Indoor Environmental Quality - Chapter 6 pptx

Both spore-forming structures and asexual spores for species inthe genera Penicillium and Aspergillus can be seen in Figures 6.2 and 6.3.Fungi may also produce spores or other structures

chapter six

Biological contaminants — mold

Biological contaminants were described in Chapter 5 in the context of illnesssyndromes and disease risks associated with exposure to airborne organismsand products of biological origin Discussion of airborne bacteria and viruseswas in the context of their role in causing infectious disease and potentiallycontributing to other problems; mites, insects and animal allergens, in thecontext of causing chronic allergic rhinitis and asthma Because mold is such

a significant indoor environment concern, this chapter is devoted to related health concerns and risk factors for mold infestation

mold-I Biology of mold

The terms mold and mildew are commonly used to describe the visiblemanifestations of the growth of a large number of organisms that are scien-tifically classified as fungi Terms such as yeast and mushrooms are used todescribe, respectively, single-celled fungi (widely used for baking and brew-ing) and the large reproductive structures of a major class of fungi that areused for food or are known for their high toxicity

Fungi form true nuclei, which distinguishes them from lower organismssuch as bacteria They differ from plants in that they do not produce chlo-rophyll and thus cannot manufacture their own food; from animals in that(except for reproductive cells in some species) they are not motile

Structurally, fungi exist as masses of threadlike filaments or hyphae Thecollective mass of hyphal filaments is described as mycelium, the vegetativepart of the organism that infests a substrate and extracts food for the organ-ism’s growth Though hyphal filaments are microscopic, the mycelium istypically visible to the naked eye Masses of mycelia can be distinguished

as fungal colonies (Figure 6.1) In many species, the hyphae are colorless; inother species they contain pigments

A Reproduction

Specialized reproductive structures develop in the life history of all fungalspecies These structures may be produced as a consequence of sexual orasexual processes Most species of fungi undergo both sexual and asexualreproduction at some time in their life history Sexual processes occur inspecialized cells or structures, with the production of spores that will ulti-mately be dispersed Sexual spores are produced in specialized “fruiting”structures The most noticeable of these are mushrooms and bracket fungi

on trees or tree debris

Asexual spores, as well as the fruiting structures (if any) on or withinwhich they are borne, vary in size and shape as well Asexual spores areproduced in large masses, particularly when a mold colony is maturing andexperiencing environmental stress, e.g., depletion of substrate, lower substratemoisture content or air relative humidity, or competition with other colonies

or species In many cases, asexual spores produce pigments which may, alongwith pigment in the mycelium, characterize colonies in culture or on naturalsubstrates Both spore-forming structures and asexual spores for species inthe genera Penicillium and Aspergillus can be seen in Figures 6.2 and 6.3.Fungi may also produce spores or other structures that are designed tosurvive harsh environmental conditions These include thick-walled, dor-mant clamydospores and hardened mycelial masses called sclerotia

A number of species of fungi grow as single cells during all or part oftheir life history Most notable of these are yeasts, which reproduce asexually

by budding and sexually by producing sac-like ascospores In genera such

as Candida, under certain environmental conditions the organism developsyeast-like growth, while under others, it develops a typical mycelium

quence of evolutionary processes, fungal spores have size and aerodynamicproperties that enhance airborne dispersal Spores vary in size (2 to 100 µm),with the smallest being single cells and the largest being multicellular Spores

of Alternaria and Cladosporium, which illustrate different spore shapes andsizes, can be seen in Figures 6.4 and 6.5

Spore dispersal may occur by passive or active release mechanisms, withsubsequent entrainment and movement by horizontal or convective air cur-rents Once airborne, spores may be carried varying distances, depending

on their aerodynamic properties as well as existing atmospheric conditions

In still air (such as may occur in houses) they settle out relatively rapidly.Based on Stokes Law, the largest and heaviest spores would settle out morequickly, with smaller spores being suspended for longer periods of time

Figure 6.2 Asexual spores and spore bearing structures of Penicillium (Courtesy of University of Minnesota.)

