Health Effects of Aerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 593

pollen concentrations in terms of standard counts (and not in terms of allergenic aerosol), which the
studies cited earlier indicate may not be the relevant measurement. Moreover, the presence of other pol-
lutants may modify these effects by decreasing the release of antigens [163], which further complicates
studies of this problem in human populations. Probably the strongest evidence to suggest that there are
human health consequences to interactions between pollen and PM comes from studies of cedar pol-
linosis in Japan, which suggest that the marked increase in allergy to cedar trees in Japan after 1964
can be traced to increases in mobile sources of air pollution [174]. A study reported by Dutch investiga-
tors observed that daily all-cause mortality and cardiovascular mortality were associated with average
weekly concentrations of common pollens, an effect that was not confounded by air pollutants (low
correlation between pollens and ambient air pollutants, which included black smoke), day of the week,
or meteorological factors [175]. The explanation for these indings is not clear, as there was no evidence
of interaction with ambient pollutants. The authors offer several speculations, some of which are sup-
ported by other data [175]. In contrast to these studies, a cross-sectional study in Switzerland found an
association between trafic density near residences and sensitization to pollen allergen (Timothy grass)
but not with symptoms of hay fever or the number of allergens to which residents (ages 18-60 years)
were sensitized [176]. The association was strongest for those who lived at their residences for 10 or
more years.
23.5.3 H ealtH e FFects a ssociated witH c Hronic e xPosure to PM
There is no universally agreed-upon duration that separates chronic from acute or subacute expo-
sures to PM aerosol. This section deines chronic exposures as those to which humans are exposed
over months to years or during deined periods of life events such as pregnancy. There is a vast
literature based on cross-sectional, population-based studies that have associated chronic exposures
to elevated pollutants and the occurrence of human disease. Due to the dificulty of the evaluation of
such data with respect to inferences about causality, this section is restricted to more recent cohort
studies of mortality and studies of effects on birth outcomes (these latter studies can be considered
as retrospective cohort studies in which the cohort is deined by onset of pregnancy and birth and
exposure histories are reconstructed after the fact). The focus is on data related to outdoor PM aero-
sol. Indoor sources are not discussed speciically—a complete discussion of health effects related
to secondhand tobacco smoke is beyond the scope of this chapter. This latter subject is reviewed
comprehensively in two recent publications from the National Cancer Institute [17,148] and is the
subject of a soon-to-be-released Report of the Surgeon General.
23.5.3.1 Effects of Exposure to Ambient PM and Birth Outcomes (Table 23.9)
In 1992, investigators from the Czech Republic reported that annual ambient levels of TSP, mea-
sured in 46 of 85 administrative districts in the Czech Republic, were associated with an increased
risk of infant mortality [177]. Studies in the United States [178] and a subsequent study in the Czech
Republic conirmed these indings [179]. Two studies of the relationship between daily changes in
TSP [180] and PM 2.5 [181] also observed increased mortality risk with increased concentrations.
The extent to which PM aerosol contributed uniquely to the increased risks could not be determined
by any of these studies, since associations were also found for gaseous pollutants (Table 23.9).
Studies from many parts of the world have identiied associations between various birth outcomes
(see footnote b of Table 23.9) and exposure to PM aerosol, measured most frequently as TSP but
also based on PM 10 and PM 2.5 and personal exposure to airborne PAHs (Table 23.9). While there is
consistency with regard to the fact that PM aerosol affects birth outcomes, there is less consistency
with regard to the speciic parameters and the critical developmental periods. Figure 23.19 presents
data from three representative studies from three different countries. The studies from the Czech
Republic (Figure 23.19a) [182] and southern California [183] found effects of exposure during the
irst trimester, but the former study failed to ind effects of exposure later in pregnancy. A study
from Beijing, China [184], observed that exposure to TSP during the third trimester increased the