Api publ 4714 2002 (american petroleum institute)

4714 RASA A Guide to Polycyclic Aromatic Hydrocarbons for the Non Specialist Regulatory and Scientific Affairs PUBLICATION NUMBER 4714 FEBRUARY 2002 Copyright American Petroleum Institute Reproduced b[.]

A Guide to Polycyclic Aromatic

Hydrocarbons for the Non-Specialist

Regulatory and Scientific Affairs

PUBLICATION NUMBER 4714

FEBRUARY 2002

Copyright American Petroleum Institute

Reproduced by IHS under license with API

`,,,,`,-`-`,,`,,`,`,,` -A Guide to Polycyclic `,,,,`,-`-`,,`,,`,`,,` -Aromatic Hydrocarbons for the Non-Specialist

Regulatory and Scientific Affairs

API PUBLICATION NUMBER 4714 FEBRUARY 2002

PREPARED UNDER CONTRACT BY:

Paul D Boehm*, Christopher P Loreti, Amy B Rosenstein, and Phillip M Rury**

Arthur D Little, Inc

Acorn ParkCambridge, Massachusetts02140-2390

*Currently at Battelle Memorial Institute, Waltham, Massachusetts

**Currently at Killam Associates, New England, Hadley, Massachusetts

Reference 69458

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Suggested revisions are invited and should be submitted to Regulatory and Scientific AffairsDepartment, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C 20005

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Copyright © 2002 American Petroleum Institute

TABLE OF CONTENTS

Page Section

0 OVERVIEW 1

1 INTRODUCTION 1

1.1 What are PAHs? 1

2 SOURCES OF PAHS 2

2.1 Primary Sources of PAHs 2

3 PAHS IN THE ENVIRONMENT 4

3.1 PAH “Environmental Delivery Systems” 4

3.2 Overview of Concentrations in the Environment 7

4 PAH TRANSPORT AND FATE 8

4.1 Transport 8

4.2 Fate Processes 8

5 ENVIRONMENTAL AND HUMAN HEALTH EFFECTS 11

5.1 Sources of Human Exposure 11

5.2 Human Health Effects 12

5.2.1 Non-carcinogenic Effects 13

5.2.2 Carcinogenic Effects 14

5.2.3 Regulatory Levels 15

5.3 Ecological Effects 15

5.3.1 Bioavailability and Uptake 16

5.3.2 Regulatory Standards Related to PAH Ecological Effects 17

5.3.3 Guidance on Screening Methods for Ecological Effects 18

6 CHEMICAL ANALYSIS OF PAHS 19

6.1 PAH Analytical Goals and Targets 19

6.2 Analytical Methods 20

6.3 PAH Source Identification (Fingerprinting) and Allocation 22

6.4 Analytical Efficiency and Costs 22

APPENDIX A 25

REFERENCES 27

Figures 2-1 Perylene, a Five-Ringed Diagenic PAH 2

2-2 Phenanthrene 3

2-3 Representative Distribution of Alkylated PAHs Formed at Different Temperatures 3

2-4 Retene 4

3-1 PAHs in Alaska North Slope Crude Oil 6

5-1 Relative Doses of Carcinogenic PAHs 13

6-1 Comparison of PAH Analyses with Two Different Target Lists 21

6-2 Schematic of Top-Level PAH Fingerprinting and Allocation Approach 23

A-1 Structures of the 16 Priority Pollutant PAHs 27

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Tables

5-1 Average Concentrations of Carcinogenic PAHs in Food, mg/kg 12

5-2 PAH Reference Doses for Non-Cancer Health Effects 14

5-3 Slope Factors and EPA Classification for Carcinogenic PAHs 15

5-4 Ambient Water Quality Criteria for PAHs 16

6-1 Extended Analytical Target List 20

6-2 Recommended Detection Limits for PAHs in Environmental Samples 22

6-3 Approximate Costs for High Quality PAH Analyses Performed by Experienced Laboratories 23

A-1 Physico-chemical Properties of Selected PAHs 27

iv

Concern about PAHs in the environment is due to their acute toxicity or carcinogenic properties, as well as their relativepersistence This concern has led to the regulation of PAHs under a number of U.S laws, including the:

Several environmental regulations relate directly to petroleum products or petroleum processing Polycyclic organic matter(POM) is one of the toxic air pollutants whose emissions reformulated gasoline are meant to reduce POM (defined as thesum of the seven carcinogenic PAHs) is also on the list of mobile source hazardous air pollutants that the EPA is proposingfor future regulation, as well as on the list of hazardous air pollutants for the EPA’s Urban Air Toxics Strategy

Toxic release inventory reporting (TRI) under EPCRA requires facilities, such as oil refineries that manufacture, process, orotherwise use as little as 10 lbs of the PAH benzo[ghi]perylene or 100 lbs of polycyclic aromatic compounds (a group of 21PAHs, substituted PAHs, and heterocyclic compounds), to report their releases to the environment Other laws andregulations on PAHs, which are described in Section 5 of this report, apply to their concentrations in the natural andworkplace environment

Polycyclic aromatic hydrocarbons (PAHs)—sometimes referred to as polynuclear aromatic hydrocarbons (PNAs),condensed ring aromatics, or fused ring aromatics—are a class of organic compounds consisting of two or more fusedaromatic rings

hydrogen and carbon atoms are labeled More commonly, PAHs are shown without labeling the carbon and hydrogen

