Episodes of high coarse particle concentrations are not associated with increased mortality

Abstract

Fine particle concentration (i.e., particles <2.5 microm in aerodynamic diameter; PM2.5), but not coarse particle concentration, was associated with increased mortality in six U.S. cities. Others criticized this result, arguing that it could result from differences in measurement error between the two size ranges. Fine particles are primarily from combustion of fossil fuel, whereras coarse particles (i.e., particles between 2.5 and 10 microm in aerodynamic diameter) are all crustal material, i.e., dust. One way to determine if coarse particles are a risk for mortality is to identify episodes of high concentrations of coarse, but not fine, particles. Spokane, Washington, is located in an arid area and is subject to occasional dust storms after crops have been harvested. Between 1989 and 1995, we identified 17 dust storms in Spokane. The 24-hr mean PM10 concentration during those storms was 263 microg/m3. Using control dates that were the same day of the year in other years (but with no dust storm on that day) and that had a mean PM10 concentration of 42 microg/m3, we compared the rate of nonaccidental deaths on the episode versus nonepisode days. There was little evidence of any risk [relative risk (RR) = 1.00; 95% confidence interval (CI), 0.81-1.22] on the episode days. Defining episode deaths as those occurring on the same or following day as the dust storm produced similar results (RR = 1.01; CI, 0.87-1.17). Sensitivity analyses, which tested more extensive seasonal control, produced smaller estimates. We conclude that coarse particles from windblown dust are not associated with mortality risk.