolizable compounds, the author sstro ngly recom mende d a procedu re that takes the met abolic rate into account ( W 0.75 ) for scalingacross speci es, i.e., dose correct ion for diff erences in body size betwee n experi mental an imals andhuman s by the caloric requireme nt ap proach (Secti on 5 3.2.3) This approac h was also consi dered toprovi de a contr ibution to reduci ng the size of the traditional safety facto r in a just ifi able way.Calabres e et al (1992) noted that the anim al-to-hum an UF is suppos ed to account for allpossi ble facto rs that co uld resul t in inte rspecies diff erences in susceptibili ty, regard less of non-carci nogeni c endpoi nt The value of the inte rspecies UF is traditi onally 10 and was stated as beingfounded on a reasonable public-heal th-based protectiv e phil osophy that assumes that an averagegroup of human s may be a s much as 10-fold more suscep tible than the average group of anima lsunder study It has been argued that the apparen t con flict between the use of body weight and that ofsurfa ce area for dose correction is not real since the animal-to-hu man UF actually incor porates theinte rspecies varia tion facto r with respec t to scaling as wel l a s other inte rspecies diff erences Con sequently, it may be infer red that the use of surfa ce area scali ng amounts to ‘‘ double counti ng ’’when the animal-to-hu man UF of 10 is also employe d This argument was inves tigated by theauthor s Their asses smen t led to the conclu sion that , under ideal circumst ances, do se adjus tmentusing body surfa ce area may lead to a close simil arity across species in the blood level of the agentreachi ng potent ial target organs However , this adjus tment will not addres s some o f the princ ipalcauses of interspeci es varia tion in respon se to xenobi otics, i.e., ce rtain met abolic and toxic odynamicfacto rs Thu s, the dose correction using the body surfac e approac h was not consi dered to eliminateall maj or causes on inte rspecies differences and had no obviou s consi stent quantitat ive inte rspeciesrelations hip to numer ous other factors that affect suscep tibility to toxi c substances Therefor e, ontheor etical grounds , the authors recommend ed that dose correction using the bod y surfa ce approac hshoul d be consi dered indepe ndent of the animal-to -human UF for the purpos e of risk manag ement.Grö nlund (1992) h as inves tigated met hods used for quantitat ive risk asses sment of non-genoto xic subst ances, with special regard to the selec tion of asses sment facto rs Grö nlund foundthat human s, in most c ases, seem to be more sensiti ve to the toxic effects of chemicals thanexperi mental animals, and that the traditi onal 10-fol d factor for interspeci es diff erences apparen tly

is too small in order to cover the real variation It was also noted that a general interspecies facto r tocover all types of chemica ls and all types of ex periment al animals cannot be expect ed It wasconclu ded that a 10-fol d facto r for interspeci es varia bility probably protects a majority, but not all ofthe popula tion , provid ed that the dose correction for differences in body size between experi mentalanim als and human s is pe rformed by the body surface area approac h (Secti on 5.3.2.2) If the dosecorrect ion is b ased on the body wei ght approac h (Secti on 5.3.2.1), the 10-fol d facto r was co nsidered

to be too small in most cases

Renwick (1993) examined the relative magnitude of toxicokinetic and toxicodynamic variationsbetween species in detail and found that toxicokinetic differences were generally greater thantoxicodynamic differences In order to allow for separate evaluations of differences in toxicokineticsand toxicodynamics, he proposed that the default interspecies UF of 10 should, by default, besubdivided into a sub-factor of 4 for toxicokinetics and a sub-factor of 2.5 for toxicodynamics.The suggested factor of 4 for differences in toxicokinetics was largely based on the extent ofabsorption and the rate of elimination or clearance in different experimental animals The suggested

facto r o f 2.5 for toxi codynam ic diff erences was not scien tificall y based, but mainly the remainin gvalue to fit the traditi onal defaul t inte rspecies assessmen t facto r of 10.

Kalberl ah and Schneid er (1998) have an alyzed the informat ion on the quanti fi cation of ation factors They noted that in interspeci es extrapo lation, two varia bles must be diff erentiate d: Thesystema tic diff erences betw een diff erent speci es, and the varia bility in the sensi tivity of the speci es.Systema tic diff erences can, e.g., be recorde d by means of allo metric approac hes, ‘‘scali ng ’’ (Secti on5.3.2) The reason s for the varia bility in sensi tivity may be due to both toxicokin etic and toxi cody-namic characteristics of a species

extrapol-In their analyses, statistics on the relevant extrapolation factor from animals to humans, asreported in the literature, were considered synoptically, and distinctions were made between:(1) publications which focused on allometrically justifiable differences; (2) publications whichexamined the toxicodynamic or toxicokinetic variability; and (3) publications which consideredthe total (gross) interspecies factor In addition, consideration of PBPK models was discussed as apossible alternative

If sufficient data are available, substance-specific PBPK models should always be givenpreference over the use of general scaling factors However, PBPK models were considered not

to replace all of the sub-factors in the interspecies comparison and should, by definition, onlyinclude toxicokinetic differences A further extrapolation factor for toxicodynamic differencesbetween the species needs to be discussed

If PBPK models cannot be used, scaling is the recommended approach Theoretical ations and the evaluation of numerous publications supported scaling according to basal metabolicrate (W0.75) When doses from experimental animal studies are expressed as mg=kg body weight,this scaling approach means an extrapolation factor of 7 for mouse-to-man, 3.9 for rat(Fischer)-to-man, 3.6 for rat(Sprague–Dawley)-to-man, 1.6 for dog(Beagle)-to-man, 3.9 for monkey(marmoset)-to-man, and 1.6 for monkey(rhesus)-to-man It was noted that these extrapolationfactors only account for toxicokinetic differences in the basal metabolic rate If an interspeciesextrapolation with an average degree of statistical certainty is undertaken (for approximately 50% ofsubstances, i.e., the 50th percentile), this extrapolation step is sufficient

consider-A second extrapolation step for toxicokinetic and toxicodynamic variability was recommended

if interspecies comparisons involve, with a certain statistical probability, the occurrence of an average sensitivity in humans as compared with the test animal species

above-The authors noted that, in comparison with the scaling factor, the traditional 10-fold factorcontains an additional extrapolation factor for possible additional toxicokinetic or toxicodynamicvariability apart from the basal metabolic rate scaling This additional factor, which can be inter-preted as the traditional 10-fold factor divided by the scaling factor, ranges from approximately 1.5for the mouse (10=7 ¼ 1.4) to approximately 6 for the rhesus monkey (10=1.6 ¼ 6.3) The authorsconsidered that the additional factor thus comprises levels of safety, which are currently nonuni-form, and this inhomogeneity is not supported toxicologically

