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

Fine particle concentration (i.e., particles <2.5 pm in aerodynamic diamete r; PM2.5), but not coarse particle concentration, was associated with incre ased mortality in six U.S. cities. Others criticized this result, arguing t hat it could result from differences in measurement error between the two s ize ranges. Fine particles are primarily from combustion of fossil fuel, wh ereas coarse particles (i.e., particles between 2.5 and 10 mu m in aerodyna mic 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 crop s 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 m u g/m(3). Using control dates that were the same day of the year in other y ears (but with no dust storm on that day) and that had a mean PM10 concentr ation of 42 mu g/m(3), we compared the rate of nonaccidental deaths on the episode versus nonepisode days. There was little evidence of any risk [rela tive risk (RR) = 1.00; 95% confidence interval (CI), 0.81-1.22] on the epis ode days. Defining episode deaths as those occurring on the same or followi ng day as the dust storm produced similar results (RR = 1.01; CI, 0.87-1.17 ). Sensitivity analyses, which tested more extensive seasonal control, prod uced smaller estimates. We conclude that coarse particles from windblown du st are not associated with mortality risk.