Figure 6.3 Asexual spores and spore-bearing structures of Aspergillus (Courtesy of University of Minnesota.)

However, spores with surface ornamentation and nonspherical shape tend

to experience significant drag and thus settle out more slowly than StokesLaw calculations would predict Settling times for particles of different aero-dynamic diameters are given in Table 6.1

When spores are released, they may become airborne as single structures

or in strands of multiple attached spores The latter is commonly the casewith asexual spores of Penicillium, Aspergillus, and Cladosporium Such clus-ters may have aerodynamic behavior characteristics that differ from singlespores In theory, they should settle out more rapidly

As is the case with most speciesof living things, fungi produce an excess

of reproductive propagules to ensure survival Not surprisingly, many donot survive the often harsh environmental conditions that exist in the timeperiod between their release from mycelia and their deposition on substrates.Ultraviolet light and low atmospheric moisture pose significant risks to their

Figure 6.4 Asexual spores of Alternaria (Courtesy of University of Toronto.)

Figure 6.5 Asexual spores of Cladosporium (Courtesy of University of Minnesota.)

viability Loss of viability during dispersion explains, in part, the often largedifferences in mold concentrations observed in concurrent airborne moldsampling using culturable/viable and total mold spore sampling methods.

In the latter case, concentrations of total airborne mold vary from severaltimes to one or more orders of magnitude higher than culturable/viablemold Spore viability ratios (concentration of live spores divided by totalnumber of spores) for species such as Aspergillus, Penicillium, and Cladospo- rium are in the range of about 0.10 to 0.15 (10 to 15%) and are much lower

in species such as Epicoccum

Spores must fall on suitable substrates for germination and subsequentinfestation to take place If environmental conditions are not suitable, sporeswill, in time, lose their viability Spores can become resuspended so that theymay have more than one chance to land on a substrate with suitable envi-ronmental conditions for growth Indoors, resuspension occurs as a conse-quence of occupant activities, e.g., moving or dusting objects, children andpets playing, and air currents generated by forced air mechanical systemssuch as fans and heating/cooling systems, etc

Different species respond to a variety of environmental conditions thataffect their dispersal As a consequence, the composition of airborne moldsamples reflects not only the presence of individual species but also dispersalmechanisms and periodicity factors The proximity of sources significantlyaffects sample concentrations Samples collected near wood lots or in houseswith significant structural deterioration tend to have high basidiospore (pro-duced by members of the Basidiomycetes) concentrations Samples collectednear crop harvesting operations may have enormously high concentrations

of Alternaria, Epicoccum, and Cladosporium In houses, samples collected nearsources of Penicillium or Aspergillus will have high concentrations of thesetwo fungal types In other cases, airborne mold spore concentrations may

be low despite the fact that significant infestation is present This is the casewith Stachybotrys chartarum, a toxigenic species with large, initially stickyspores, which may cling together and settle out rapidly (Figure 6.6)

C Nutrition

Fungi obtain food by parasitizing other organisms (mainly plants), or fromdecomposing organic matter, or, in specialized cases (lichens), mutualisticsymbiosis with algae Most fungi are saprobes, obtaining their nutrient

Table 6.1 Settling Velocities for Particles of Different Aerodynamic Diameters Particle diameter ( µ m) Settling rate (cm/s)

needs from nonliving organic matter Most saprobes are saprophytes, that

is, they decompose dead plant matter Some species are both parasitic andsaprobic, living on dead organic matter but invading living tissues when ahost is present

Most parasitic fungi are plant pathogens A number of these are notablebecause they pose major threats to agricultural crops, which require contin-ued selective breeding programs or intensive use of fungicides for theirprotection A small number of species can parasitize humans, causing infec-tions of the skin (e.g., athlete’s foot) or more serious diseases such as histo-plasmosis and life-threatening pulmonary infections (aspergillosis)

