Daily deaths are associated with combustion particles rather than SO2 in Philadelphia

Objectives-To assess whether the association between SO2 and daily deaths in Philadelphia during the years 1974-88 is due to its correlation with airborne particles, and vice versa. Methods-There is a significant variation in the relation between total suspended particulate (TSP) and SO2 in Philadelphia by year and season. Firstly, 30 separate regressions were fitted for each pollutant in the warm and cold season of each year. These regressions controlled for weather, long term temporal patterns, and day of the week. Then a meta-regression was performed to find whether the effect of SO2 was due to TSP, or vice versa. Results-Controlling for TSP, there was no significant association between SO2 and daily deaths. By contrast, in periods when TSP was less correlated with SO2, its association with daily deaths was higher. However, all of the association between TSP and daily deaths was explained by its correlation with extinction coefficient, a measurement of the scattering of light by fine particles, which has been shown to be highly correlated with fine combustion particles in Philadelphia. Conclusions-The association between air pollution and daily deaths in Philadelphia is due to fine combustion particles, and not to SO2.