The authors also noted that the database for the derivation of a standard value is very limited.The few available data support a factor of 2–3 additional (multiplicatively) to the scaling factor inorder to cover, for approximately 95% of the substances (i.e., the 95th percentile), a possibly greatersensitivity of humans compared with experimental animals The overall interspecies assessmentfactor would then be 8–12 for a rat study and 14–21 for a mouse study

When dose levels are expressed as the concentration in the medium (mg=m3air or mg=kg feed),

a scaling factor is not relevant In such cases, only the toxicokinetic and toxicodynamic variability,i.e., an interspecies factor of 2–3 should be applied

Finally, it was emphasized that substance-specific modifications are always possible when data

on toxicokinetics and toxicodynamics permit more precise statements on interspecies differences.Renwick (1999) noted that, from the perspective of the late 1990s, it is nạve to expect a single10-fold assessment factor to allow for differences between different test animals and humans It wasalso noted that the 10-fold interspecies factor is applied to an intake expressed in mg=kg body

weight, but most physiologi cal a nd many biochemic al proces ses correl ate better with body surfa cearea It was there fore conclu ded that interspeci es facto rs of about 3–4 and 8–1 0 woul d be necess aryfor rats and mic e, respec tively, to convert the exter nal dose expressed in mg=kg bod y weight into adose based on body surfa ce area, see also Secti on 5.3.2.2.

Ver meire et al (1999) have asses sed the remaining interspeci es uncert ainty, i.e., the uncert aintynot related to differences in metaboli c size that c an be account ed for by allo metric scaling (Section

2001, Vermei re et al 2001) The reanal yses indicated that the defaul t factor of 3 for rema ininguncert ainty, as well as the traditional factor of 10 for interspecies differences in general, in manycases does not sufficiently account for the remaining interspecies differences

Analyses of species differences in the toxicokinetics of compo unds elimin ated by a single majormet abolic pathway in human s have been perfor med by Ren wick and cowo rkers using publisheddata for compo unds in four test species (dog, rabbi t, rat, and mouse)

Walt on et al (2001a) exami ned data for compo unds elim inated by the cytoch rome P450isoen zymes CYP1A2 in humans Absorpt ion, bioava ilability, and route of excret ion were generallysimil ar between humans and the test species for each of the substances (caffeine, p araxanthine ,theobr omine, and theoph ylline) However , interspeci es differences in the route of metabolism , andthe enzym es invol ved in this proces s, wer e identi fied The magni tude of difference in the internaldose, between speci es, show ed that values for the mouse (10.6) and rat (5.4) exceed ed the fourfolddefaul t facto r for toxicokin etic s, wher eas the rabbit (2.6) and the dog (1.6) were below this value

In a second study (Wal ton et al 2001b), the magnitud e of the interspecies diffe rences in theinte rnal dose of compounds for whi ch g lucuronidat ion is the major pathway of metaboli sm ineith er human s or in the test speci es was deter mined The re were maj or inte rspecies diff erences inthe natur e of the biol ogical processes that in fluence the inte rnal dose including route of metabolism ,the extent of pre-s ystemic metaboli sm, and enter ohepatic recir culation There was also a widevaria bility in the magnitud e of diff erences in the internal do se for all of the test speci es The meanvalues for the clear ance ratios compa red to human s wer e 4.5 for the mous e, 9.1 for the rat, 8.7 forthe rabbit, and 9 7 for the dog Thu s, the fourfold default factor was exceeded for all the speci es.Walt on et al (2004) d etermine d the extent of inte rspecies d ifferences in the internal dose ofcompo unds, which are eliminated primarily by renal excretion in humans Renal excretion was alsothe main route of elimination in the test species for most of the compounds Interspecies differenceswere apparent for both the mechanism of renal excretion (glomerularfiltration, tubular secretion,and=or reabsorption), and the extent of plasma protein binding Both of these may affect renalclearance and therefore the magnitude of species differences in the internal dose For compoundswhich were eliminated unchanged by both humans and the test species, the average difference in theinternal dose between humans and animals were 1.6 for dogs, 3.3 for rabbits, 5.2 for rats, and 13 formice This suggests that for renal excretion the differences between humans and the rat, andespecially the mouse, may exceed the fourfold default factor for toxicokinetics

The analyses thus indicated that pathway-related factors for different species could be derivedfor some pathways; however, according to Walton et al (2001c), the pathway of elimination inhumans for most compounds did not reliably predict the pathway in animals Thus, there would beconsiderable uncertainty in using a species-specific pathway-related factor, unless there are detaileddata for both humans and the animal species

ECETOC (2003) recommended that in the absence of any substance- or species-specificmecha nism or PBPK model ing (Section 4.3.6), allomet ric scali ng based on metabolic rate (W 0.75 )(calo ric requi reme nt a pproach , Section 5.3 2.3) is consi dered to provide an a ppropriate defaul t for

an assessment factor for interspecies differences with respect to systemic effects Allometric scalingwas stated as being a tool for estimating interspecies differences of internal exposure or body burdenand to provide indirectly information on differences in sensitivity between species Typical scalingfactors for interspecies adjustment were noted as 7 for mouse, 4 for rat, and 2 for dog; however,

adjus tments of these scali ng facto rs may be ne cessary especiall y for direc tly acting and met ally acti vated=inactivat ed c ompounds.

