Air Quality Guidelines Global Update 2005 - World Health ...

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AIR QUALITY GUIDELINES
exposures to ambient PM. The toxicologically significant secondary inorganic
particles were probably those having strong acidity, namely sulfuric acid, ammonium
bisulfate and, under some unusual atmospheric conditions, nitric acid.
Some of these same PM characteristics have been associated with adverse health
effects in epidemiological studies. Nonetheless, Schlesinger & Cassee (219) concluded
that the toxicological evidence did not support a role for secondary inorganic
aerosols in adverse health outcomes noted in the epidemiological studies.
They reached this conclusion because the ambient concentrations of particulate
sulfate are generally much lower than those associated with apparent health effects
in the controlled studies. Furthermore, the physicochemical characteristics
of the sulfate used in the laboratory studies may have differed from the characteristics
of the particles to which human populations are actually exposed in ambient
air. Of course, this same limitation probably holds for other PM surrogates
used in controlled exposure studies. Other studies also indicate that sulfate may
not be of significant toxicological importance in terms of overall effects of PM
exposure. In a study using CAPs, effects on a biomarker of cellular stress in human
bronchial epithelial cells exposed to concentrated fine PM were not correlated
with ambient sulfate, even though sulfate constituted 65% of the particulate
mass (132). Effects were, however, clearly correlated with certain metals, such
as vanadium and nickel from the PM associated with residual oil combustion.
Similarly, in the study of Huang et al. (193), metals were associated with release of
cytokine whereas sulfate was not.
Furthermore, toxicological effects associated with primary metal sulfates
should not be attributed to a generic species (“sulfates”), as secondary sulfate
compounds, thought to be primarily ammonium salts, have negligible health effects
(220).
Crustal-associated chemicals
Early studies often used ash from the Mount St Helens volcanic eruption as a
surrogate for crustal material. One advantage of using this material is that it isolates
the inorganic metal oxides from humic and other organic material associated
with soil dust normally found in PM. The studies using volcanic ash tended
to show that such particles were relatively inert and generally non-inflammatory
(221).
Some recent studies have examined effects of other components generally associated
with crustal PM. Using fine particulate mode CAPs from the Boston
area, Clarke et al. (222) noted that aluminium and silicon were correlated with
increased pulmonary neutrophils in lung lavage samples and increased total
peripheral blood white cell counts in dogs. However, since crustal PM does not
comprise a large fraction of material in the fine particulate mode, the investigators
hypothesized that the findings indicated that the aluminium–silicon factor
was a surrogate for other particles. Batalha et al. (223) showed that exposure