Air pollution and daily mortality in two U.S. counties: season-specific analyses and exposure-response relationships

I used generalized additive models to analyze the time series of daily total nonaccidental deaths and deaths due to vascular disease over the period 1987-1995 in two major metropolitan areas, Cook County, Illinois, and Los Angeles County, California, in the United States. In both counties I had monitoring information on PM(10), CO, SO(2), NO(2), and O(3). In Los Angeles, monitoring information on PM(2.5) was available as well. In addition to full-year analyses, I performed season-specific analyses. I present the results of both single- and multipollutant analyses. Although components of air pollution were associated with total nonaccidental and vascular disease mortality in both counties, the results indicate considerable heterogeneity of these associations in the two locations and also from season to season. In Los Angeles County, the gases, particularly CO and SO(2) but not ozone, were more strongly associated with mortality than was particulate matter, which exhibited only weak and inconsistent associations with both mortality endpoints. Both PM(10) and the gases were associated with total and vascular disease mortality in Cook County. The association of the gases with both mortality endpoints appeared to be stronger and more robust than that of PM(10). Exposure-response analyses using flexible smoothers showed significant departures from linearity, particularly for PM effects.