abolic-ECET OC also considered that the scaling approac h migh t not acco unt completely for speci es variation in biol ogical sensitiv ity and might not addres s speci al cases of higher sensitivity inhuman s due to toxi cokinetic or toxi codynamic diff erences between animals and human s Thedataba se for determin ation of this (addi tional) asses smen t factor for inte rspecies sensitivity wasconsi dered a s smal l and most like ly c onfound ed by intraspecie s varia bility It was c oncluded that ,although resi dual inte rspecies variability may rema in following allomet ric scaling, this is large lyaccount ed for in the default asses smen t factor for intraspeci es varia bility re flecting the inher entinterdepend enc y of the inte r- and intraspeci es factors

inter-For local effect s, a defaul t asses smen t factor of 1 for interspecies extrapolat ion for water-solubl egases an d vapors was consi dered to be suf ficient ly conser vati ve, as well as for aerosols since therespirato ry rate of rodents leads to a great er respirato ry tract burden as compared to h umans.The WHO=IPCS (1994, 1996, 1999) has adopted the approac h set forth by Renwi ck (1993) , i.e.,the UF for interspeci es (animal- to-human) extra polation shoul d be split into defaul t values of 4 fortoxicokin etic s and 2.5 for toxicodynam ics, see Section 5.2.1.3 In sit uations wher e appropriat etoxicokin etic and=or toxi codynam ic data are available for a particula r compo und, the relev ant UFshoul d be replaced b y the data- deriv ed factor If a data- derived facto r is intr oduced, then thecommon ly used 10-fold facto r would be replaced by the product of that d ata-derive d facto r andthe rema ining defaul t factor It is also noted that for some c lasses of compo unds, a d ata-derive dfacto r for one mem ber of the class may be applicab le to a ll mem bers thereby producing a group-based data-deriv ed factor

In 1988, the US-EPA adopte d the ADI approac h with respect to the deriv ation of RfDs andRfCs with a 10-fold UF to accoun t for interspeci es extra polation (US-EPA 1988, 1993), see Secti on

interspecies UF is generally presumed to include both toxicokinetic and toxicodynamic aspects.Much of the RfC methodology (US-EPA 1994) focused on improving the science underlyingthe animal-to-human UF, segregating it into toxicokinetic and toxicodynamic components andproviding generalized procedures to derive DAFs (domimetric adjustment factors, represents amultiplicative factor used to adjust an observed exposure concentration in a particular laboratoryspecies to an exposure concentration for humans that would be associated with the same delivereddose) Application of a DAF in the calculation of a human equivalent concentration (HEC) wasconsidered to address the toxicokinetic aspects of the animal-to-human UF, i.e., to estimate fromanimal exposure information the human exposure scenario that would result in the same dose to agiven target tissue The RfC methodology recommended dividing the default interspecies UF of 10

in half, one-half (100.5) each for toxicokinetic and toxicodynamic considerations The methodologyincluded a DAF to account for toxicokinetic differences in calculating the HEC, thus reducing theuncertainty about the remaining toxicodynamic component through application of the partialanimal-to-human UF (100.5, which is typically rounded to 3) It was also noted that seldom arethere data available to inform toxicodynamic differences One-half the default 10-fold interspecies

UF (i.e., 100.5) was assumed to account for such differences, but more specific data should be usedwhen available and theflexibility for applying a factor greater than 10 should be recognized Unlessdata support the conclusion that the test species is more or equally susceptible to the pollutant as arehumans, and in the absence of any other specific toxicokinetic or toxicodynamic data, a defaultfactor of 3 (in conjunction with HEC derivation) or 10 is applied

It was also noted, in the 2002 review of the RfD and RfC processes (US-EPA 2002), thatcurrently, no procedures parallel to the inhalation RfC methodology exist for deriving either oral ordermal human equivalents from animal data Default factors (usually of 10) are routinely applied toaddress the issue of animal-to-human extrapolation Thus, no parallel to the HEC, i.e., a humanequivalent dose (HED), is derived nor are other adjustments applied to the animal oral or dermaldose Instead, assumptions are made regarding the comparability of ingested or applied dose, based

on a mg=kg body wei ght basis The calor ic requireme nt approac h for dose correct ion was analyz edand this p rocess was recommend ed as a possible candid ate for estimating cross -speci es toxicoki-neti c relat ionships in the absence of adequate toxi cokinetic infor mation That is, W 0.75 facto rs could

be applied as DA Fs for d eriving a human equivalent dose (HED) This procedu re would parallel theone used for deriv ing the HEC It was noted that, as wi th the HEC, this proces s applies only totoxi cokinetic aspect s of cross -speci es extra polation and does not addres s the toxi codynam ic differ-ences that may ex ist between speci es As with the HEC, considerat ion of toxi codynam ic differenceswas propos ed to be throu gh applicati on of a portion of the animal-to-hu man extra polation (10 0.5 ,which is typi cally rounded to 3)

TNO has sugges ted a defaul t interspeci es assessmen t facto r composed of a scaling facto rallo wing for differences in basal met abolic rate ( W 0.75 ) for oral studies dependi ng on thespeci es (mouse 7, rat 4, rabbit 2.4, dog 1.4), and a facto r of 3 for rema ining v ariability (Hakkert

et al 1996) For local skin and respi ratory tract effects, an asses smen t facto r of 3 was sugges ted,

as an adjus tment for differences in body size is inappr opria te for local effects, see also Se ction5.2.1.7

KEMI (2003) has suggested that a speci es-sp eci fic defaul t facto r shoul d be used for inte rspeciesextra polation regarding metaboli c size Thi s facto r shoul d be based o n diff erences in calor icdeman d ( W 0.75 ) and the facto r for extrapolat ion from rats is 4, from mice 7, from guinea pigs 3,from rabbi ts 2.4, and from dogs 1.4 The rema ining varia bility can be described by a distrib ution,i.e., a probabi listic approac h, see Secti on 5.3.3 Which percent ile of the distrib ution, i.e., percent

of substances to be covered by the factor should be chosen is a matter of judgment The compositeinterspecies assessment factor would be 48 (123 4) if the 95th percentile (i.e., 95% of substances

to be covered) is chosen in the case of extrapolation from a study in rats The traditional defaultfactor of 10 only covers the 73rd percentile of the distribution, i.e., 73% of the substancesare covered

