DOES WEATHER CONFOUND OR MODIFY THE ASSOCIATION OF PARTICULATE AIR-POLLUTION WITH MORTALITY - AN ANALYSIS OF THE PHILADELPHIA DATA, 1973-1980

Because weather has the potential to confound or modify the pollution- mortality relationship, researchers have developed several approaches for controlling it in estimating the independent effect of air polluti on on mortality. This report considers the consequences of using alter native approaches to controlling for weather and explores modification of air pollution effects by weather, as weather patterns could plausi bly alter air pollution's effect on health. We analyzed 1973-1980 tota l mortality data for Philadelphia using four weather models and compar ed estimates of the effects of TSP and SO2 on mortality using a Poisso n regression model. Two synoptic categories developed by Kalkstein wer e selected-the Temporal Synoptic Index (TSI) and the Spatial Synoptic Classification (SSC)-and compared with (1) descriptive models develope d by Schwartz and Dockery (S-D); and (2) LOESS, a non-parametric funct ion of the previous day's temperature and dew point. We considered mod el fit using Alkaike's Information Criterion (AIC) and changes in the estimated effects of TSP and SO2. In the full-year analysis, S-D is be tter than LOESS at prediciting mortality, and S-D and LOESS are better than TSI, as measured by AIC. When TSP or SO2 was fit alone, the resu lts were qualitatively similar, regardless of how weather was controll ed; when TSP and SO2 mere fit simultaneously, the S-D and LOESS models give qualitatively different results than TSI, which attributes more of the pollution effect to SO2 than to TSP. Model fit is substantially poorer with TSI. This pattern was repeated in analyses of summer and winter months, which included SSC. In summary, using synoptic weather categories in regression models does not meaningfully change the assoc iation between mortality and air pollution indexes. We also found litt le evidence that weather conditions modified the effect of pollution, regardless of the approach used to represent weather. (C) 1998 Academi c Press.