Saharan Dust and Associations between Particulate Matter and Daily Mortality in Rome, Italy

Background: Outbreaks of Saharan-Sahel dust over Euro-Mediterranean areas frequently induce exceedances of the Europen Union's 24-hr standard of 50 mu g/m(3) for particulate matter (PM) with aerodynamic diameter <= than 10 mu m (PM10). Objectives: We evaluated the effect of Saharan dust on the association between different PM fractions and daily mortality in Rome, Italy. Methods: In a study of 80,423 adult residents who died in Rome between 2001 and 2004, we performed a time-series analysis to explore the effects of PM2.5, PM2.5-10, and PM10 on natural, cardiac, cerebrovascular, and respiratory mortality. We defined Saharan dust days by combining light detection and ranging (LIDAR) observations and analyses from operational models. We tested a Saharan dust-PM interaction term to evaluate the hypothesis that the effects of PM, especially coarse PM (PM2.5-10), on mortality would be enhanced on dust days. Results: Interquartile range increases in PM2.5-10 (10.8 mu g/m(3)) and PM10 (19.8 mu g/m(3)) were associated with increased mortality due to natural, cardiac, cerebrovascular, and respiratory causes, with estimated effects ranging from 2.64% to 12.65% [ 95% confidence interval (CI), 1.18-25.42%] for the association between PM2.5-10 and respiratory mortality (0- to 5-day lag). Associations of PM2.5-10 with cardiac mortality were stronger on Saharan dust days (9.73%; 95% CI, 4.25-15.49%) than on dust-free days (0.86%; 95% CI, -2.47% to 4.31%; p = 0.005). Saharan dust days also modified associations between PM10 and cardiac mortality (9.55% increase; 95% CI, 3.81-15.61%; vs. dust-free days: 2.09%; 95% CI, -0.76% to 5.02%; p = 0.02). Conclusions: We found evidence of effects of PM2.5-10 and PM10 on natural and cause-specific mortality, with stronger estimated effects on cardiac mortality during Saharan dust outbreaks. Toxicological and biological effects of particles from desert sources need to be further investigated and taken into account in air quality standards.