The major ecological role of fungi is decomposition, a role shared withbacteria and insect larvae During decomposition, fungal hyphae infest sub-strates, producing extracellular enzymes that digest complex organic mole-cules into smaller and simpler molecules that are absorbed and metabolized.Decomposition is a successional phenomenon, with different species domi-nating as the decomposing substrate changes As a consequence, substratesamples often show the presence of a number of species

Fungal species have varied growth requirements Therefore, individualspecies may be prevalent on some substrates and in certain environmentsbut not others These growth requirements affect species prevalence in bothoutdoor and indoor environments

Many saprobic fungi can use a variety of substrates Substrate tion in individual situations is dependent on their ability to compete withother species, as well as the presence of suitable temperatures and availabil-ity of water

for initial infestation and subsequent growth Though water is required forgrowth, fungal species have a broad tolerance range for its availability Theterm water activity (aw) is used to describe the moisture content of substrates.Water activity is the relative humidity of the substrate expressed as a decimalfraction (e.g., 95% = 0.95 aw) The range of water activities that individualspecies grow under varies from approximately 0.55 to 1.0 aw Species thatrequire high water activities (>0.95 aw) are described as hydrophilic; those thatcan tolerate lower water activities are xerophilic.

Fungi grow over a range of temperature conditions (<40 to 140°F, <5 to60°C) Mesophilic fungi, the largest group, have an optimum temperaturerange of 68 to 86°F (20 to 30°C) Thermophilic fungi that are human patho-gens grow well at temperatures of 95 to 104°F (35 to 40°C); true thermophilicfungi grow at temperatures of 113 to 140°F (45 to 60°C), e.g., compost heaps.Cryophilic fungi can grow at relatively low temperatures (<40°F, <5°C), e.g.,

on materials in cold storage and in regions having cold climates

E Classification

Up until the last several decades, fungi were classified as members of thePlant Kingdom because some contain cellulose in their cell walls In today’sclassification, the approximately 70,000 identified species are placed in eitherthe Kingdom Protista or Kingdom Fungi The slime molds, which haveaffinities to both fungi and animals (division Myxomycota), and the watermolds (division Oomycota), which produce motile spores, are classified inthe Kingdom Protista The Kingdom Fungi includes five major fungal types

or divisions These are the Chytridiomycota, Zygomycota, Ascomycota,Basidiomycota, and Deuteromycetes The Chytridiomycota are a primitivegroup of fungi which do not produce airborne spores and thus pose littlerisk of human exposure and health effects

Fungal species in the Zygomycota produce thick-walled sexual spores(zygospores) and asexual spores called sporangiospores The mycelium ischaracterized by the absence of cross walls (i.e., it is nonseptate) Zygomycotaspecies are saprobic and can grow on a variety of substrates, particularly inareas with high relative humidity Two major genera in this group (Rhizopus

and Mucor) are commonly isolated from indoor environment samples Inculture, they rapidly overgrow colonies of other genera Genera in this groupcolonize soil and house dust Rhizopus stolonifera colonizes bread, and thus

is referred to as bread mold

The Ascomycota represent a large and widely distributed group oforganisms characterized by the production of sexual spores in a sac-likestructure called an ascus Members of this group are, for historical reasons,called ascomycetes They range from the single-celled yeasts to species thatproduce mushroom-like fruiting bodies (morels) Many members of theAscomycota reproduce asexually by producing conidia (spores) in largemasses on specially differentiated hyphal structures The asexual sporesproduced by Ascomycota are common indoor air contaminants (e.g., those

from Penicillium and Aspergillus) Many of the fungal allergens identified bymedical scientists have been associated with species of Ascomycota.The division Basidiomycota, historically known as basidiomycetes,includes a large variety of fungal types and species They are characterized

