with a smooth of time in the model, an
association between PM10 and both symptom
categories remains statistically significant
for the Odean Circle adults as well as
for the children for upper respiratory symptoms.
We expect these groups have higher
exposure to outdoor pollutants than the
nurses because they spend less time in airconditioned
buildings. We give greater
weight to the results without the smooth of
time variable because there appears to be a
good chance the smooth is capturing some
of the real effect of the decline in PM10
concentrations during the study period and
thus causing the PM10 coefficient to understate
the true effect of PM10 on respiratory
symptoms. However, the uncertainty about
how to interpret the results when the
smooth of time is included cannot be fully
resolved without running similar panel
studies for longer periods of time and
during different seasons.
It is difficult to identify any potentially
confounding omitted factor that is correlated
with both PM10 and frequencies of reported
symptoms. It appears the decrease in symptom
incidence over time was a real phenomenon
and not due to response bias. Diary
compliance was extremely high throughout
the study. In addition, for all three panels,
symptoms decreased during the study period
and daily symptom incidences were highly
correlated across the three panels.
The results reported here show statistically
significant PM10 effects in three different
population groups in Bangkok, but these
groups had some unique characteristics that
make it difficult to generalize to the entire
population of Bangkok. Daily symptom
diary studies could be extended to other
population groups whose exposure circumstances
vary. Those who commute may have
even higher exposures than the Odean
Circle adults in this study because of their
time spent in traffic. Others who work in
air-conditioned offices and have air conditioning
in their homes may have even less
exposure than the nurses in this study. In
addition, this symptom diary was completed
during the high-pollution months. It would
be useful to test for this association during
other seasons and with other panels. For
Bangkok and other cities outside of the
industrialized West, further diary work
would also be enhanced by obtaining daily
information on indoor sources of PM,
including exposure to cooking sources. In
many countries, these sources represent a
significant burden (13