The air pollution disasters in London in 1952, the Meuse valley in 1930, and in Donoroa, Pennsylvania, in 1948 made it clear that extremely high levels of particulate-based smog could produce large increases in the daily mortality rate. Recent studies of fluctuations in daily air pollution and daily mortality have reported associations at much lower concentrations in London during the 1960s and in Philadelphia, Steubenville, Santa Clara, St. Louis, Utah valley, Detroit, and eastern Tennessee in the 1970s and 1980s. Whether these associations are causal or not is a matter of considerable public health concern. If the detailed pattern of the deaths at these lower concentrations appeared similar to the pattern in London, this would strengthen the argument for causality. To examine this issue, the death certificates from Philadelphia were examined on the 5% of the days with the highest particulate air pollution and the 5% of the days with the lowest particulate air pollution during the years 1973-1980. There was little difference in weather between the high and low pollution days, but total suspended particulate matter concentrations averaged 141 micrograms/m3 on the high pollution days versus 47 micrograms/m3 on the low pollution days. The relative risk of dying on the high pollution days was 1.08 P < 0.0001. The relative increase was higher for COPD (1.25) and pneumonia (1.13). Deaths were also elevated for heart disease and stroke; however, there was a substantial increase in the reports of respiratory factors as contributing causes for those underlying causes of death. Dead-on-arrival deaths and deaths outside of hospitals and clinics were also disproportionately increased. This paralleled the pattern seen in London in 1952. The age pattern of the relative risk of death was also similar. This adds to the evidence that the association is causal.