Environmental Engineering Reference
In-Depth Information
of 1.2-1.8 (Section 12.4.1). 281 For certain microbial species, exposure may occur mainly through
fragments rather than intact cells or spores (Section 12.4.1.3). The ratio of fungal spores depositing
in the respiratory tract to the indoor concentration has been estimated to be higher for the extratho-
racic region of the lung (0.70-0.80) than for the bronchial (0.41-0.60), bronchiolar (0.12-0.40), or
alveolar-interstitial (0.01-0.24) regions. 155
As for deposition, the mechanisms that clear other particles also operate for those of biological
origin (see Chapters 5 and 24). However, differences have been noted among microbial species that
have not yet fully been explained. A review of studies on lung clearance concluded that the spores
of most culturable fungi are eliminated within a few days but that for others clearance was not
complete even after a week. 233 Furthermore, alveolar macrophages were able to either kill fungi or
prevent germination with the exception of A. fumigatus .
12.5.2  d ose -r esPonse  r elationsHiPs
Animal studies can provide exposure-response data under well-controlled and deined conditions
when a biological agent is delivered at an environmentally relevant concentration and route of expo-
sure. Experimental inhalation studies have been used to estimate infectivity in humans, deine the
pathogenesis of airborne diseases, study the mechanisms by which biological components exert their
effects, and understand the action of anti-inlammatory agents and antibiotics. 326,327 The number of
inhalation tests on humans or animals as well as ield exposure studies is increasing, especially for
endotoxin and glucans. 86 Based on what was known, toxin concentrations in spores and fragments
of fungi that grow on damp building materials have been estimated to be on the order of ∼10 −5
moles. 225 They observed time- and toxin-dependent transcription and expression of inlammation-
associated genes and inlammatory responses in mouse lungs intratracheally exposed to eight fun-
gal toxins (4 × 10 −5 moles toxin/kg lung weight). These observations could provide a biological basis
for some of the inlammatory health effects that occupants of damp buildings experience.
However, extrapolation from animal models to humans is not straightforward due to interspe-
cies differences. 327 There is relative certainty about potential risks for some bioaerosols, such as
infectious agents, whereas the uncertainty is much greater for other bioaerosols, such as endo-
toxin and other bacterial and fungal toxins. Furthermore, some effects that have been inferred
from cross-sectional population studies require further conirmation preferably from longitudinal
studies. The contribution of biological agents to the adverse health effects observed for PM has yet
to be evaluated.
12.5.3  i nteractions
Complicating studies of human responses to bioaerosol exposures are simultaneous exposures to
gaseous and particulate air pollutants that may irritate or damage the respiratory tract, altering
people's sensitivity to bioaerosols. Any practice or condition that impairs the natural defenses of the
lung increases a person's susceptibility to infection. 328 For example, inluenza is often a precursor to
secondary bacterial pneumonia because the virus temporarily destroys ciliated cells.
Smoking also depresses the effectiveness of the ciliated cell defense. This damage might explain
why persons with a long history of smoking have a higher risk of bacterial and fungal infections as
well as other adverse health effects as compared with nonsmokers. The presence of (1) culturable
bacteria and fungi in tobacco; (2) 3-hydroxy fatty acids (3-OHFA), muramic acid, and ergosterol in
tobacco and tobacco smoke; and (3) bacterial metagenomes from environmental bacteria and com-
mensals as well as the potential presence of human pathogens in tobacco may contribute to these
observed effects. 329,330
The contribution of biological agents to the adverse health effects that are seen with expo-
sure to airborne PM is only beginning to be evaluated. Airborne particles can provide adsorptive
and absorptive surfaces for inorganic and organic gases and vapors. Particles of vehicle exhaust
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