 epidemiologic studies of the link between particulate matter ( pm ) concentrations and mortality rates have yielded a range of estimates , leading to disagreement about the magnitude of the relationship and the strength of the causal connection . 
 previous meta - analyses of this literature have provided pooled effect estimates , but have not addressed between - study variability that may be associated with analytical models , pollution patterns , and exposed populations . to determine whether study - specific factors can explain some of the variability in the time - series studies on mortality from particulate matter [ less than / equal to ] 10 microm in aerodynamic diameter ( pm(10 ) ) , we applied an empirical bayes meta - analysis . 
 we estimate that mortality rates increase on average by 0.7% per 10 microg / m(3 ) increase in pm(10 ) concentrations , with greater effects at sites with higher ratios of particulate matter [ less than / equal to ] 2.5 microm in aerodynamic diameter ( pm(2.5))/pm(10 ) . 
 this finding did not change with the inclusion of a number of potential confounders and effect modifiers , although there is some evidence that pm effects are influenced by climate , housing characteristics , demographics , and the presence of sulfur dioxide and ozone . 
 although further analysis would be needed to determine which factors causally influence the relationship between pm(10 ) and mortality , these findings can help guide future epidemiologic investigations and policy decisions.imagesfigure 1 
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