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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 μg/m³ increase in TSP concentration. While mortality peaked in the cold months in all locations, in the majority of the studies airborne …