Environmental Engineering Reference
In-Depth Information
levels of some metals in ambient PM in the region were also lower during the year
that the mill was closed, it was suggested that PM-bound metals may be associated
with changes in the incidence of respiratory ailments. The results of these studies in
the Utah Valley are so central to the hypothesis that metals may be causally related
to the respiratory and cardiovascular illnesses associated with particulate air pollu-
tion, that archived particulate matter from those 3 years has been used as the source
of exposure in several of the toxicology studies that we review in this paper.
Over the past 20 years, numerous studies have been conducted to characterize
metal constituents in ambient PM and to apportion the relative contributions of
source categories (e.g., industrial, motor vehicle, crustal elements) to the composi-
tion of ambient PM in various geographic regions. A relatively smaller number of
epidemiologic studies have been conducted that explored associations between spe-
cifi c transition metals and health effects in members of the general population. There
is a growing body of
in vivo
and
in vitro
toxicology studies, in which the potential has
been tested for PM metals to induce infl ammation and alter gene expression along
pathways potentially involved in respiratory and cardiovascular illnesses. To our
knowledge, the adequacy of the current body of evidence to determine the risk posed
by specifi c metals in ambient PM has not been critically evaluated, although previous
reviews of the literature have been published (Schlesinger
2007
). In particular, the
review by Chen and Lippmann (
2009
) served as a spring-board for our current work.
Chen and Lippmann (
2009
) detailed available epidemiological studies through
2006, summarizing the health effects of PM-associated metals on exposed popula-
tions. They found that there were limited data on metallic elements as tracers for
source apportionment of health effects, and many of the studies that they cited did
not report metal-specifi c data that were associated with PM. They concluded that
extrapolations would have to be made, based on the source and the known general
components of that source, to estimate what metals might be causing the health
effects. However, such extrapolation is only an indirect means of identifying particular
metals or their amounts that constitute a signifi cant factor from a given source.
2
Approach
Our objective was to better understand how the metal content of ambient air PM
contributes to the health effects associated with particulate air pollution. We searched
the peer-reviewed literature that was published from 1993 to October, 2012, to
review the health effects of specifi c transition metals that were associated with air-
borne particulate matter in normal and/or at-risk sub-populations. We narrowed the
selection of articles to those containing exposure assessment and dose-response
data, such that results from this review can be integrated into a future risk analysis.
Although the strongest associations for health effects that are linked to ambient par-
ticulates reported in the epidemiological literature are for fi ne particles having aero-
dynamic diameters of
m (PM
2.5
), we reviewed all relevant studies regardless of
the size category. These size categories included the following particulate classes:
coarse particles (with an aerodynamic diameter larger than PM
2.5
but smaller than
10
≤
2.5
ʼ
ʼ
m), ultrafi ne particles (particles with an aerodynamic diameter
≤
0.1
ʼ
m), and
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