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and is made more complex by the aging of the aerosol through atmospheric chemical processes
as it moves away from its sources. In addition, the PM aerosol is experienced in the presence
of other oxidant pollutants (oxides of nitrogen, ozone) and stress (e.g., infections, diet) that may
contribute to and/or “account for” effects that are attributable to the PM aerosol. Animal studies
provide direct evidence for the occurrence of such complex interactions. Intranasal instillation of
ambient urban PM exposed to 100 ppb of ozone (O
3
) prior to instillation in rats produced greater
inlammation and biochemical evidence of cellular injury than a similar concentration of PM not
pre-exposed to O
3
[116]. Inhalation exposure of rats to urban PM and ozone demonstrated syner-
gistic effects of O
3
and PM on lung inlammation, an effect that was not seen for the stimulation
of endothelin (potent vasoconstrictor produced by vascular endothelial cells and thought to con-
tribute to ambient PM aerosol cardiovascular events) that was associated only with PM exposure
[117]. This latter study points to the additional complexity that ambient PM likely has differential
effects on different organ systems and metabolic processes that depend upon the chemical con-
text in which the aerosol is presented to the host. A further complexity relates the qualitative and
quantitative spatial heterogeneity of the aerosol constituents between areas in close geographi-
cal proximity. The effect of heterogeneity of the biological effects of PM aerosol collected from
different parts of the same city is illustrated in a study conducted in Mexico City in which PM
10
was collected from parts of the city with different sources of PM and different concentrations of
O
3
[118]. Signiicant differences were observed in the effects of PM
10
from the different locations
on cell viability, occurrence of apoptosis, evidence of DNA damage (comet assay), and secretion
of the pro-inlammatory cytokines IL-6 (Figure 23.17) and TNF-α. While controlled exposure
studies of human subjects do permit the assessment of biological responses to PM aerosol expo-
sure, they cannot provide direct evidence for acute health effects, since PM aerosol is presented
N
20
IL-6 secretion
16
North
South
Center
12
*
8
C
*
4
S
N
0
20
40
80
(a)
(b)
Concentration (µg/cm
2
)
20 km
Mean ± SD PM10 (µ/m
3
)
North - 122 ± 4
Central 107 ± 3
South 76 ± 2
FIGURE 23.17
Effects of PM
10
collected from three different locations in Mexico City with different
levels of PM
10
and different concentrations of ozone. IL-6 secretion was measured in mouse monocytes. (a)
Map of Mexico City; the metropolitan area is composed of two regions: the Federal District (black outline)
and the Estade de Mexico ( outline). Air quality monitoring stations cover the area shown in . The most
polluted areas for particles and ozone are in the northeast ( ) and the southwest ( ), respectively. We col-
lected particles for this study in northern (industrial), central (business), and southern (residential) zones;
sites of collection are marked with squares. Mountains ( triangles) surround the city, mainly in the
south. (Adapted from Gobierno del Distrito Federal [15]; from Alfaro-Moreno, E. et al.,
Environ. Health
Perspect
., 110, 715, 2002.)
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