Environmental Engineering Reference
In-Depth Information
components can lead directly to the production of reactive oxygen species (metabolites—ROM)
[38,39]. ROM (superoxide radical [O
22
•
], hydrogen peroxide [H
2
O
2
], hydroxyl radical [OH
•
]; some-
times included are singlet O
2
and hypochlorous acid), in turn, are potent inducers of inlammation
[40] and initiators of lipid peroxidation and DNA strand breaks [41], which may be important in
carcinogenesis (summarized in Figure 12 of Ref. [42]). Since a large body of research suggests
that oxidant stress may be one of the principal (if not the principal) inal common pathways for the
effects of ambient aerosols on human health, this section begins with a consideration of the evidence
for this view. The section then considers pathways less proximate: inlammation, immunological
modulation, and particle overloading effects.
23.4.1 P
article
a
erosol
i
nduction
oF
o
xidative
d
aMage
As epidemiological evidence of health effects related to particulate air pollution mounted, animal
studies were undertaken to try to identify the components of PM and mechanisms that might explain
the epidemiological observations. Some of the most important of these were animal exposure studies
undertaken with residual oil ly ash (ROFA), which focused on the role of transition metals. ROFA is
generated from the burning of fossil fuels and has a high concentration of metals, particularly transi-
tion metals ([43]; reviewed in Ref. [44]). These metals, which can exist in more than one valence state,
can participate in electron cycling that can lead to the production of highly reactive OH through the
Haber-Weiss reaction (Table 23.5) [34]. This generation can take place in acellular and cellular sys-
tems [44,45]. First-row transition metals (titanium, vanadium, chromium, manganese, iron, cobalt,
nickel, and copper) are found in highest concentrations in crustal material and in the atmosphere as
a consequence of human-generated pollutants [45]. A series of studies by U.S. EPA investigators and
collaborators (selected references include [45-49]) established the role of the oxidative potential of
the transition metal content of ROFA to generate ROM and the speciicity of these metals as causes
of lung injury in rodents. The metal contents of dusts from a variety of sources other than ROFA
were also shown to produce lung injury that was speciic to the generation of ROM [46,47,50]. A
recent, comprehensive evaluation of the role of metals in the toxicology of PM generated from coals
from three different locations in the United States and from PM derived from several formulations of
gasoline and diesel fuel (Utah, Illinois, North Dakota) provides strong support for the toxicological
importance of the oxidant properties of transition metals [51]. These investigators documented that
bioavailable iron (nmol/mg of coal ly ash as measured after ferric ammonium citrate extraction)
was greatest for PM ≤1 μm, but bioavailable iron could also be found in PM
10-2.5
. Chelation of iron
signiicantly reduced evidence of oxidative damage as measured by malondialdehyde [51].
The ability of PM
10
to generate ROM unrelated to transition metals has been well documented
[52,53]. Investigators at the University of California, Los Angeles, CA, have conducted an extensive
series of studies that establish that organic components of PM lead to the production of ROM (reviewed
in Ref. [54]). In particular, polycyclic aromatic hydrocarbons (PAHs) and PAH-derived quinones found
in diesel exhaust particles (DEP) have been shown to produce
in vitro
evidence of oxidant stress [55].
TABLE 23.5
Haber-Weiss Reaction: Ferrous Iron-Catalyzed
Generation of Hydroxyl Radicals
O
i
−
3
+
2
+
+
Fe
→ +
O
Fe
2
2
Fe
2
+
+
H
O
→
Fe
3
+
+
HO OH
i
+
−
2
2
O
i
−
+
H O
→ +
O HO OH
i
+
−
2
2
2
2
All irst-row transition metals can participate in this reaction.
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