PAHs most commonly encountered in the environment contain two to seven fused benzene rings, although PAHs with agreater number of rings are also found The “ultimate” PAH is graphite, an inert material comprised of planes of fusedbenzene rings

Like all hydrocarbons, PAHs contain only hydrogen and carbon However, closely related compounds called heterocycles,

in which an atom of nitrogen, oxygen, or sulfur replaces one of the carbon atoms in a ring, are commonly found with PAHs

CH C C CH

CH CH CH HC HC CH

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considered “priority pollutants” under the “Clean Water Act” are found in Figure A-1 in the Appendix

PAHs often occur with aliphatic (straight chain) hydrocarbons attached to the rings at one or more points These compoundsare referred to as “branched” or “alkylated” PAHs The aliphatic chains are depicted as lines attached to the PAH with the

methylnaphthalene

Because there are numerous possible combinations of the location of the alkyl chain on the parent PAH, the number ofchains on the molecule, and the length of the chains, alkylated PAHs are often classified by the number of alkyl carbons

PAHs are produced in nature through four generalized pathways: 1) low temperature diagenesis of organic matter (part ofthe changes undergone by a sediment after its initial deposition); 2) the formation of petroleum and coal; 3) incomplete orinefficient combustion at moderate to high temperatures (pyrolysis); and, 4) biosynthesis by plants and animals These

These include the combustion of fossil fuels and biomass, such as wood, as well as chemical production that results in theformation of PAHs

Because the type distribution of PAHs depends on the temperature of formation, the characteristic distributions of thesedifferent sources can be used to help distinguish among different sources of PAHs in the environment Once produced, PAHs

described in Section 3

Diagenic PAHs Diagenic PAHs are those produced by natural processes that are set in motion when organic matter isdeposited in nature—in soils or sediments These processes, collectively called diagenesis, begin shortly after deposition ofthe organic matter These are low temperature processes that occur after oxygen is depleted, and are believed to involvemicroorganisms, such as bacteria, though non-biological processes may occur in tandem Relatively few individual PAHsare produced by these early diagenic processes One of the most notable PAHs produced in this manner is the five-ringedPAH, perylene, shown in Figure 2-1 Perylene is commonly found in sediments of rivers, lakes, and oceans at a depth in thesediment where oxygen is reduced

Fossil Fuel (Petroleum and Coal) PAHs Over geological time and within petroleum reservoirs and coal beds in geological

higher temperatures than the formation of diagenic PAHs) within deeply buried layers of sediments Petrogenic PAHs areformed, for example, when biological organic matter from plankton is converted to petroleum These processes also canform coal, although the starting biological material (e.g., higher plants and animals) may be different Petroleum orpetrogenic PAHs and coal-derived PAHs are “fossil fuel” PAHs The nature of the processes, which convert organic matter

Figure 2-1 Perylene, a Five-Ringed Diagenic PAHS

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to fossil fuels, involves semi-random chemical processes This fact results in the complexity of many PAH structures thatare found in fossil fuels Hundreds to thousands of individual PAHs may be produced by nature during the processes thatform petroleum Nevertheless, while their compositions vary greatly, crude oils have in common the existence of two tosix+ ringed PAHs, with a preponderance of alkylated structures associated with the two to four ringed compounds

The types of PAHs formed as fossil fuels include a complex variety of parent (i.e., unsubstituted), and alkylated PAHs Series

homologous series of PAHs includes, for example, phenanthrene itself, plus a series of alkylated homologues of phenanthrenewith many alkyl substitutions (see Figure 2-2) The relative abundance of the alkylated PAHs of petrogenic PAHs far exceeds

greater than” parent PAHs is a main feature of petrogenic PAHs.This is illustrated in the first chart of Figure 2-3 for typicalalkyl homologue distributions

Pyrogenic PAHs High temperature processes such as the combustion of fossil fuels or the burning of wood also form PAH(Figure 2-3) During higher temperature processes, those organic compounds that escape complete combustion (oxidation

with certain woods are examples of pyrogenic processes PAHs are also formed on meats by barbecuing (see Table 5-1).Included in this pyrogenic category are the products of high temperature processing of coals in coal gasification processes

Figure 2-2 Phenanthrene( indicates possible site of alkylation)

Figure 2-3 Representative Distribution of Alkylated PAHs Formed at Different Temperatures

(based on Chrysene)

0 0.2 0.4 0.6 0.8 1 1.2

C0 C1 C2 C3 C4 C0 C1 C2 C3 C4 C0 C1 C2 C3 C4

Number of Alkyl Carbons on Aromatic Rings

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Because the high temperature processes tend to destroy the more reactive alkylated PAHs, the production of unsubstituted

more numerous” than alkylated PAHs. As a result, PAH distributions that are produced by the hotter and more rapidpyrolysis or combustion-related processes are markedly different from those produced by petrogenic processes (see Figure2-3) Also, pyrogenic PAHs are characterized by the higher abundance of the four, five, and six+ ringed PAHs relative toPAHs found in most petroleums

Biogenic PAHs Certain PAH precursor compounds are biosynthesized in nature, although the direct biosynthesis of PAHs

per se, remains uncertain Although the contribution of microorganisms to the production of PAHs in nature has beenreported, their contribution may be more to the oxygen-containing aromatic compounds rather than to PAHs themselves