by their sexual reproduction processes, which produce basidiospores onstructures called basidia Basidiospores are the primary means of dispersal,though asexual spores may also be formed Basidiomycetes include yeastslike Sporobolomyces, which colonize indoor substrates; rusts and smuts, whichare plant pathogens; many species of mushrooms, bracket fungi, and puff-balls; and a variety of other wood-decaying fungi which grow on structuraltimbers in human dwellings The basidiomycetes rarely form fruiting bodies

in culture (sexual or asexual) and, as a consequence, are difficult to identify

In addition, many do not grow well in culture

The Deuteromycetes, also known as fungi imperfecti or asexual fungi,include a group of organisms classified by their asexual reproductive struc-tures and apparent absence of sexual structures This is an artificial classifi-cation, since the sexual stage is not commonly observed under culture con-ditions Many deuteromycetes have been determined to be ascomycetes,while only a few are basidiomycetes Classifications in this group have beenproblematic, since the asexual stage of Deuteromycete genera such as

Aspergillus may be associated with different genera in the Ascomycota Booksused to identify species such as Aspergillus and Penicillium typically use theDeuteromycete name rather than the more scientifically correct name, which

is based on the sexual stage

II Biologically significant fungal compounds

As fungi grow, they produce a variety of secondary metabolites These areoften species-dependent, but a number of compounds appear to be generic

to fungi These include pigments which may color spores or other structures

or diffuse into substrates, a variety of volatile organic compounds (MVOCs)with their distinctive musty or earthy odors, alkaloids, antibiotics, and myc-otoxins Many secondary metabolites are produced by older parts of fungalcolonies, particularly cells that are being restricted in their growth

Some metabolites may have important direct functions It is notable thatgenera found outdoors, such as Cladosporium, Epicoccum, Alternaria, and Pith- omyces, are dark in color, producing black spores Many mushrooms producedark spores as well The major benefit of dark spores and mycelium would

be to protect the organism from ultraviolet light Other metabolites may serve

to reduce competition with different organisms (antibiotics and bacteria),other species, or members of the same species (alkaloids, mycotoxins)

A MVOCs

During their growth, all fungal colonies release volatile organic compounds(VOCs or, more specifically, MVOCs to denote their microbial origin) that

are characteristically microbial or fungal in the sense that they are responsiblefor the musty, mildewy, earthy, or mushroomy odors associated with moldinfestations Commonly reported fungal VOCs include: 3-methylfuran, hep-tanone, 1-octen-3-ol, octan-3-ol, 2-octen-1-ol, octen-3-one, octan-3-ol, 2-methyl-1-butanol, 2-hexanone, geosmin (1,10-dimethyl-trans-9-decalol), ace-tone, 2-butanol, dimethyl trisulfide, methanol, 1-propanol, 4-decanol, 2-methylisoborneol, trimethylhexane, 3,3-dimethyl-2-oxetanone, 3,3-dimethyl-1-octene, ethyl 2,4-dimethylpentone, and 2-methoxy-3-isopropylpyrizine Ascan be seen, most fungal MVOCs are alcohols and ketones.

Microbial VOCs produced by mold colonies vary with individual species

as well as substrates colonized The most frequently reported MVOC inbuilding investigations is 2-octen-1-ol This compound, as well as 1-octen-3-ol, 2-hexanone, and light alcohols, tends to be associated with water-damaged materials Fungal VOCs, reported as total MVOCs, range in con-centration from 50 to 126 µg/m3 in problem buildings, while outdoor levelsaverage 8.6 µg/m3 Their odor is readily detectable despite relatively lowconcentrations (even in mold-infested buildings), indicating a relatively lowodor threshold

B Fungal toxins

Many fungal species produce toxins which, on exposure, adversely affectthe physiological functioning of other organisms Toxins such as antibioticsinhibit the growth of bacteria and provide a competitive advantage to theorganism producing the antibiotic Other toxins may inhibit the growth offungal species in a somewhat similar fashion