5.3.5 INTERSPECIESEXTRAPOLATION(ANIMAL-TO-HUMAN): SUMMARY ANDRECOMMENDATIONSThe rationale for extrapolation of toxicity data across species is founded in the commonality ofanatomic characteristics and the universality of physiological functions and biochemical reactions,despite the great diversity of sizes, shapes, and forms of mammalian species

For extrapolation of data from animal studies to humans, account should be taken of specific differences between animals and humans

species-Ideally, the interspecies extrapolation should be based on substance-specific information;however, for most substances, only limited or no data are available Therefore, an assessment factor

is usually applied in the interspecies extrapolation The traditionally used default interspeciesassessment factor is 10, possibly divided into a sub-factor of 4 for differences in toxicokineticsand a sub-factor of 2.5 for differences in toxicodynamics as proposed by Renwick (1993) andadopted by the WHO=IPCS (1994) The validity of the interspecies default factor of 4 for toxico-kinetics has been assessed by Walton et al (2001a,b) for each of the test species (dog, rabbit, rat,and mouse); the authors concluded that their assessment supports the need to replace the genericdefault factor by a compound-related value derived from specific, relevant, quantitative data.Two variables must be differentiated in interspecies extrapolation:

. Consideration of the systematic differences between different species

. Consideration of the variability in the sensitivity of the species

Systematic differences can be accounted for by means of allometric‘‘scaling.’’ The reasons for thevariability in sensitivity may be due to both toxicokinetic and toxicodynamic characteristics of aspecies and may imply both higher and lower sensitivity for man when compared to experimental

animals These inte rspecies differences can thus be seen as differences in metaboli c size andrema ining species-sp eci fic differences

Differe nces in met abolic size, the maj or part of toxi cokine tic differences , can be account edfor by allo metric scaling (Section 5.3.2) To account for diff erences in metaboli c size (differenc es

in body size betw een human s an d experiment al anim als), three meas ures of body size are used

in pract ice as the basis for the extra polation: body weight, body surfa ce area, and calor icrequi rement The se met hods can be described by an allomet ric equation in whi ch body weighthas to be raised to the power of 1, 0.67, and 0 75, respective ly Scaling on the basis of bodyweight (Secti on 5.3.2.1) most often provi des the quanti tative basis for the c orrection of dosesfor differences in body size between experi mental animals and hu mans However , scaling on thebasis of body surfa ce area (Section 5.3.2.2) or calor ic requi reme nt (Section 5.3.2.3 ) is consi dered

as more appropr iate compa red to extra polation based on body weight There is a general ,internati onal, an d scienti fic recommend atio n to the calor ic requi reme nt approac h, i.e., scali ngaccordi ng to the basal metaboli c rate ( W 0.75 ); scaling facto rs for the most comm only usedexperi mental animals are provi ded in Table 5.4 If suffi cient data are availab le, subst ance-speci fic PBPK model s shoul d always be given prefer ence over the use of g eneral scaling factors.However , PBPK model s only account for toxi cokine tic differences and thus cannot repla ce all ofthe sub-fac tors in the interspeci es extrapolat ion It shoul d be reme mber ed that allo metric scaling isrelev ant for oral exposur e (and derm al e xposure if syst emic effect s occur) only as (sm all)animals and human s breat he at a rate related to their need for oxygen, i.e., at a rate depend-ing on their calor ic requi rement; therefore, they are autom atically being exposed to chemicalsoccurr ing in the breat hing atmospher e at a rate similar to that of the caloric requireme nt(Section 5.3.2.4)

The average sensiti vity of human s (aft er scali ng for caloric requireme nt) is consi dered to becompa rable to other speci es How ever, an extra assessmen t facto r is need ed to accou nt for therema ining interspeci es differences , which include differences in toxicody namics as well as inspeci es-speci fic toxi cokinetics Usuall y, a default facto r is used; however, there is no general,internati onal, and scien tifi c recom menda tion to a stand ard value for the remainin g interspeci esdifferences (Section 5.3.4) The TNO in the Netherl ands has propos ed a stand ard value of 3(Hakkert et al 1996 ), and Kalberl ah and Schneid er (1998) a factor of 2 –3 addit ional (multip lica-tively) to the scaling facto r Con siderati on of toxi codynamic differences was propos ed by theWHO=IPCS (1994) throu gh ap plication of a defaul t facto r of 2 5 as proposed by Renwick (1993) ,and by the US-EPA (2002) throu gh appli cation of a portion of the anim al-to-hum an extra polationdefaul t facto r of 10, i.e., 10 0.5 , which is typicall y rounded to 3 ECETO C (2003) conclu ded that ,although resi dual inte rspecies variability may rema in following allomet ric scaling, this is large lyaccount ed for in the defaul t asses sment facto r for intraspeci es varia bility, re flecti ng the inher entinterdepend enc y of the inte r- and intraspeci es factors However , the analys es pe rformed byVermei re et al (1999, 2001) and Rennen et al (2001) indi cate that a default facto r of 3 forrema ining uncertaint y, in many cases, does not suf ficient ly account for the rema ining interspeci esdifferences (Secti on 5.3.3) For these reason s, Vermei re et al (1999, 2001) and KEMI (2003) haverecommended that the remaining variability should be described by a distribution, i.e., a probabilis-tic approach; which percentile of the distribution, i.e., percent of substances to be covered by thefactor, should be chosen is a matter of judgment

In conclusion, if no substance-specific data are available, it is recommended as a default tocorrect for differences in metabolic size (differences in body size between humans and experimentalanimals) by using allometric scaling based on the caloric requirement approach (see Table 5.4) Theassessment factor accounting for remaining interspecies differences should preferentially bedescribed probabilistically as suggested by Vermeire et al (1999, 2001) and KEMI (2003), or adeterministic default factor of 2.5 could be used for extrapolation of data from rat studies to thehuman situation

5.4 INTRASPECIES EXTRAPOLATION (INTERINDIVIDUAL,

HUMAN-TO-HUMAN)