It is well-known that other PAH precursor compounds (e.g., abietic acid) exist in abundance in certain tree resins (e.g.,conifer resins) and that specific PAHs are formed from the diagenesis or the combustion of these resins For example,

retene, (Figure 2-4), a specific C4-phenanthrene isomer, is ubiquitous in residues from these plants and can be found in high

found in abundance where organic conifer residues exist These specific, singular PAHs are found in coastal sedimentsaround the world

Once they are produced by primary processes, PAHs may be introduced into the environment through a number ofpathways—secondary sources or “PAH delivery systems.” These include:

PAHs can enter the environment on local, regional, and global scales Point sources, such as municipal or industrial outfalls,are on the local scale, and are generally made up of mixtures of PAHs that are combustion or oil-related Non-point sources(e.g., rainfall runoff or atmospheric deposition) are found on regional scales, and are also made up of PAHs from multipleprimary sources Wide-field atmospheric deposition is a global source that distributes primarily pyrogenic PAHs (Ohkouchi

et al., 1999) to remote regions of the earth Airborne transport of PAHs on soot particles from forest fires and thecombustion of coal and oil has been established as a major mechanism for the distribution and delivery of PAHs to soils andsediments on regional and global scales

Understanding how PAHs enter the environment is important in conducting environmental impact assessments and riskstudies On a global basis and in areas remote from urban influence, PAHs from pyrogenic processes transported over largedistances are the principal source of background concentrations—more important than petrogenic PAH inputs—though thelevels tend to be very low On more localized scales, background PAH concentrations may be much higher, and PAHs fromurban runoff together with combustion-related PAH inputs are very important contributors to most receiving environments

In selected geologically-active environments, oil seeps and erosion from oil source rocks and coal result in elevated

Figure 2-4 Retene

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concentrations from natural sources of PAHs The background concentrations of PAH are of particular significance whenthe potential effects (i.e., incremental addition) of PAHs from new oil and gas exploration projects or effects of oil spills arebeing evaluated as part of new project plans or environmental impact and damage assessments

Natural Oil Seeps and Source Rock Erosion

In local areas (e.g., Santa Barbara Channel, Gulf of Alaska, Caspian Sea) and on a global basis, natural oil seeps fromgeologic formations that are exposed at the earth’s surfaces contribute significant quantities of petrogenic PAHs to theenvironment When these PAHs are bound tightly to particles by sorption they are deposited to sediments and become part

of the natural PAH background The significant background of PAHs in the sediments of Prince William Sound, the site ofthe Exxon Valdez oil spill, is almost totally the result of seep oil-related material and petroleum source rocks (shales) that are

input of PAHs from coal seam erosion mix with oil source rock erosion to establish a large PAH background concentration

in the sediments

Petroleum Spills and Releases

The PAHs that enter the environment from petroleum spills, leaks, and other releases are, by definition, petrogenic.Although oil spills receive considerable attention from industry, regulators, and the media, they typically do not account for

a large fraction of PAHs entering the environment This is so because spills account for only a fraction of the oil releasedinto the environment, and because the concentrations of PAHs in the oil are less than those of other types of hydrocarbons The PAH content of crude oils varies widely, ranging from <10% – >30% of the oil Typical crude oils contain a smallpercentage of PAHs compared to other hydrocarbon fractions, as is shown in Figure 3-1 Similarly, while some refinedproducts, such as diesel fuels, may consist of 25% – 30% aromatic compounds, most of these are monocyclic aromatics, notPAHs

The concentration of individual PAHs may be as high as 5000 to 10,000 ppm in oils The concentration of benzo[a]pyrene,

a well-known and heavily monitored carcinogen, however, is typically <1 ppm in crude oil but up to roughly 50 ppm insome heavy refined products

Composition and property data for crude oil and oil products can be found at:

http://www.etcentre.org/main/e/db/db.html

at the Association for Environmental Health and Sciences website:

http://www.aehs.com/

Urban Runoff and Municipal Wastewaters (Mixtures of PAH Sources)

A significant fraction (20% – 30%) of the petrogenic PAHs that enter the environment is from urban runoff that makes itsway into coastal environments from stormwater runoff, which is often routed through municipal sewerage systems (NRC,1985) The largest fractions of PAHs in many urban harbors (e.g., Boston, Los Angeles, San Francisco) come from chronicdischarges of urban runoff through publicly-owned sewage treatment works and storm sewer outfalls

Urban runoff consists of a considerable amount of pyrogenic PAHs, as well as petrogenic PAHs The combustion of fossilfuels leads to soot, which when deposited to land leads to rainwater runoff with a significant presence of combustion-relatedPAHs Runoff from parking lots and roadways are important local sources of PAHs to receiving waters and to municipaltreatment plants, owing to the significant quantities of crankcase oil drippings, soot, and automotive tire particles thataccumulate in heavily trafficked areas (PAHs are present in carbon black used to make tires)

Industrial Pyrogenic PAH Sources: Aluminum Smelting, Manufactured Gas Plants, Creosote Pilings

The smelting of aluminum by the Soderberg and other processes produces waste materials, wastewaters, and air emissionsthat are rich in PAHs (Naes and Oug, 1998) These PAHs originate in the materials (i.e., coal tar pitches) used to make theelectrodes utilized in the smelting process Air emissions and wastewater sludges from these processes contain significantquantities of pyrogenic PAHs

