Airborne coarse particles and mortality

Some recent epidemiologic studies suggest a stronger effect of fine particles (PM2.5) than of coarser particulate matter. To examine the support for such a differential effect, the authors conducted a daily time-series analysis of mortality in relation to measurements or PM2.5, PM10, and PM 10-2.5 in southwestern Mexico City in the years 1992-1995. A generalized linear model based on Poisson regression was used to control for weather and periodic cycles, and the average concentration of the previous five days was the index 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 correlated with each other (correlation coefficient 0.52). All three particle size fractions were associated individually with mortality: a 10-mu g m(-3) increase 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 1.48% 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 particles was stronger for respiratory diseases than for total mortality cardiovascular diseases, or other noninjury causes of death. These patterns persisted after adjustment for O-3 and NO2. When both PM2.5 and PM 10-2.5 were included simultaneously in the regression model, the effect of PM10-2.5 remained 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 attributable to specific combustion or biogenic materials within the coarse particle mass. Understanding these relationships will require analyses of the composition of coarse particles. The findings also suggest a need to that the relative effects or coarse and fine particles on mortality should be examined in more cities with a wider variety of climates, population characteristics, and air pollutants.