| Estimating the Mortality Impacts of Particulate Matter: What Can Be Learned from Between-Study Variability? Jonathan I. Levy,1 James K. Hammitt,2 and John D. Spengler3 1Departments of Environmental Health and Biostatistics, 2Center for Risk Analysis, 3Environmental Science and Engineering Program, Harvard School of Public Health, Boston, Massachusetts, USA Abstract
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  10 µm in aerodynamic diameter (PM10) , we applied an empirical Bayes meta-analysis. We estimate that mortality rates increase on average by 0.7% per 10 µg/m3 increase in PM10 concentrations, with greater effects at sites with higher ratios of particulate matter 2.5 µm in aerodynamic diameter (PM2.5) /PM10. 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 PM10 and mortality, these findings can help guide future epidemiologic investigations and policy decisions. Key words: air pollution, confounding, empirical Bayes, epidemiology, hierarchical linear models, meta-analysis, mortality, particulate matter. Environ Health Perspect 108:109-117 (2000) . [Online 27 December 1999] http://ehpnet1.niehs.nih.gov/docs/2000/108p109-117levy/ abstract.html Address correspondence to J.I. Levy, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115 USA. Telephone: (617) 432-4845. Fax: (617) 432-3349. E-mail: jilevy@hsph.harvard.edu We thank N. Laird for her analytical assistance, D. Dockery for providing access to the Six Cities data and giving valuable feedback, and J. Evans for helping to frame the paper and the analyses of confounders. Funding for this research was provided by Owens Corning and KST-Hokkaido. Received 18 May 1999 ; accepted 19 August 1999.
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