Fungal toxins can adversely affect humans and other animals Theseinclude the highly poisonous toxins formed in the fruiting structure of avariety of mushroom species and mycotoxins produced by the mycelia ofmany common fungi

1 Mycotoxins

Mycotoxins are generally produced when fungal mycelia are subject to ent limitation Well-known mycotoxins and species producing them are sum-marized in Table 6.2 Fungal genera found indoors, such as Penicillium and

nutri-Aspergillus, commonly produce mycotoxins Best known of these are toxin A and aflatoxin B, both relatively large molecules Ochratoxin A is acolorless crystalline compound with a dihydroisocoumarin moiety linked tothe amino acid phenylalanine and an atom of chlorine attached to the iso-coumarin ring The aflatoxins are a family of substituted coumarins contain-ing a dihydrofuran moiety

ochra-Of particular note are mycotoxins produced by Stachybotrys chartarum,

a species of fungi that grows well on substrates containing cellulose (such

as hay, straw and, in buildings, ceiling tile and gypsum board) S chartarum

has a characteristically dark mycelium and large, initially sticky asexualspores Mycotoxins produced by S chartarum include highly toxic members

of the trichothecene family such as saratoxin H and G and verucarin A and

B, as well as others such as trichovarin A and B

The trichothecenes are macrocyclic compounds with both olefinic andepoxy groups In addition to Stachybotrys, the trichothecenes are produced

by species in the genera Trichothecium, Fusarium, Myrothecium, Trichoderma,and Cephalosporium Trichothecenes associated with species of the genus

Fusarium (which occurs widely in nature as both a saprobe and parasite onplants) have received considerable scientific attention Mycotoxins produced

by Fusarium species include the highly toxic compounds T-2 and DON ynivalenol) Fusarium spores are commonly collected in outdoor samplesand less commonly indoors

(deox-Though mycotoxins are produced by mycelia, high concentrations areoften found in spores Concentrations of aflatoxin as high as 200 ppm w/whave been reported in the asexual spores of A flavus and A parasiticus Veryhigh mycotoxin levels are also found in the spores of Stachybotrys As such,spores have the potential to cause significant risk of mycotoxin exposure.Limited studies have confirmed the presence of mycotoxins in airbornesamples in problem environments

Table 6.2 Mycotoxins Produced by Common

Fungal Species Genus/species Mycotoxin

Penicillium sp Patulin

P verrucosum Ochratoxin A

Citrinin Citroviriden Emodein Egliotoxin Verruculogen Secalonic acid D

Fusarium sp Zearalenone

Fumonisins

Paecelomyces variatii Patulin

2 Fungal glucans

Glucans are polyglucose polymers present in the cell walls of mold hyphaeand spores and certain bacteria They are very soluble in water and retaintheir biological activity when mold spores are no longer viable

Because of their macromolecular size and diversity, glucans are not easilymeasured in environmental samples except by bioassay procedures Thoughthey are likely to be ubiquitous in suspended aerosols both in indoor andoutdoor air, little information is available on the range of airborne glucanconcentrations indoors

III Exposure assessments

Numerous studies have been conducted to assess airborne mold levels inboth outdoor and indoor environments These studies have attempted tocharacterize the presence and prevalence of genera (and in some cases, spe-cies) present in airborne samples as well as total mold spore concentrations.Most studies have been based on the use of culturable/viable samplingmethods, wherein airborne mold spores/particles are impacted onto spe-cially-formulated nutrient agar media Such sampling is used to identify anddescribe the dominant types that grow on the culture media used and pro-vide a relative measurement of their abundance It has the advantage ofproviding investigators with the opportunity to identify mold colonies tothe species level However, culturable/viable sampling methods have sig-nificant limitations Media used differ in their ability to support the growth

of different species and genera In addition, an often high percentage ofairborne mold spores is not viable and cannot grow on any culture media

