Environmental Engineering Reference
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in isolation or in higher than ambient concentrations and the exposure is inite. Moreover, such
controlled studies cannot offer even indirect evidence for health effects related to exposure over
long periods of time.
Exposure assessment in studies of human health : Since the actual distribution of source-
speciic dosages of aerosol received by a population cannot be known, epidemiological studies
have had to focus on measures of exposure to estimate health effects related to exposure to the
toxic components of the ambient PM aerosol. Two strategies have been used to try to ascribe
health effects either to the PM aerosol itself or speciic components of it. By far the most com-
mon approach is the comparison of the effect estimates from statistical models that include
only PM aerosol or the mass concentration of a speciic component with results from the same
models with other elements of the aerosol or ambient gas concentrations included. This approach
is hampered by the relatively high correlations that often exist between components of the aero-
sol or other ambient gases and can result in unstable or biased effect estimates (e.g., see Refs
[119,120]). A less common approach is to group components of the aerosol by sources without
attempting to attribute health effects to any speciic component from the source [121]. A varia-
tion of this approach is to identify marker components of the aggregate PM mix to narrow the
source contribution to health effects. The use of black smoke (correlated with elemental carbon)
and nitrogen dioxide (NO 2 ) as a marker for exposure to diesel exhaust PM [122,123] and trafic-
related pollution [124], respectively, are examples.
Most studies of health effects of PM aerosol have based their assessment of health effects on
mass concentrations of total suspended particulates (TSP; older studies) PM 10 , PM 2.5 , and PM 10-2.5
or components such as element carbon (often measured as black smoke) (e.g., see Ref. [2]). Attempts
to attribute health effects to one mass fraction or another remain somewhat clouded. Some inves-
tigations in areas in which PM 2.5 dominates the PM 10 mass have suggested that most health effects
related to the anthropogenic component of the PM aerosol are conined to the PM 2.5 component
[125-128]. Studies that have focused on the sulfate and acid component of PM 10 (e.g., Refs [129-
131]), by implication, emphasize the contribution of ine PM (see Figures 23.1 and 23.2) in which
most of the sulfate and acid sulfates (and salts of acids from nitric acid) are found [132]. However,
studies from environments in which PM 10-2.5 makes up a larger fraction of the PM 10 mass have
found that a variety of health effects are more closely associated with this coarse fraction [133,134].
Similar indings have been reported from environments whose PM aerosol mass is not dominated
by PM 10-2.5 [135,136]. Further, claims have been made for a unique role of the ultraine component
of the PM aerosol in causing health effects [137-139]. The speciicity of the results has been ques-
tioned in a discussion of one study (see the panel discussion in Ref. [138]) and questioned by the
indings in other studies [140,141]. Thus, despite the toxicological properties attributed to ultraine
particles (refer to earlier text), it has been dificult to quantify what proportion of health effects
attributed to the anthropogenic PM aerosol are related speciically to ultraine particles or to the
number of particles rather than their mass.
23.5.2  s ources oF  PM a erosol and  H uMan  H ealtH  e FFects
Mobile sources : Much of the early epidemiological research on the human health effects of ambient
aerosol focused on ixed industrial sources and used home heating fuels (e.g., soft coal) that made
major contributions to the ambient aerosol. The rapid growth of motor vehicle use has led to a
research focus on mobile sources of the anthropogenic PM aerosol and its health effects (e.g., see Ref.
[142]). This focus has fostered the extensive research on the immunotoxicology of DEP discussed
previously. This emphasis derives from a large number of epidemiological studies that have observed
increased risks of a variety of health outcomes related to exposure to trafic and trafic-related PM
aerosol. Table 23.8 summarizes a representative sample of these studies. In general, most of these
studies support an association between one of several different metrics of exposure to mobile sources
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