Threshold dependence of mortality effects for fine and coarse particles inPhoenix, Arizona

Daily data for fine (<2.5 mu m) and coarse (2.5-10 mu m) particles are avai lable for 1995-1997 from the U.S. Environmental Protection Agency (EPA) res earch monitor in Phoenix, AZ. Mortality effects on the 65 and over populati on were studied for both the city of Phoenix and for a region of about 50 m i around Phoenix. Coarse particles in Phoenix are believed to be natural in origin and spatially homogeneous, whereas fine particles are primarily veh icular in origin and concentrated in the city itself. For this reason, it i s natural to focus on city mortality data when considering fine particles, and on region mortality data when considering coarse particles, and most of the results reported here correspond to those assignments. After allowing for seasonality and long-term trend through a nonlinear (B-s pline) trend curve, and also for meteorological effects based on temperatur e and specific humidity, a regression of mortality was performed on PM usin g several different measures for PM. Based on a linear PM effect, we found a statistically significant coefficient for coarse particles, but not for f ine particles, contrary to what is widely believed about the effects of coa rse and fine particles. An analysis of nonlinear pollution-mortality relati onships, however, suggests that the true picture is more complicated than t hat. For coarse particles, the evidence for any nonlinear or threshold-base d effect is slight. For fine particles, we found evidence of a threshold, m ost likely with values in the range of 20-25 mu g/m(3). We also found some evidence of interactions of the PM effects with season and year. The main effect here is an apparent seasonal interaction in the coarse PM e ffect. An attempt was made to explain this in terms of seasonal variation i n the chemical composition of PM, but this led to another counterintuitive result: the PM effect is highest in spring and summer, when the anthropogen ic concentration of coarse PM is lowest as determined by a principal compon ents analysis. There was no evidence of confounding between the fine and co arse PM effects. Although these results are based on one city and should be considered tentative until replicated in other studies, they suggest that the prevailing focus on fine rather than coarse particles may be an oversim plification. The study also shows that consideration of nonlinear effects c an lead to real changes of interpretation and raises the possibility of sea sonal effects associated with the chemical composition of PM.