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In the first half of the 20th century, manufactured gas plants (MGP) produced gas from coal, resulting in residues called

“coal tar” These coal tars contain large quantities (up to 70% – 80% by weight) of pyrogenic PAHs as a result of the hightemperature processing of coal in the plants These PAHs have entered the environment via groundwater flow and runoff tocoastal rivers and sediments

Also in the category of industrial pyrogenic PAHs are those derived from creosote, a coal tar derivative The use of creosote

as a wood preservative in wood pilings and telephone poles, especially in locations adjacent to urban harbors during the20th century, has resulted in the release of creosote to the aquatic environment Since creosote is heavier than water, oncereleased it tends to become associated with sediments Creosote-derived PAHs can be an important and dominant localizedsource of PAHs to the environment—in Puget Sound, in Boston Harbor, and even in parts of Prince William Sound

Industrial Wastewaters and Waste Sites

Industrial wastewater effluents typically contain low quantities of PAHs, the levels of which are regulated by dischargepermits However, refineries, chemicals, plastics, and dyestuff manufacturing plants, among others, discharge PAHs toreceiving waters and emit them to the atmosphere as fugitive emissions at low-to-moderate quantities High molecularweight PAHs are also produced in modern petroleum coking operations

At many waste disposal sites throughout the world, historically unregulated waste handling and disposal have resulted inthe presence of a complex PAH “soup.” Typically, such sites were the scene of oily waste disposal operations, coking, orcoal gasification These PAHs can migrate from the site, enter groundwater, or seep into surface waters

Figure 3-1 PAHs in Alaska North Slope Crude Oil

Benzo[g,h,i]perylene Dibenzo[a,h]anthracene Indeno[1,2,3,-c,d]pyrene Perylene

Benzo[a]pyrene Benzo[e]pyrene Benzo[k]fluoranthene Benzo[b]fluoranthene C4-Chrysenes C3-Chrysenes C2-Chrysenes C1-Chrysenes Chrysene Benzo[a]anthracene C3-Fluoranthenes/pyrenes C2-Fluoranthenes/pyrenes C1-Fluoranthenes/pyrenes Pyrene

Fluoranthene C3-Dibenzothiophenes C2-Dibenzothiophenes C1-Dibenzothiophenes

Dibenzothiophene C4-Phenanthrenes/anthracenes C3-Phenanthrenes/anthracenes C2-Phenanthrenes/anthracenes C1-Phenanthrenes/anthracenes

Phenanthrene Anthracene

C3-Fluorenes C2-Fluorenes C1-Fluorenes Fluorene

Biphenyl Acenaphthene Acenaphthylene

C4-Naphthalenes

C3-Naphthalenes C2-Naphthalenes C1-Naphthalenes

Naphthalene

Other CompoundsChrysenes

DibenzothiophenesPhenanthrenesFluorenesNaphthalenes

Alaska North Slope Crude

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Air

PAHs are found in ambient air in both gaseous form and on particles PAHs having two or three rings are foundpredominantly in the gaseous phase, those with five or more rings in the particle phase, and those with four rings in bothphases (ATSDR, 1995) Most of the particle-phase PAHs are found on particles having aerodynamic diameters of 0.1 – 3.0microns, meaning they are easily respirable

PAHs are ubiquitous in ambient air, being found in urban, suburban, and rural locations PAH emissions to the atmosphereare primarily anthropogenic in origin, and the PAHs—particularly those of heavier molecular weight—tend to associate

be dispersed widely from their sources into areas with little human activity

A wide range of PAHs can be found in the atmosphere More than 100 species have been identified in the urban air of the

1981)

Water

The widespread transport of PAHs in the atmosphere and their subsequent deposition to the ground, as well as the directdischarge of waters containing PAHs to surface water bodies means that PAHs are found widely in fresh surface water andsea water Concentrations of PAHs in natural waters vary greatly depending on the local sources Typical concentrations ofbenzo[a]pyrene have been reported to range from 5.6 to 150 ng/l in U.S rivers and 0.6 to 110 ng/l in German rivers (Neff,

range of 0.1-830 ng/l Relatively few data compilations exist on concentrations of individual PAHs in surface water.Reported concentrations of PAHs in sea water vary widely, in part due to differences in methods of sampling and analysis

In general, PAH concentrations at locations far off shore and away from natural oil seeps or anthropogenic releases are low

or non-detectable, with higher concentrations observed in coastal and estuary areas (Manoli and Samara, 1999)

Concentrations of two to six-ringed PAHs in Chesapeake Bay were found to typically range between 0.1 and 2 ng/l (Manoliand Samara, 1999) In areas affected by oil seeps or spills, concentrations could be greater In water samples collected near

a seep in the Gulf of Mexico, total PAH concentrations of 28 ng/l were reported, and concentrations of semivolatile PAHsranging from 150 to 520 ng/l have been reported in water near a shallow water seep off of southern California (Neff, 1997) Reported concentrations of PAHs in groundwater are typically lower than those in surface waters This is to be expectedsince suspended sediment, to which the heavier PAHs tend to sorb, occur at lower concentrations in groundwater than in

median carcinogenic PAH concentration in groundwater of 1.2 ng/l, with a range of 0.2 to 6.9

