Environmental Engineering Reference
In-Depth Information
1
Introduction
There is a large body of community-level epidemiologic evidence showing positive
associations between increases in morbidity and mortality from respiratory and car-
diovascular causes, and increases in the mass of ambient particulate matter (PM) in
air in the preceding 1-3 days (Dockery and Pope
1994
; Schwartz and Morris
1995
;
Laden et al.
2000
). The evidence has been suffi ciently convincing to support the
development of ambient air quality standards and regulations to reduce air particu-
late emissions in the U.S., Canada and Europe. Ambient PM is a dynamic and com-
plex mixture that varies in composition over both time and location; and, it is not
clear which components of ambient PM are most active in producing respiratory
and cardiovascular health effects. Identifying which components of ambient PM
present the greatest risk is potentially important for refi ning strategies to protect
public health (Grahame and Schlesinger
2007
).
Rohr and Wyzga (
2012
) recently summarized the epidemiological, controlled
human exposure, and toxicological literature related to the role of PM components
in health effects. They concluded that the epidemiological studies that they reviewed
most strongly implicated carbon-containing PM components, but they did not fully
exonerate any major component class. They also noted that the toxicology literature
suggests that several elements including aluminum, silicon, vanadium, and nickel
have been associated with health effects, in addition to carbon-containing compo-
nents. In this critical review, we review studies in which metal components have had
a role in producing health effects attributed to ambient PM.
In late 2013 the Health Effects Institute (HEI) National Particle Component
Toxicity (NPACT) Initiative published two comprehensive reports of coordinated
toxicological and nationwide epidemiologic studies of the health effects of PM and its
components. Report No. 177 presents the fi ndings of Lippmann and colleagues
(
2013
) from studies with mice and human cell lines which identifi ed the coal combus-
tion, residual oil combustion, traffi c, and metals source categories as most frequently
associated with cardiovascular effects. In Report No. 178 Vedal and colleagues (
2013
)
investigated the cardiovascular effects of traffi c related PM by evaluating data from
two national epidemiologic studies-the Multi-Ethnic Study of Atherosclerosis
(MESA) and the Women's Health Initiative Observational Study (WHI-OS). Their
analysis of the epidemiologic data was supported by toxicological studies, in which
mice were exposed to mixtures of diesel and gasoline emissions. Both the particulate
and gas phases of motor vehicle exhaust were found to play a role in cardiovascular
effects. These reports add signifi cant new information regarding the cardiovascular
effects of PM. However, as observed by the HEI review panel implicating (or exoner-
ating) any specifi c PM component is diffi cult given the complexity of the exposures.
Several epidemiologic studies conducted during the late 1980s and 1990s (Pope
1989
,
1991
,
1992
) introduced the hypothesis that metal constituents of ambient PM
may contribute to the increases in respiratory and cardiovascular morbidity and
mortality that are associated with small increases in ambient PM. Pope (
1989
)
reported that the incidence of respiratory ailments in the community near an open
hearth steel mill in the Utah Valley decreased during a year (1987) when the mill
was closed for repairs, compared to the two adjacent years (1986, 1988). Since the
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