Risk asses sments are usual ly based on data from studies in anim als of sim ilar age In addition, theanim als are initial ly healt hy an d are fed wi th the same feed, etc The NOAEL from animal studies isextra polated to a tole rable intake that is co nsidered to be without appreci able health risk for thegeneral popula tion This rais es the quest ions whether it is possible to general ize to the averagehuman popula tion or whethe r there is any particula r vulnerable subpopu lation that should be takeninto consi derat ion in the risk asses sment

This secti on gives an overvi ew regarding the biol ogical varia tion between human indi viduals(Secti on 5.4.1 ) The n a numbe r of analys es perfor med regard ing the validit y of the defaul tasses sment facto r of 10 are revie wed (Section 5.4.2 ) Finally, the key issues are summari zed andour recommend atio ns are presen ted (Secti on 5.4.3)

5.4.1 B IOLOGICAL V ARIATION

In compa rison wi th the genetic ally relativel y homog enous inbred and outbr ed strains of tal animals used for toxicolog ical testing, a considerabl y great er varia bility in the respon ses tochemi cals c an be expect ed in the heter ogeneous human popula tion This is due in part to g eneticfacto rs, but also to acquir ed suscep tibility factors as well as to previ ous or simultaneous expo sure tomul tiple compo unds (industrial chemi cals, food addit ives, pesticide s, drugs), all of which may have

experimen-an impact on the NOA EL for the diff erent individua ls of a popula tion The foll owing facto rs mayplay a role for the mark ed diff erences wi th respec t to the inte rindivid ual sensi tivity in the respon ses

to chemicals :

. Gene tic facto rs (enzym e po lymorphi sms, hereditary metaboli c disor ders)

. Age and develo pment (physi ology, organ sensi tivity)

. Gend er

. Health and disease status (diet, stress, lifestyle)

. Speci fic const itution and situati on (weight, propor tion of fat, pregnan cy)

The inte rindivid ual varia bility re flects diff erences in the ex tent o f exposur e, in toxicokin etics as well

as in toxicodynam ics The variab ility due to facto rs which in fluence the extent o f exposur e(physi ological differences in the intake, e.g., inhal ation rates ) can be considered by means ofsuitable param eters for the internal exposur e (absor bed dose, area under the curve AUC, plasmaconcent ration) if suf ficient infor mation is avail able With respec t to toxicok inetic facto rs, interindi-vidual differences in the met abolism of chemi cals are general ly considered as the most signi ficantexplanatory factor Hardly any knowledge is available with respect to the factors that influencetoxicodynamics In the following, a brief overview of the factors playing a role for the toxicokineticand toxicodynamic differences is presented

5.4.1.1 Age and Development

A human being changes, anatomically, biochemically, and physiologically, during its lifetime fromconception to death, and there may be windows of increased vulnerability or periods of humandevelopment when chemical exposures may substantially alter organ structure or function Someorgan systems show specific vulnerability to chemical toxicity during development, as organmaturation is an ongoing process throughout the embryo-fetal period and during infancy, childhood,pubertal period, and adolescence, and will not be completed until adulthood For example, thereproductive and endocrine systems mature slowly, reaching a peak immediately prior to adulthood,the immune system develops both pre- and postpartum, and the nervous system develops both lateduring pregnancy and in the infant and child (Nielsen et al 2001)

The concept that infants and children may be a sensitive subgroup relates to their relativeimmaturity compared to adults Children, as well as the unborn child, have in some cases appeared

to be uniquely vulnerable to toxic effects of chemicals because periods of rapid growth anddevelopment render them more susceptible to some specific toxic effects when compared to adults

In addition to such toxicodynamic factors, differences in toxicokinetics may contribute to anincreased susceptibility during these periods It should be noted, however, that during the develop-mental and maturational periods the susceptibility to exposure to xenobiotics in children may behigher, equal, or even lower than in adults Except for a few specific substances, not very much isknown about whether and why the response to a substance may differ between age groups It shouldalso be borne in mind that, in terms of risk assessment, children are not simply small adults, butrather a unique population (Nielsen et al 2001)

In general, the fetus is not protected against xenobiotics that circulate in the maternal blood aschemicals, which pass maternal membranes, are also likely to pass the placental barrier The humanfetus and the placenta possess metabolic capacity, but the contribution of these metabolizing entities

to the total kinetics is probably minimal The period of organogenesis (the embryonic stage fromapproximately 2 to 8 weeks of gestation) is considered to be the developmental phase most sensitive

to exogenously induced classical malformations in single organs During the fetal stage (from the9th week of pregnancy until birth), which is characterized by differentiation, growth, and physio-logical maturation, the fetus becomes increasingly resistant to the actions of teratogens Exposure tochemicals during this period is most likely to result in effects on growth and functional maturation.Receptors and other molecular targets for chemicals affecting future functions are developingcontinuously, so that the fetus may be even more sensitive than the embryo to some toxic effects(Nielsen et al 2001)

Infancy is the period from birth up to 12 months of age In early infancy (0 to 4 months), theorgans are still rather immature and various maturation processes take place The complexity ofall these factors makes it difficult to predict the net effect of xenobiotics on toxicokinetics; however,the maturation of the gastrointestinal system, liver, and kidneys has generally taken place within

6–12 months after birth By late infancy (4 to 12 months), most processes related to metabolicactivity and excretion are probably comparable to those of adults for most substances Because ofthe immature function of the organs, neonates and young infants may have lower biotransformationand elimination capacities This may render these individuals less able to detoxify and excretexenobiotics and thereby more vulnerable to toxicants On the other hand, if toxicity is caused by atoxic intermediate produced via biotransformation, young infants may be less sensitive In child-hood (1 to 12 years), metabolism and excretion of xenobiotics may be equal to or even higher than

in adults due to the higher basal metabolic rate and larger relative liver size It is difficult togeneralize about age-dependent deficiencies in the metabolism of xenobiotics because the variousenzyme systems mature at different time points The age at which metabolism is similar to the adultvalue may be different for each substance In general, the most prominent differences in toxicoki-netics are seen in children less than 1 year of age, especially in thefirst few days and weeks of life.However, children do not seem to represent a special group from a toxicokinetic viewpointregarding variability among children, as the toxicokinetic variability among children generallyappears to be of a similar magnitude as the variability among adults (Nielsen et al 2001)

Generally, it appears that effects of xenobiotics on organs or endpoints may be similar inchildren and adults, e.g., liver necrosis observed in adults will also be observed in children Asregards toxicodynamics, age-dependent differences are primarily related to the specific and uniqueeffects that substances may have on the development of the embryo, fetus, and child in that thephysiological development of the nervous, immune, and endocrine=reproductive systems continuesuntil adolescence (12 to 18 years) Furthermore, receptors and other molecular targets for variousxenobiotics are continuously developing during the embryonic, fetal, and infant periods This maycause age-dependent differences in the outcome of receptor–xenobiotic interactions and even result

in opposite effects of xenobiotics in infants and adults The available data are insufficient to evaluate

the toxicodynamic variability among children as well as the differences between children and adults(Nielsen et al 2001).

