ults, for an interquartile change (75th–25th
percentile) in PM10 of approximately 45
μg/m3, the odds ratio is 1.66 [95% confidence
interval (CI) = 1.52–1.82] for lower
respiratory symptoms and 1.94 (95% CI =
1.77–2.12) for upper respiratory symptoms.
Lower effect magnitudes were observed for
the panels of nurses and children, but PM10
was associated with statistically significant
increases in frequencies for both symptom
categories for all three panels, with and
without daily weather variables included in
the models.
The inclusion of a variable indicating the
presence of a symptom on the prior day
caused virtually no change in the estimated
PM10 effects for lower respiratory symptoms
for all three panels relative to the basic model
with daily weather variables, but attenuated
the estimated effect of PM10 on upper respiratory
symptoms in all three panels. Omitting
the hottest 25% of the days tended to increase
the estimated association, whereas omitting
the coldest 25% of the days lowered the estimate,
but the PM10 effect remained statistically
significant in nearly all cases. The effects
of PM10 on the likelihood of a new symptom
episode were examined in an analysis that
included only those days for which there were
no symptoms reported on the previous day.
The results indicate an association exists for
both adult panels and for both outcomes. The
magnitude and statistical significance of the
PM10 effect remained comparable to that
found in the original model for all days.
However, the results for children showed no
statistically significant effect of PM10 on new
symptoms. We also ran a model in which
cough was not included as a lower respiratory
symptom. The results were similar to those
obtained for lower respiratory symptoms
when cough was included.
The last row in Table 4 shows the PM10
results estimated with a fixed-effects model
that included the daily weather variables. The
fixed-effects model allows the baseline symptom
incidence to vary for each individual and
corrects for the correlations among repeated
responses from the same individuals. The
PM10 results for the fixed-effects models for
the two adult panels were little changed from
the results using the basic model with daily
weather variables. However, the PM10 results
for the children were about 50% lower, but
still statistically significant. Overall, the central
estimate for the odds ratios for a 45 μg/m3
change in PM10 calculated from the sensitivity
test results for the Odean Circle adults ranges
from 1.38 to 1.66 for lower respiratory symptoms
and from 1.38 to 1.94 for upper respiratory
symptoms. For the nurses the central odds
ratios from the sensitivity tests range from 1.10
to 1.27 for lower respiratory symptoms and
from 1.11 to 1.35 for upper respiratory
symptoms. For children the odds ratio for
probability of a symptom day ranges from
1.08 to 1.66 for lower respiratory symptoms
and from 0.99 to 1.53 for upper respiratory
symptoms. No association was found for
asthma episodes, but these were infrequent in
the data.
Table 5 shows the results of two alternative
assumptions that give the likely highest
and lowest PM10 effects and reflect alternative
approaches of accounting for time trends in
the data (Figure 1). The first specification
includes age, sex, educational level, having a
chronic respiratory condition, and having no
air conditioning in the home, but does not
include any terms for time and temperature
(which generally increased over time). This
model reflects the hypothesis that the decreasing
trend in symptoms is entirely attributable
to the decreasing trend in PM10 concentrations
during the study period. At the other
extreme, loess smoothers of symptoms over
both time and temperature are added to the
model. The smooth of symptoms over time
causes a significant attenuation in the estimated
odds ratio for PM10 for all three of the
panels. In this model the downward trend in
symptoms is captured by the smooth variable,
essentially implying only a small portion of
the downward trend is attributable to
decreasing concentrations of PM10. It is
notable that even with this extreme assumption,
a statistically significant association
between symptoms and PM10 is still observed
for the Odean Circle adult panel.
Dichotomous samplers located at Odean
Circle and at Chulalongkorn Hospital during
part of the study period provided a limited
number of days of PM2.5 concentrations. We
therefore estimated a PM2.5 coefficient and a
PM10 coefficient for the same days to compare
their associations with symptoms. As
summarized in Table 6, statistically significant
associations were found between respiratory
symptoms and both PM10 and PM2.5,
measured as 4-day moving averages, for both
adult panels. The odds ratios for interquartile
ranges (45 μg/m3 for PM10 and 26 μg/m3 for
PM2.5) are comparable in magnitude for all
the comparisons. For children, however, the
PM2.5 results are not statistically significant.