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Daily mortality and fine and ultrafine particles in Erfurt, Germany
part I: role of particle number and particle mass.
GSF Institute of Epidemiology, Neuherberg,
Germany.
Increases in morbidity and mortality have been
observed consistently and coherently in association with ambient air
pollution. A number of studies on short-term effects have identified
ambient particles as a major pollutant in urban air. This study,
conducted in Erfurt, Germany, investigated the association of mortality
not only with ambient particles but also with gaseous pollutants and
indicators of sources. Part I of this study concentrates on particles.
Data were collected prospectively over a 3.5-year period from September
1995 to December 1998. Death certificates were obtained from the local
authorities and aggregated to daily time series of total counts and
counts for subgroups. In addition to standard data for particle mass
with diameters < or = 2.5 microm (PM2.5)* or < or = 10 microm
(PM10) from impactors, a mobile aerosol spectrometer (MAS) was used to
obtain size-specific number and mass concentration data in six size
classes between 0.01 microm and 2.5 microm. Particles smaller than 0.1
microm were labeled ultrafine particles (three size classes), and
particles between 0.1 and 2.5 microm were termed fine particles (three
size classes). Concentrations of the gases sulfur dioxide (SO2),
nitrogen dioxide (NO2), and carbon monoxide (CO) were also measured. The
daily average total number concentration was 18,000 particles/cm3 with
88% of particles below 0.1 pm and 58% below 0.03 microm in diameter. The
average mass concentration (PM2.5) was 26 microg/m3; of this, 75% of
particles were between 0.1 and 0.5 microm in diameter. Other average
concentrations were 38 microg/m3 for PM10, 17 microg/m3 for SO2, 36
microg/m3 for NO2, and 600 microg/m3 for CO. Ambient air pollution
demonstrated a strong seasonality with maximum concentrations in winter.
Across the study period, fine particle mass decreased, whereas ultrafine
particle number was unchanged. The proportion of ultrafine particles
below 0.03 microm diameter increased compared with the proportion of
other particles. During the study, concentrations of SO2 and CO also
decreased, whereas the concentration of NO2 remained unchanged. The data
were analyzed using Poisson regression techniques with generalized
additive modeling (GAM) to allow nonparametric adjustment for the
confounders. Both the best single-day lag and the overall association of
multiple days fitted by a polynomial distributed lag model were used to
assess the lag structure between air pollution and death. Mortality
increased in association with level of ambient air pollution after
adjustment for season, influenza epidemics, day of week, and weather. In
the sensitivity analyses, the results proved stable against changes of
the confounder model. We saw comparable associations for ultrafine and
fine particles in a distributed lag model where the contribution of the
previous 4 to 5 days was considered. Furthermore, the data suggest a
somewhat more delayed association of ultrafine particles than of fine
particles if single-day lags are considered. The associations tended to
be stronger in winter than in summer and at ages below 70 years compared
to ages above 70 years. Analysis of the prevalent diseases mentioned on
death certificates revealed that the overall association for respiratory
diseases was slightly stronger than for cardiovascular diseases. In
two-pollutant models, associations of ultrafine and fine particles
seemed to be largely independent of each other, and the risk was
enhanced if both were considered at the same time. Furthermore, when the
associations were summed for the six size classes between 0.01 and 2.5
microm, the overall association was clearly stronger than the
associations of the individual size classes alone. Associations were
observed for SO2, NO2, and CO with mortality despite low concentrations
of these gases. These associations disappeared in two-pollutant models
for NO2 and CO, but they remained stable for SO2. The persistence of the
SO2 effect was interpreted as artifact, however, because the SO2
concentration was much below levels at which effects are usually
expected. Furthermore, the results for SO2 were inconsistent with those
from earlier studies conducted in Erfurt. We conclude that both fine
particles (represented by particle mass) and ultrafine particles
(represented by particle number) showed independent effects on mortality
at ambient concentrations. Comparable associations for gaseous
pollutants were interpreted as artifacts of collinearity with particles
from the same sources.
PMID: 11918089 [PubMed - indexed for MEDLINE]
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