Children’s exposure pattern differs from that of adults and children may be more heavilyexposed than adults to certain chemicals in the environment as they, on a body weight basis,breathe more air, drink more water, and eat more food than adults; additionally, their behaviorpatterns, such as play close to the ground and hand-to-mouth activities, can increase their exposure.The differences in exposure patterns between children and adults are often used as an argument forincreased susceptibility of children to chemicals; however, it should be recognized that suchdifferences are not related to increased vulnerability to chemicals but are purely related to anincreased internal exposure (Nielsen et al 2001)

Irrespective of other differences, there might be different conditions during adulthood, such aspregnancy, that might change the susceptibility to chemicals During pregnancy, many physio-logical changes occur in the maternal organism as a consequence of, and in order to support, therapid growth of the fetus and reproductive tissues These changes may in different ways influencethe toxicokinetic handling of a chemical Absorption of chemicals from the gastrointestinal tractincreases and hepatic metabolism decreases during pregnancy; this may favor retention of chemicalsleading to enhanced toxicity Also, the higher fat content during pregnancy increases the potentialfor greater body burden of lipophilic chemicals There are also signs of metabolic changes andincreasing activity in certain endocrine organs (Nielsen et al 2001)

The numerous physiological and biochemical changes occurring during aging can modify thetoxicokinetics and toxicodynamics of chemicals in the elderly, resulting in either higher or lowertoxicity Enhanced effects caused by xenobiotics in the elderly (from 60–65 years), may be due todecreases in renal excretion, hepatic extraction, plasma protein binding, or volume of distribution forwater-soluble chemicals Many elderly people have impaired renal function and chemicals that areexcreted primarily by the kidneys will persist longer in the blood of older individuals than in youngerpeople Elderly people have decreased hepatic mass and bloodflow Moreover, phase I enzymes oftendecline in rate of function The decrease in cardiac output that accompanies aging gradually changesthe regional distribution of bloodflow in the body As a result, transport of chemicals to the liver andkidneys is slower, delaying inactivation of many chemicals Elderly people also have a higherpercentage of body fat, on average, than younger adults, so lipophilic chemicals are more extensivelystored in older people and accumulate to a greater extent Furthermore, various organ systems alterfunctionally with increasing age In the nervous system, the density of neurons and dendrites isreduced, the receptor densities on the cell surfaces are reduced, the immune system reacts more slowly

to xenobiotics, the cardiovascular system is limited functionally as a result of arterioscleroticsymptoms, and the respiratory tract has a smaller area for gas exchange The responses of targettissues in the elderly may also be enhanced because of a less unused functional capacity in reserve As

a result, chemical exposures impact more keenly on tissue systems already stretched to the limit Inaddition, aging can potentially alter not only the structure of genes, but also the way in which theyfunction Changes in the DNA are often thought to be integral to aging It is clear that not onlymutations but also chromosomal rearrangements accumulate with age (WHO=IPCS 1993, Kalberlahand Schneider 1998, Dybing and Søderlund 1999, Beltoft et al 2001)

Clewell et al (2002b) have reviewed and evaluated the potential impact of age-specificpharmacokinetic differences on tissue dosimetry A large number of age-specific quantitativedifferences in pharmacokinetic parameters were identified The majority of these differences wereidentified between neonates=children and adults, with fewer differences being identified betweenyoung adults and the elderly

5.4.1.2 Gender

Women and men differ from each other in some constitutive and physiological parameters Forexample, weight, tidal volume, and the water and fat content of the body differ between genders

However, the differences become smaller when normalized according to body weight or bodysurface area Many physiological parameters are altered in women during pregnancy due to specificprocesses of adaptation to the needs of the circulatory system of pregnant women and fetuses, e.g.,the blood output of the heart increases by 50% for only a slight increase in the body surface area.Data indicate that differences in physiological parameters that may influence the toxicokinetics ofchemicals are generally below a factor of 2 (Kalberlah and Schneider 1998).

Clewell et al (2002b) have reviewed and evaluated the potential impact of gender-specificpharmacokinetic differences on tissue dosimetry A large number of gender-specific quantitativedifferences in pharmacokinetic parameters were identified The majority of these differences wereidentified between neonates=children and adults, with fewer differences being identified betweenyoung adults and the elderly

Differences are also obvious if chemicals have specific mechanisms of action that affect specific differences, i.e., as a result of interaction with hormonal regulation, specific damage to thesex organs, or adverse effects on organs in the development of the infantile organism (Kalberlah andSchneider 1998)

gender-5.4.1.3 Genetic Polymorphism

Enzyme levels and activities within the human population can vary considerably and many of theenzymes involved in the metabolism of xenobiotics are polymorphically distributed in the humanpopulation Genetic polymorphism (from Greek: poly‘‘many’’, morph ‘‘form’’) is defined as theoccurrence of at least two different alleles, with allele frequencies exceeding 1% at a particularlocus The allelic variants include point mutations as well as deletions and insertions and geneticpolymorphism may cause an increase, a decrease, or no change in enzymatic activity

Genetic polymorphism may result in‘‘poor metabolizers’’ (i.e., individuals who have only alimited or no capacity to metabolize a given chemical via a specific enzymatic pathway), and

‘‘extensive metabolizers’’ (i.e., individuals who have a sufficient capacity to metabolize a givenchemical via a specific enzymatic pathway) and individuals of a particular group may thereforerespond differently to exposure to chemicals

