Effect Modification of the Association of Long-Term PM2.5 Exposure and Cause-Specific Mortality: An Analysis of 64 Million U.S. Medicare Beneficiaries
Bingyu Wang, Ki-Do Eum, Justtin Manjourides, Fatemeh Kazemiparkouhi, Helen Suh, Virgil Pavlu
Introduction. Long-term exposure to fine particulate matter (PM2.5) has been consistently associated with mortality; however, our understanding of how these associations are modified by certain characteristics, such as urbanicity and race, is more limited.
Methods. We considered demographic and mortality data for over 64 million Medicare enrollees across all 40 thousand ZIP codes in the conterminous United States from 2000 to 2008. We linked these data to ZIP code and month-specific PM2.5 exposures estimated using GIS-based spatio-temporal models and fit the data using Cox proportional hazard model that were modified using data aggregation and limited-memory BFGS optimization. This modification allowed our Cox PH model to analyze data simultaneously for all 64 million Medicare beneficiaries within 10 minutes (Intel Xeon E5-2680, 56 cores). We used these models to estimate the association of PM2.5 on all cause, cardiovascular (CVD), respiratory disease and cancers (including specifics: ischemic heart disease, cerebrovascular disease, congestive heart failure, COPD, pneumonia, upper respiratory infection (URI), lung cancer), with strata for age, race, gender, and ZIP code of residence. We examined effect modification by age, race, gender, land use, and region and assessed the linearity of the dose-response relationship for each cause and by modifier.
Results. We found significant positive associations of PM2.5 and all causes of death, except URI, with mortality risk ratios (RR) for non-accidental, CVD, respiratory, and cancer mortality equaling 1.244 (95% CI: 1.238, 1.251), 1.683 (95% CI: 1.669, 1.696), 1.241 (95% CI: 1.223, 1.260) and 1.160 (95% CI: 1.147, 1.172) per 10 $\mu$g/m3 increase in PM2.5, respectively. We showed risk of death to be higher for beneficiaries living in urban as compared to non-urban areas, for men as compared to women, for younger ages, and individuals living in the northeastern US. PM2.5 associated RRs were similar for whites and non-whites.