Air Pollution and Daily Mortality: a Review and Meta Analysis

The air pollution disaster in London in 1952 established that very high levels of particulate-based smog can cause dramatic increases in daily mortality. Recently, more than a dozen studies at much lower particle concentrations have reported evidence that exposure to lower levels of airborne particles results in smaller, but nonzero increases in daily mortality. These studies were examined in a meta-analysis. A primary focus of the meta-analysis was to examine effect size estimates across large differences in both the levels of potential confounding factors and in their correlation with airborne particle concentration. In the primary meta-analysis, airborne particle concentration was a significant risk factor for elevated mortality (RR = 1.06, 95% CI = 1.05-1.07). The relative risk is for a 100 micrograms/m3 increase in TSP concentration. While mortality peaked in the cold months in all locations, in the majority of the studies airborne particle concentrations were highest in the warm months, indicating that seasonal patterns were not responsible for the observed associations. The relative risk was 1.06 (95% CI = 1.05-1.07) when the analysis was restricted to cities with summer peaking pollution. The relative risk was identical in cities with above average annual temperatures and cities with colder climates. It was also identical in drier and more humid climates, and similar across a wide range of correlations between temperature and airborne particle concentrations. These results suggest that inadequate weather control was not responsible for the association. A detailed examination of data from Philadelphia showed that control for season and weather was adequate for removing all long-term seasonal and subseasonal patterns from the mortality data, and that using a very flexible nonlinear fit to the weather factors did not disturb the association with TSP. The most reasonable interpretation of this pattern of results is that the association is causal. This is supported by other studies which have reported that particulate air pollution was associated with lung function deficits, increased symptoms, and increased hospitalization.