The polymorphisms in genes encoding xenobiotic metabolizing enzymes are far more ent than those seen in genes encoding enzymes of endogenous importance with defined physio-logical functions Because of genetic drift, where a small population has migrated and thenexpanded, interethnic differences in the distribution of the genes encoding xenobiotic metabolizingenzymes are sometimes large (KEMI 2003)

promin-It should be noted that a large variation in metabolic capacity not necessarily corresponds to anequal variation in toxicity

There are many examples in the literature showing a more than 10-fold variation in metaboliccapacity depending on genetic polymorphism in the involved enzymes (Kalberlah and Schneider1998)

Data reviewed in Beltoft et al (2001) show interindividual levels and differences in theactivities of cytochrome P450 (CYP) enzymes within the human population Polymorphisms invarious CYP enzymes have been described and may result in poor metabolizers For example,approximately 2%–7% of the Caucasian population and 18%–23% of Japanese are reported to bepoor metabolizers due to genetic alterations The CYP enzymes are haemeprotein oxidoreductasesthat utilize electrons (from NADPH) to reduce molecular oxygen and oxidize molecules containing,e.g., carbon, nitrogen, oxygen, sulphur, phosphorus, and=or halogens The CYP enzymes belong tothree main P450 gene families: CYP1, CYP2, and CYP3 The highest concentration of CYPenzymes involved in biotransformation of xenobiotics is found in the liver, but CYP enzymes arepresent in virtually all tissues

The glutathione S-transferases (GST) enzymes are dimeric enzymes that catalyze the tion of glutathione (GSH) to electrophilic xenobiotics in order to inactivate them and facilitate their

conjuga-excretion from the body The isoenzymes are divided into at least seven classes, and markedinterindividual differences exist in the expression of some of these classes Genetic polymorphisms,deletion of the whole gene (null allele), have been detected in two of the classes (GSTM1 andGSTT1) of this enzyme family and ethnic differences have been reported with the frequency of theGSTM1 and GSTT1 genotypes ranging from 5.8% to 58% and from 12% to 62%, respectively,among different ethnic groups (Beltoft et al 2001).

The paraoxonase enzyme (PON1), involved in the metabolism of organophosphate pesticidesand aryl ester compounds, is widely distributed in mammals, including humans Differences inobserved rates of hydrolysis of the toxic metabolite (paraoxon) of the pesticide parathion betweenindividuals have been reported to vary by at least 20-fold There is also a large interindividualvariability of the level of circulating PON1 Human PON1 exists in two polymorphic forms:PON1R192, which hydrolyzes paraoxon at a high rate; and PON1Q192, which hydrolyzes paraoxon

at a low rate Significant differences in gene frequencies between different ethnic groups have beenreported with a frequency for the PON1R192 allele of 0.31 in Caucasian populations, 0.41 inHispanic populations, and 0.66 in Japanese populations (Beltoft et al 2001)

Other metabolic enzymes that show polymorphic differences in that they can occur as genetichigh-activity and low-activity variants include acetylcholinesterase, butyrylcholinesterases,flavin-dependent monooxygenase, alcohol dehydrogenase, epoxide hydrolase, and arylesterase (Beltoft

et al 2001)

Altered enzyme levels and activities may thus render some individuals more susceptible toexposure to chemicals than the general population It could therefore be hypothesized that even avery low exposure to a chemical may be associated with various biological responses in suchsusceptible individuals as altered enzyme levels and activities may influence the individual’s ability

to detoxify a chemical or increase the conversion of a chemical to a toxic metabolite Whether and towhat extent an altered enzyme level or activity will increase an individual’s risk of experienceadverse effects from exposure to chemicals is generally not known

In addition to polymorphisms in biotransformation enzymes, some genetically determinedvariations in toxicodynamic processes have also been described Genetic polymorphisms are present

in many receptor genes, for example the D2 dopamine receptor or the b2-adrenoceptor, but thefunctional significance of these variations and the importance for cell signaling is uncertain (KEMI2003) The extreme polymorphism of the immune system can explain why chemicals alone orconjugated with tissue macromolecules are recognized very differently by different individuals(Weigle 1997, as cited in Dybing and Søderlund 1999)

Humans also display marked interindividual variability in the capacity to repair damaged DNA.This variation is partly due to genetic factors, and genetic polymorphisms have been found inseveral proteins involved in DNA repair (Kalberlah and Schneider 1998, KEMI 2003)

5.4.1.4 Health and Disease

A poor nutritional state can have a considerable influence on the metabolism of xenobiotics(Kalberlah and Schneider 1998)

Nutritional factors such as a poor or an unbalanced diet may influence the intake and probablyalso the biotransformation of food contaminants Ethnic differences due to variations in, e.g., dietaryhabits have also been reported (KEMI 2003)

Starvation may be associated with an increase in the permeability of the blood–brain barrier(Hawkins 1986, as cited in Dybing and Søderlund 1999) Tissue antioxidant status may becompromised under nutritional deficiencies and starvation (Godin and Wohaieb 1988, as cited inDybing and Søderlund 1999)

In general, a diseased state can be expected to influence the sensitivity to chemicals This is ofspecial concern when the target organ of the toxic effects is affected by the disease or when themetabolism and elimination are disturbed

Liver disease may decrease hepatic metabolism resulting in enhanced responses to parentchemicals; however, for many compounds, metabolism is only slightly impaired in moderate tosevere liver disease Disease-induced alterations in clearance and volume of distribution often act inopposite directions with respect to their effect on half-life Bioavailability may be markedlyincreased in liver disease with portal=systemic anastomosis (the connection of normally separateparts so they intercommunicate) so that orally administered chemicals bypass hepatic first-passmetabolism Altered receptor sensitivity has been observed for some chemical substances in livercirrhosis When liver tissue repair is inhibited by chemical co-exposure, even an inconsequentiallevel of liver injury may lead to fulminating liver failure from a nonlethal exposure of hepatotoxic-ants (Several articles, as reviewed by Dybing and Søderlund 1999.)