Aquatic Sediments

The low water solubility of PAHs with more than three rings results in greater concentrations of these compounds beingpresent in sediments than dissolved in the water This is true for both surface and groundwaters Manoli and Samara (1999)report, for example, that total particulate PAH concentrations in the Seine were an order of magnitude higher than thedissolved concentrations, and that sediments in the Slave River (Canada) often exceeded regulatory thresholds whiledissolved concentrations rarely exceeded analytical detection limits

Observed concentrations of PAHs in marine sediments span almost four orders of magnitude National Oceanographic andAtmospheric Agency (NOAA) Status and Trends data range from 0.002 to 232 mg/kg (dry weight) for total carcinogenicPAHs, and other published studies report values ranging from 0.003 to 232 mg/kg Highest concentrations are found inurban harbors around the U.S Background concentrations are at the low end of these ranges with total PAHs off thesouthern New England coast in the range of 0.01 to 0.02 mg/kg for sediment cores 24 to 35 cm deep (Neff, 1997) Thesurficial sediment concentration in these cores was 0.1 mg/kg, consistent with the high end of the range of 0.001 to 0.1 mg/

kg reported by Boehm and Farrington (1984) for sediments from Georges Bank off the Massachusetts coast

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Marine sediments in the immediate area of oil and gas production operations have greater concentrations of PAHs Brooks

et al (1990) found nearshore coastal Texas sediments to have average PAH concentration of 0.029 mg/kg Averagesediment concentrations at 10 and 25 meters from a multi-well platform were 0.494 and 1.82 mg/kg, respectively, andconsisted primarily of two ring aromatics, indicating that the PAHs were petrogenic in origin

The PAH background found in sediments in areas of Alaska have been found to be 0.1-1.0 mg/kg in the nearshore Beaufort

Soil

PAHs emitted to the atmosphere are eventually deposited to the ground For this reason, PAHs are widely distributed insoils Concentrations of individual PAHs are typically 10 to 100 times higher in urban soils than in rural soils This would

respectively

primarily to road dust

At sites contaminated by PAHs from such industrial operations as wood preserving and wood treatment, creosoteproduction, coke production, and gas works, soil concentrations may be even greater than those in road dust.Concentrations of individual PAHs including naphthalene, phenanthrene, dibenz[a,h]anthracene, fluoranthene and fluorene

in the thousands of ppm (mg/kg) range have been reported (ATSDR, 1995)

The transport of PAHs in the environment depends on movement of the environmental media in which they are containedand on the physicochemical properties of the PAHs In the atmosphere, PAHs move with the air mass containing them untilthey are deposited to a surface In surface waters, dissolved PAHs are transported with the water, and PAHs associated withsediments will follow the movement or deposition of the sediments In groundwater, the transport of PAHs depends on thepartitioning of the PAHs between the water and the soil, including the presence of substances in the water that may enhancethe apparent solubility and mobility of the compounds

For each of the PAH transport modes, various fate processes influence their movement within a particular environmentalmedium, their transfer from one environmental medium to another, and their degradation The principal fate processesaffecting PAHs are:

The chemical and physical properties of each of the PAHs and the prevailing environmental conditions determine thesignificance of each of these processes for each PAH The interrelationships among these processes determines the fate ofPAHs in the environment

Chemical and physical properties relevant to the environmental fate of PAHs are listed in Table A-1 Property data for PAHsand other chemicals are available in the Hazardous Substance Data Bank (HSDB) of the TOXNET toxicology data networkmaintained by the National Library of Medicine:

http://toxnet.nlm.nih.gov/

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Data specifically on PAHs can be found on a Japanese chemistry web site:

http://chrom.tutms.tut.ac.jp/JINNO/DATABASE/00alphabet.html

Dissolution

The dissolution of PAHs into water is one of the most important processes affecting their fate in the environment, and the

aqueous solubility of PAHs is a key parameter in understanding their movement PAHs as a chemical class have relatively

low solubilities (see Table A-1, Appendix) This is particularly true for the heavier PAHs, as solubility decreases with an

increasing number of aromatic rings and molecular weight Alkylation tends to further reduce solubility compared to the

parent compound, with greater degrees of alkylation leading to greater reductions in solubility (Neff, 1978)

The solubility of PAHs covers a wide range Dissolution of naphthalene and three-ring PAHs in water may be significant, as

the solubilities of these compounds are in the tenths of mg/l or greater range For heavier PAHs, like benzo[a]pyrene,

solubilities are three orders of magnitude less, and dissolution and transport in water would be much less significant

The solubility of PAHs decreases with increasing salinity, such that the solubility of some compounds in seawater is half of

that in freshwater (Neff, 1997) Temperature tends to have the opposite effect, with solubilities rising with the temperature

Several mechanisms can enhance the mobility of PAHs (and other hydrophobic organic compounds) in water beyond what

would be expected based on their solubility in distilled water One of these mechanisms is the tendency of PAHs to

associate with particulate matter in the water The degree of association depends on the type of particles and the size of the

PAH Organic particles are more effective in sorbing PAHs than inorganic particles, and the heavier, less water soluble

PAHs are more likely to be found in the particulate phase than are the lighter PAHs A study in the Chesapeake Bay, for

example, found that for lower molecular weight compounds, fluorene through pyrene, most were present in solution, while

heavier PAHs tended to be associated with particulate matter (Neff, 1997)