As a consequence, levels determined from airborne sampling are only ameasure of the abundance of viable spores present and their culturability

on the sampling media used

Compared to culturable/viable sampling, total mold sampling methodswhich collect mold spores and other particles on greased microscope slideshave been little used to assess airborne mold concentrations However,because these sample values potentially represent most mold spores andhyphal fragments, including both viable and nonviable mold structures, theyare a better indicator of airborne mold concentrations Since allergenicity isindependent of viability, total mold spore sampling is also a better indicator

of potential health risks It is not possible at this time to identify spores tospecies, nor in many cases to genus either, using total mold spore sampling.Despite limitations associated with airborne mold sampling, studiesconducted to date can be used (to a limited degree) to describe dominantmold types present in outdoor and indoor air and their relative abundance

A Outdoor prevalence

Significant differences in mold types present and total colony counts havebeen reported in comparison studies of outdoor and indoor airborne mold

spores and structures Such differences are typically the case when studies

are conducted in the winter and under closed building conditions There is

only limited intrusion of outdoor airborne mold into indoor spaces when a

building is under closure conditions Mold taxa commonly reported in

out-door air samples include the phylloplane fungi (grown on the surface of

leaves): Cladosporium, Alternaria, Epicoccum, and Aureobasidium; Dresclera,

Fusarium, and a number of species of basidiomycetes; and plant pathogens

such as Botrytis, Helminthosporium, and Ustilago Less commonly the mold

taxa Aspergillus and Penicillium are reported Dozens of mold types are

present in samples collected outdoors; however, individual samples tend to

be dominated by a relatively few taxa, with Cladosporium and basidiomycetes

being most abundant

Outdoor mold concentrations as determined by culturable/viable

sam-pling methods vary from nondetectable (under cold, snow-covered

condi-tions) to tens of thousands of colony-forming units/cubic meter (CFU/m3)

High concentrations are reported during the autumn when decomposing

plant material is abundant and grain harvesting is taking place Under

mod-erate weather conditions, outdoor culturable/viable mold concentrations are

typically in the range of hundreds to thousands of CFUs/m3 With the

exception of the winter season, outdoor mold levels are almost always higher

than those measured indoors under closure conditions

B Indoor prevalence

Indoor mold taxa and their concentrations are also quite variable This is

due to factors such as the intrusion of outdoor mold spora, the presence or

absence of sources of mold infestation, the nature of mold infestation and

its location, and building type In the latter case, residential buildings

typi-cally have both higher concentrations and a greater range of taxa present

than mechanically ventilated buildings While few differences in both mold

taxa present and airborne concentrations are seen between indoor and

out-door samples when significant intrusion of outout-door mold spora occurs,

significant differences between indoor and outdoor samples may be

observed under closure conditions

Mold taxa commonly reported and found in the highest concentrations

indoors are, in declining order: Cladosporium, Penicillium, and Aspergillus.

Less commonly reported and in fewer numbers are the taxa: Alternaria,

Fusarium, Epicoccum, yeasts, basidiomycetes, Mucor, Rhizopus, Aureobasidium,

Chaetomium, Acremonium, Monilia, Pithomyces, Paecilomyces, Trichoderma,

Scop-ulariopsus, and rarely, Stachybotrys Abundance, however, varies with the

nature and degree of infestation

Highest airborne mold concentrations are reported in buildings where

significant mold infestation has occurred, most notably houses that have

experienced severe flood damage Mold concentrations in flood-damaged

houses may be in the tens of thousands of CFUs/m3 In buildings

experi-encing less significant water damage and mold infestation, total airborne

culturable/viable mold concentrations >1000 CFU/m3 are common

Con-centrations <1000 CFU/m3 are typical in buildings in which there is no

apparent mold infestation; when these are new houses and mechanically

ventilated buildings, levels are usually <300 CFU/m3 (Table 6.3)

