EFFECTS OF AMBIENT FINE AND COARSE PARTICLES

ON MORTALITY IN PHOENIX, ARIZONA¹

Merlise A. Clyde, Peter Guttorp, and Erin Sullivan

Abstract

Considerable attention has been given to the health effects of ambient air borne particulate matter as the Environmental Protection Agency (EPA) revises the National Ambient Air Quality Standards (NAAQS). Much of the recent focus has been on the effects of fine particles, with the establishment of additional monitoring platforms to measure both fine and coarse particles for epidemiological studies. Much of the evidence supporting the 1997 standards is based on statistical models using generalized additive models for time series data. Among the statistical concerns raised by National Research Council is the issue of whether observed statistical associations are a result of multiple testing and selection effects due to model selection. We propose a method based on Bayesian Model Averaging to estimate relative risks that incorporates both uncertainty due to estimation and model choice. This incorporates uncertainty regarding the choice of confounding variables, choice of pollutant (fine and coarse particles), which lags of all variables should be included in the model and degrees of freedom used in estimation of the overall temporal trend. This approach is illustrated using fine and coarse particulate matter (PM) data from the National Exposure Research Laboratory research monitoring platform in Phoenix, AZ for the time period of May 1995 to June 1998. We consider elderly non-accidental mortality for three regions of increasing size in the Phoenix metropolitan area, as well as using accidental mortality as a control population. We find evidence of a particulate matter effect on elderly mortality in the three geographic region; the strongest effects are for the Phoenix metropolitan region where the posterior probability of a PM effect is 0.95. We find no effect of PM on accidental mortality. Taking into account model uncertainty, 95% probability intervals for the relative risk (RR) with a 1 IQR increase in both fine and coarse particulate matter levels is [1, 1.049) for the metropolitan area. While previous scientific information would suggest that fine particles should have a larger effect on health outcomes than coarse particles, we find instead that the effect is primarily associated with coarse particles. The posterior probability that the effect is due to fine particles ranges from 0.04 to 0.16 over the three regions in Phoenix.