Airborne coarse particles and mortality

Some recent epidemiologic studies suggest a stronger effect of fine particl es (PM2.5) than of coarser particulate matter. To examine the support for s uch a differential effect, the authors conducted a daily time-series analys is of mortality in relation to measurements or PM2.5, PM10, and PM 10-2.5 i n southwestern Mexico City in the years 1992-1995. A generalized linear mod el based on Poisson regression was used to control for weather and periodic cycles, and the average concentration of the previous five days was the in dex or particle exposure. The mean concentrations of PM2.5 and PM10 were 27 .4 mu g m(-3) and 44.6 mu g m(-3), respectively, and the mean concentration of PM 10-2.5 was 17.2 mu g m(-3). PM10 was highly correlated with both the fine and coarse fractions, but PM2.5 and PM10-2.5 were rather weakly corre lated with each other (correlation coefficient 0.52). All three particle si ze fractions were associated individually with mortality: a 10-mu g m(-3) i ncrease in PM10 was associated with a 1.83% increase in total mortality (95 % CI -0.01-2.96), and an equal increment in PM2.5 was associated with a 1.4 8% increase in deaths (95% CI 0.98-2.68%). The largest effect was observed for a 10-mu g m(-3) increment in PM10-2.5; mean daily mortality increased 4 .07% for each 10 mu g m(-3) (95% CI 2.49-5.66%). The effect of coarse parti cles was stronger for respiratory diseases than for total mortality cardiov ascular diseases, or other noninjury causes of death. These patterns persis ted after adjustment for O-3 and NO2. When both PM2.5 and PM 10-2.5 were in cluded simultaneously in the regression model, the effect of PM10-2.5 remai ned about 4% per 10 mu g m(-3) (95% CI 1.96-6.02%), while the effect of PM2 .5 was virtually eliminated (0.18% change). These associations may be attri butable to specific combustion or biogenic materials within the coarse part icle mass. Understanding these relationships will require analyses of the c omposition of coarse particles. The findings also suggest a need to that th e relative effects or coarse and fine particles on mortality should be exam ined in more cities with a wider variety of climates, population characteri stics, and air pollutants.