Measurement of airborne mold concentrations as determined by total

mold spore/particle sampling methods in either indoor or outdoor

environ-ments has been limited The method is amenable to identifying and

quanti-fying major mold genera, but few studies have reported quantitative

expo-sure by mold type Most information is available as total mold spore counts

In general, total mold spore levels in residential structures vary from 5000

to 15,000 spores/particles per cubic meter (S/m3); in mechanically ventilated

buildings without any visible mold infestation, airborne mold varies from

1000 to 3000 S/m3 (Table 6.3) In moderately infested residences,

concentra-tions of 15,000 to 30,000 S/m3 can be expected Heavily contaminated school

buildings (25,000 to 200,000 S/m3) and flooded residences (200,000 to

1,000,000 S/m3) have been among the highest reports of airborne mold

When very high concentrations are observed, they are typically dominated

by Aspergillus or Penicillium or both

IV Health concerns

Exposure to airborne mold and mold-produced environmental contaminants

may pose significant health risks to humans These include well-known

mycotic diseases as well as other less well-defined health effects

There are three basic categories of mycotic disease: infections,

aller-genic/immunological illness, and nonallergic illness

Table 6.3 Culturable/Viable and Total Mold Spore Concentration Ranges

Observed in Buildings

Building type Conditions

Concentration ranges Culturable/viable

(CFU/m 3 )

Total mold spores (S/m 3 )

Not mold infested (avg.)

>300<1000 >3000<10,000 Moderately infested >1000<3000 >10,000<30,000 Heavily infested >3000 >30,000 Mechanically ventilated

nonresidential

Not mold infested (avg.)

<300 1000–3000 Moderate, localized

infestations

>1000<3000 >3000<10,000 Heavily infested >3000 >30,000

A Infections

A number of fungal species can cause infectious disease However, compared

to bacteria and viruses, fungal infections have played a relatively minorhistorical role in causing human suffering and death In the past two decadesthere has been a significant decline in skin disease, and a significant increase

in systemic disease, caused by fungal infections This is due, in the lattercase, to significant increases in the population of individuals who are sus-ceptible to systemic fungal infections This susceptible population includesindividuals with immune system deficiencies who would have normallydied early in life; patients on immunosuppressive drugs; and other at-riskpatients with cancer, on long-term antibiotic therapy, or with certain infec-tious diseases Acquired immunodeficiency syndrome (AIDS) is the mostprevalent of such infectious diseases The epidemic increase in AIDS infec-tions has resulted in a corresponding increase in life-threatening fungal

infections, most notably by species of the genera Candida and Cryptococcus.

Fungi that cause systemic disease are saprobic organisms that becomeinfectious when disruption of a human host’s physiology, microflora, orimmune system occurs Immune-compromised individuals who have AIDS

or are being treated with immunosuppressive drugs are of special concernbecause they are potentially exposed to both true pathogens and opportu-nistic fungi (Table 6.4) in both indoor and outdoor air Exposure to oppor-tunistic fungi is of major concern in hospital environments where special airfiltration systems are often installed to protect patients Most notable are

efforts to control exposure to Aspergillus fumigatus, which is responsible for most aspergillosis infections Other Aspergillus species of concern include A.

flavus, A niger, and A terreus Outbreaks of nosocomial (hospital-acquired)

aspergillosis have been associated with new construction as well as tion activities

renova-Life-threatening fungal infections in immune-compromised and other

patients are often associated with Candida spp Candida grows in a yeast-like

fashion and is a normal part of the microflora of humans As such, filtration

Table 6.4 Pathogenic and Opportunistic Fungal Species

that Cause Systemic Infections

Cryptococcus neoformans Candida sp.

Histoplasma capsulatum Aspergillus sp.

Coccidiodes inmitis Trichosporon sp.

Blastomyces dermatidis Fusarium sp.

Paracoccidiodes braziliences Pseudoallescheria boydii

Mucor sp.

Rhizopus sp.

Absidia sp.

Rhizomucor sp.