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

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AIR QUALITY GUIDELINES
Osornio-Vargas et al. (241) exposed murine cells to PM10 and PM2.5 recovered
from filters sampling air in Mexico City to compare cytotoxic and proinflammatory
effects of these two size fractions. The PM induced different biological
effects depending on the specific sampling site and particle size.
In spite of these limitations, the toxicological evidence is clear in linking adverse
biological responses to PM exposure. Toxicological studies have shown that
the ultrafine, fine and coarse size modes may result in biological responses that
could plausibly contribute to the health outcomes observed in epidemiological
studies. The findings of epidemiological studies of acute and chronic health effects
suggest that PM2.5, which includes PM in the ultrafine size mode, is associated
with a range of adverse health outcomes. However, there are only limited
epidemiological data on either ultrafine or coarse PM to complement the toxicological
studies of these size fractions.
The toxicological evidence does provide an indication that aspects of PM other
than mass alone determine toxicity. In terms of chemical species, the strongest
toxicological consistency is with secondary inorganic PM, namely sulfates and
nitrates at above-ambient levels, but this consistency is opposed by a lack of effect
in controlled exposure studies within the ambient concentration range. The findings
of the controlled exposure studies contrast with some of the epidemiological
findings. There are many potential explanations for this lack of coherence across
different lines of investigation and their resolution will require new, interdisciplinary
research approaches that better combine toxicology and epidemiology. One
such explanation may involve metal sulfates, as noted above.
Controlled exposure studies strongly suggest that transition metals are a chemical
component of PM with toxic potential. Experimental studies generally used
fairly high exposure concentrations, leaving unclear the relevance of their findings
to ambient exposure. Furthermore, the concentration of such metals varies
widely geographically, but is generally quite low in ambient air in the United
States. A potential role for transition metals at relatively high concentrations in
determining risk for health outcomes was demonstrated in parallel toxicological
studies and in epidemiological studies of PM associated with steel mill emissions
in the Utah Valley. However, recent long-term exposure toxicology suggests that
acute biological effects may be due to transition metals at lower ambient concentrations
(132).
Experimental studies have indicated that the organic constituents of PM are
also likely to be toxicologically active. While the current evidence is not sufficient
to develop an unequivocal conclusion as to health hazards from specific organic
compounds, the best candidate in this regard is the PAHs or their nitro- and oxyderivatives.
Most studies with biogenic organic carbon-containing aerosols examined
bacterial endotoxin, a cell wall component. Endotoxin is present in the
coarse mode and may be responsible, at least in part, for the toxicity of this size
mode observed in some studies. Parallel epidemiological evidence is still lacking.