Other mechanisms that can enhance the amount of PAH in solution are the presence of organic solvents or dissolved

organic matter (humic material), and association with colloids (sometimes referred to as microparticles) These

mechanisms tend to be most important for the heavier PAHs, those with five or more rings, which have low solubility The

presence of other organic compounds acting as solvents is important in the case of spills of crude oil and other petroleum

products where PAHs enter the environment as part of a mixture of hydrocarbons, many with much greater solubilities than

the PAHs

Evaporation

All PAHs are solids at normal environmental temperatures Their tendency to evaporate is indicated by their vapor pressure

which varies over eight orders of magnitude (Table A-1, Appendix) (The vapor pressure of a compound is the pressure

exerted by its vapor in equilibrium with its liquid or solid phase The vapor pressure indicates the tendency of a compound

in the solid or liquid phase to volatilize It increases sharply with temperature.) Naphthalene is the most volatile PAH, with

a vapor pressure less than 0.1 mm Hg at room temperature Benzo[a]pyrene, by contrast, is one of the least volatile, and has

The differing vapor pressures of PAHs have several implications for their environmental fate After their release into the

environment, a large fraction of the low molecular weight, more volatile PAHs (naphthalene through phenanthrene) will

evaporate over a period of days In one behavior study of PAHs in soil, 20% of methylnaphthalene and 30% of naphthalene

were volatilized, while volatilization was found not to be significant for heavier PAHs (ATSDR, 1995)

The vapor pressure of PAHs is one of the key factors that determines whether they will be present principally in the vapor

phase or tend to associate with airborne particles once they are released into the atmosphere As previously discussed, the

lighter and more volatile PAHs (up to three rings) are found principally in the gas phase, while heavier and less volatile

PAHs (five or more rings) are found principally associated with particles

For compounds dissolved in water, the Henry’s Law constant is used as a measure of the tendency to volatilize from water

into the atmosphere The Henry’s Law constant is the equilibrium ratio of the concentration of the compound in air to the

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The Henry’s Law constants for PAHs vary widely The lower molecular weight PAHs, naphthalene through anthracene,

volatile Some of the heavier molecular weight PAHs, such as indeno[1,2,3-cd]pyrene, dibenz[a,h]anthracene, and

Because the Henry’s Law constant depends both on the vapor pressure and aqueous solubility of the compound, both of

which decrease with molecular weight, compounds with low solubilities and vapor pressures do not always have low

Henry’s Law constants Some of the higher molecular weight PAHs have Henry’s Law constants as large as the lower

molecular weight PAHs For example, the Henry’s Law constant for benzo[a]pyrene, a five-ringed PAH, is nearly the same

as that for phenanthrene, a three-ringed PAH, and thus it too would be considered moderately volatile in water

Sorption to Soils and Sediments

In surface waters and groundwaters neutral organic chemicals will tend, to varying degrees, to sorb onto organic matter

compound in soil or sediment (normalized by the fraction organic carbon) to that in water at equilibrium

where

values for the common PAHs range from less than 2 for acenaphthylene to more than 6 for PAHs with five or more rings, a

difference of more than 10,000 times (Table A-1, Appendix)

more soluble and less likely to be sorbed to soil and sediments In groundwater systems, this means they will have a greater

tendency to be carried with the groundwater, while the heavier PAHs will tend to be retained on the soil Similarly, in

surface waters, more of the lower molecular weight PAHs will be present in the aqueous phase, while most of the higher

molecular weight PAHs will be sorbed to suspended or bottom sediments

equilibrium concentration in octanol to its concentration in water in a two-phase system containing small quantities of the

Appendix)

Biodegradation

Biodegradation of PAHs in aquatic environments depends on the characteristics of the environment and the specific PAHs

Under aerobic conditions, microbial metabolism may be significant (ATSDR, 1995) Some PAHs can be completely

metabolized by some strains of bacteria (Neff, 1978) Fungi have also been found to fully or partially degrade PAHs, and

algae have been found to oxidize benzo[a]pyrene to several degradation products (ATSDR, 1995) Under anaerobic

conditions, however, biodegradation of PAHs is reported to be extremely slow (Neff et al., 1994).

The higher molecular weight PAHs cannot act as the sole source of carbon for bacteria (Neff, 1978) Nevertheless, bacteria

can catalyze their oxidation Through the process of co-metabolism, microorganisms may degrade PAHs in the process of

degrading other compounds

In addition to the nature of the microbial population present, the rate and extent of biodegradation is affected by such factors

as the presence of nutrients and oxygen, the organic carbon content, and the physical and chemical characteristics of the

PAHs themselves (ATSDR, 1995) PAHs in soil are less susceptible to degradation than low molecular weight alkanes and

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monocyclic aromatic compounds, but more susceptible than highly condensed cycloalkanes, resins, and asphaltenes (Neff

et al., 1994) In most soils, some of the higher molecular weight PAHs, such as chrysene, dibenzanthracene, and perylene

have been found to degrade very slowly (Bossert and Bartha, 1986), over periods of months to years

Chemical and Photochemical Degradation

Laboratory studies have demonstrated the susceptibility of PAHs to undergo chemical or photochemical oxidation under

atmospheric conditions (Baek et al., 1991) Degradation products include oxy- and hydroxy PAH derivatives as well as

nitrated PAHs, some of which are mutagenic (ATSDR, 1995)