Renal impairment results in reduced clearance of many chemicals or their metabolites that areeliminated largely in the urine; the decline in excretion is directly related to the glomerularfiltrationrate (Dybing and Søderlund 1999) A number of diseases, including preexisting renal disease,systemic hypertension, and diabetes, are significant risk factors for chemical-induced renal disease(Ritter et al 1995, Bennett 1997, as cited in Dybing and Søderlund 1999)

Individuals with lung diseases such as asthma and chronic obstructive lung disease are generallyregarded as sensitive groups with regard to air pollutants (KEMI 2003)

In addition to diseases of important organs such as the lungs, the liver, and the kidneys,hereditary or acquired characteristics such as immunodeficiency and hypersensitivity may also

influence sensitivity to xenobiotics For example, atopics (individuals with immunologically ated allergy) may develop life-threatening reactions to a chemical at an exposure level that isinsignificant for the population in general (KEMI 2003)

medi-5.4.1.5 Lifestyle

The toxic effects of alcohol, tobacco smoke, and drugs may modify the toxic responses to chemicals.Ethanol can increase the levels of many enzymes involved in metabolism of xenobiotics.Prolonged ethanol intake causes irreversible damage in the central nervous system and in theliver, resulting in marked decreased capacity for detoxification of xenobiotics and thereby increasedsensitivity to a number of chemicals (KEMI 2003)

Reduced antioxidant capacity has been found in several tissues of alcoholics (Bjorneboe andBjorneboe 1993, as cited in Dybing and Søderlund 1999)

Tobacco smoke is considered to be one of the more severe confounders in epidemiologicalstudies, due, e.g., to its ability to affect enzyme activities and to cause various health effects (KEMI2003)

Tobacco smoke contains more than 3800 different compounds About 10% of these constitutethe particulate phase, which contains nicotine and tar The remaining 90% contains volatilesubstances such as carbon monoxide, carbon dioxide, cyanides, various hydrocarbons, aldehydes,and organic acids Although all of these substances affect the smoker to some degree, nicotine isgenerally considered to be the primary substance responsible for the pharmacological responses tosmoking (Nielsen et al 2001)

5.4.2 ASSESSMENTFACTOR FORINTRASPECIESVARIATION(HUMAN-TO-HUMAN):

DEFAULTVALUE

The intraspecies (interindividual) assessment factor is generally recognized as providing an ation from the average human being to the sensitive human being, assuming that most (notnecessarily all) human responses to a chemical fall within a 10-fold range The rationale for thetraditionally used default interindividual assessment factor of 10 is not known

extrapol-If the N=LOAEL has been derived from an animal study, animal intraspecies differences havealready to some extent been accounted for in that N=LOAEL Ideally therefore, the assessment

factor for interindividual variation should reflect the additional interspecies variability, i.e., thedifference between variability in the human population and variability in the animal population.The variability within the experimental animals is, however, assumed to be small and in addition,difficult to quantify Therefore the interindividual assessment factor is generally not corrected foranimal variation.

In the following text, various studies will be described which attempt to establish a scientificrationale for the selection of the interindividual assessment factor Based on these studies, it can

be concluded that the factor for interindividual variability should preferentially be describedprobabilistically However, at present there is no database-derived distribution of the interindividualfactor and thus a deterministic default factor of 10, split evenly into a sub-factor of 3.16 forboth toxicokinetics and for toxicodynamics, respectively, could be used to account for the inter-individual variability in the human population Alternatively, a pathway-related UF could be applied

in case the pathway(s) of the metabolism of the chemical in humans and the particular enzyme(s)are known

Calabrese (1985) examined the range of human responses with respect to (1) the degree

of variation in the metabolism of xenobiotics, (2) the binding of toxic substances to moleculessuch as hemoglobin and DNA, (3) the activity level of selected cellular enzymes, and (4) thedifferential risks to diseases in the human population Considerable differences were found amonghuman subjects in their capacity to metabolize xenobiotics The range of responses varied widely,depending on the substance, enzyme, and organ considered In addition, it was apparent thathuman variation may range up to two or three orders of magnitude indicating that human variation

in the metabolism of various xenobiotics can exceed a factor of 10 The author concluded thatthe commonly used safety factor of 10 appeared to provide protection for the majority of thepopulation (80%–95%) The remaining 5%–20% exhibited responses outside the 10-fold range

of variation

Hattis et al (1987) examined the variability in key pharmacokinetic parameters (eliminationhalf-lives (T½), area under the curve (AUC), and peak concentration (Cmax) in blood) in healthyadults based on 101 data sets for 49 specific chemicals (mostly drugs) For the median chemical, a10-fold difference in these parameters would correspond to 7–9 standard deviations in populations

of normal healthy adults For one relatively lipophilic chemical, a 10-fold difference wouldcorrespond to only about 2.5 standard deviations in the population The authors remarked that theparameters studied are only components of the overall susceptibility to toxic substances and did notinclude contributions from variability in exposure- and response-determining parameters The studyalso implicitly excluded most human interindividual variability from age and diseases When theseother sources of variability are included, it is likely that a 10-fold difference will correspond to fewerstandard deviations in the overall population and thus a greater number of people at risk of toxicity.According to Kalberlah and Schneider (1998), a factor of 10 ensures protection of much greaterthan 99% of the population if the average behavior of the chemicals is taken as the basis In the case

of the chemical for which a high degree of variability was observed, a factor of 10 signifiedprotection of about 99% of the population

Reanalysis of the data of Hattis et al (1987) showed that the variation between individuals forthe elimination half-life was quite small (Schaddelee 1997, as cited in Vermeire et al 1999, 2001)

Defining the interindividual factor as the ratio of the P50(50th percentile) and P05(5th percentile)resulted in a factor of 1.4 It was emphasized that although it appeared from this analysis that a10-fold factor would be sufficient for pharmacokinetic variation, the real median to sensitive humanvariability is underestimated because variation also exists in pharmacodynamics and only data ofhealthy volunteers were available

Grönlund (1992) has investigated methods used for quantitative risk assessment of genotoxic substances, with special regard to the selection of assessment factors Grönlund foundthat the 10-fold factor suggested for interindividual variability probably protects a majority but notall of the population