Photochemical degradation is considered the most important degradation mode for PAHs in the atmosphere, although the

degradation rates vary widely (Baek et al., 1991) PAHs appear to be less susceptible to photodegradation when sorbed to

naturally occurring particles, such as soot However, while sorbed to particles, PAHs may undergo chemical oxidation withnitrogen dioxide, ozone, and sulfur trioxide (ATSDR, 1995)

Chemical and photochemical oxidation are also important degradation routes for PAHs in surface waters Thesusceptibility of PAHs to undergo photodegradation in water varies widely from PAH to PAH (Neff, 1978), and does notcorrelate directly with the size of the molecule Naphthalene, for example, undergoes aqueous photolysis much moreslowly that anthracene (ATSDR, 1995)

In contrast to the behavior of particle bound PAHs in the atmosphere, PAHs sorbed to particle surfaces are much morephotochemically reactive in water than dissolved PAHs, all other things being equal (Neff, 1978) Since PAHs, particularlythose with higher molecular weight tend to be partitioned to sediments, degradation rates for PAHs on suspended sedimentswould be faster than suggested by experimental data based on dissolved PAHs

The rate and extent of photodegradation in aqueous environments depend on the intensity of the light For PAHs in bottomsediments, photodegradation will occur only if the depth and clarity of the water allow light of sufficient intensity to reachthe sediments In soil, biodegradation of PAHs is the primary degradation mode (ATSDR, 1995) because they are lesssusceptible to photodegradation

The chemical or photochemical degradation of PAHs does not mean that they are completely mineralized (converted to

be formed Their greater solubility will lead to greater mobility of these compounds as compared to the parent PAHs More

importantly, the toxicities of many of the degradation products are potentially greater than the parent compounds (Ankley et

al., 1995).

Humans are exposed to PAHs in air, water, soil, and food, as well as in such consumer products as cosmetics andasphaltic materials used in roofs and driveways Occupational exposure to PAHs also occurs Persons working with coaland coal products are potentially exposed to PAHs, as are workers in other industries, such as mining, oil refining,metalworking, chemical production, transportation, and the electric industry Those who smoke cigarettes or accompanypeople who smoke receive additional doses of PAHs Most studies of human exposure to PAHs have focused on thePriority Pollutant PAHs and, for that reason, most of the information presented in this section does as well (The U.S.EPA has designated 16 PAHs as priority pollutants (16-PP PAHs) in water and classifies seven of them as known humancarcinogens—see Figure A-1, Appendix.)

For non-smoking adults who are not exposed to PAHs occupationally or through the use of consumer products, the greatest

doses of carcinogenic PAHs come from food Menzie et al (1992), using median concentrations from the literature, have estimated a total adult dose of carcinogenic PAHs of 3.12 µg/day, over 95% of which comes from food (see Figure 5-1) For

people who smoke a pack of unfiltered cigarettes each day, the total median dose of carcinogenic PAHs was estimated to be

5-8 µg/day, roughly double that for non-smokers

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The dose of PAHs received in food depends on the type of diet consumed An average non-vegetarian diet results in the

lowest potential dose, a vegetarian diet a larger dose, and a diet heavy in meat, an even larger dose (Menzie et al., 1992;

ATSDR, 1995) This pattern is explained by the tendency of vegetables, particularly leafy vegetables, to accumulate PAHsfrom the atmosphere Fish and meat that have been smoked or broiled may contain PAH concentrations as high as those invegetables Concentrations of carcinogenic PAHs that have been measured in foods are listed in Table 5-1

Although fish consumption advisories have been issued in the United States due to elevated PAH concentrations inuncooked fish (ATSDR, 1995), a large majority of the exposure to PAHs in cooked fish result from the smoking orcharcoal broiling of the fish, and the PAHs originally present in the fish would account for a negligible fraction of total

exposure Santodonato et al (1981) reported total benzo[a]pyrene doses from food in the range of 0.16 to 1.6 µg/day The

estimated dose from consuming one meal (114 g) of Great Lakes fish containing 50 parts per trillion of benzo[a]pyrene

would be less than 0.006 µg, a small fraction of the 0.16 to 1.6 µg/ day total benzo[a]pyrene dose, and negligible compared to the total daily dose of 3 µg shown in Figure 5-1.

The next greatest means of human exposure are through the incidental ingestion of soil and the inhalation of air For both ofthese sources, the dose received varies over two orders of magnitude

Concentrations of PAHs in indoor air may be as high or higher than outdoor air concentrations, particularly in homescontaining wood or kerosene stoves, or where smokers are present Concentrations of PAHs more than double those ofoutside air have been reported in homes without smokers; with smokers present, concentrations of selected PAHs were up

to 10 times as high (ATSDR, 1995) The range of concentrations of PAHs in indoor air attributed to wood burning stoves is

approximately half of that for second hand tobacco smoke (Menzie et al., 1992).

Occupational exposure to PAHs occurs in a variety of industries including petroleum production and refining,metalworking, coke production, anode manufacture, and aluminum production Inhalation and, to a lesser extent, dermalcontact are the routes of exposure The types of PAHs to which workers are exposed varies with the type of industrialoperation In the petroleum industry, most of the exposure to PAHs tends to be for two- or three-ring compounds In onestudy of a refinery de-asphalting unit, for example, more than 85% of the exposure was to naphthalene and its derivatives,and 9% to three-ring compounds At the catalytic cracker, PAHs with five or more rings contributed less than 0.1% of thetotal, but at the delayed coker unit, they contributed up to 2.5% (ATSDR, 1995)

PAHs have a variety of potential human health effects based on evidence from occupational and laboratory animal studies

In general, they have low acute toxicity Adverse effects for chronic exposures may include cancers, reproductive effects,

Table 5-1 Average Concentrations of Carcinogenic PAHs in Food, mg/kg

Food Average Minimum Average Maximum Charcoal Broiled or Smoked:

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immunosuppression, and skin irritations The cancer potency of PAHs is related to the specific structure of the molecule,although no PAHs having fewer than four rings are considered by the U.S EPA to be carcinogenic

PAHs can enter the body by various routes The lungs can absorb PAHs, although the rate and completeness of PAHabsorption is not known PAHs are absorbed slowly when swallowed or contacted by the skin The rate at which they areabsorbed is influenced by the presence of other compounds and the level of fat in the exposed tissue PAHs tend to be stored

in the kidneys, liver, and fat, and are metabolized by the body into different substances Results from animal studies showthat PAHs do not tend to be stored for longer than a few days

The specific non-carcinogenic and carcinogenic effects of exposure to selected PAHs have been extensively studied, andtoxicity values based on animal research, industrial exposures or accidents, and/or epidemiological studies have beenpublished by government bodies such as the EPA Sources for toxicity values include:

Agency (EPA, 1997a)

Detailed summaries of published toxicity literature are compiled by the Agency for Toxic Substances and Disease Registry(ATSDR):

For substances suspected of causing non-cancer effects, a reference dose (RfD) is developed by the EPA, based on publisheddata The RfD is expressed as a chronic intake level (in units of mg/kg/day—milligrams of chemical per kilogram of body

Figure 5-1 Relative Doses of Carcinogenic PAHs

Source: Menzie et al, 1992

3

0 0.5 1 1.5 2 2.5 3 3.5

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weight per day), or “threshold,” below which no adverse effects are expected Because each RfD incorporates safety anduncertainty factors, a dose above the RfD does not mean that adverse effects will necessarily occur Naphthalene, for example,has a safety and uncertainty factor equal to 3000 It is calculated as the product of four other safety and uncertainty factors: 10

to extrapolate from rats to humans, 10 to protect sensitive humans, 10 to extrapolate from subchronic to chronic exposure, and

3 for database deficiencies

Table 5-2 lists available non-cancer reference doses for PAHs These toxicity values are used in human health riskassessments to estimate the potential for non-cancer health effects As the table shows, reference doses for different PAHsvary by more than an order of magnitude Because several of the non-carcinogenic PAHs do not have published EPA RfDs,the toxicity value for naphthalene is often used for PAHs with known non-carcinogenic effects

Many PAHs have been found to be carcinogenic in laboratory animals, causing tumors both at the site of application andsystemically These carcinogenic PAHs have generally been found to be active in mutagenicity assays (tests designed todetect DNA mutations in bacteria) In humans, PAHs are associated with cancer of the lung and skin, and possibly withurologic, gastrointestinal, laryngeal, and pharyngeal cancers Adverse effects on the liver and kidney have been associatedwith exposure to carcinogenic PAHs

To quantify potential cancer effects of exposure to PAHs, cancer slope factors (CSFs) have been developed by the EPA.CSFs are used to calculate conservative (or upper bound) estimates of cancer risks They relate the lifetime probability ofexcess tumors (in excess of what would be expected without the additional exposure) to the lifetime average exposure dose

of a chemical The CSF is multiplied by the average lifetime exposure (expressed in milligrams of chemical per kilogram ofbody weight) to arrive at the cancer risk factor This risk factor represents the probability of an individual contracting acancer due to exposure to the carcinogen over the individual’s lifetime

In contrast to the approach for non-cancer effects, a non-threshold model is assumed for carcinogens That is, it is assumedthat even a very low level of exposure to the chemicals could cause an effect The EPA has also developed a Weight-of-Evidence classification for carcinogenicity based on an evaluation of the likelihood that the agent is a human carcinogen Because several of the carcinogenic PAHs do not have EPA-published CSFs, the value for benzo[a]pyrene is often used forPAHs that are known to have carcinogenic effects The use of this value is likely to be conservative, as benzo[a]pyrene isthought to be the most carcinogenic PAH Alternatively, a relative potency approach is used to express the carcinogenicity

of PAHs relative to benzo[a] pyrene Use of a relative potency approach accounts for the differing toxicological properties

of the various PAH compounds

Table 5-3 provides the EPA classification, available oral cancer slope factors, and cancer risk factors for the PAHs that arecarcinogenic priority pollutants Cancer slope factors are also published for the dermal and inhalation routes of exposure

As this table illustrates, the carcinogenicity of PAHs, as indicated by the cancer slope factor, varies by three orders ofmagnitude, as would the risk factor for receiving an equal lifetime dose of each of the listed PAHs

Table 5-2 PAH Reference Doses for Non-Cancer Health Effects

Non-carcinogenic PAHs Oral Reference Dose (RfD)