Air pollution and mortality in a large, representative U.S. cohort: multiple-pollutant analyses, and spatial and temporal decompositions.

BACKGROUND

Cohort studies have documented associations between fine particulate matter air pollution (PM) and mortality risk. However, there remains uncertainty regarding the contribution of co-pollutants and the stability of pollution-mortality associations in models that include multiple air pollutants. Furthermore, it is unclear whether the PM-mortality relationship varies spatially, when exposures are decomposed according to scale of spatial variability, or temporally, when effect estimates are allowed to change between years.

METHODS

A cohort of 635,539 individuals was compiled using public National Health Interview Survey (NHIS) data from 1987 to 2014 and linked with mortality follow-up through 2015. Modelled air pollution exposure estimates for PM, other criteria air pollutants, and spatial decompositions (< 1 km, 1-10 km, 10-100 km, > 100 km) of PM were assigned at the census-tract level. The NHIS samples were also divided into yearly cohorts for temporally-decomposed analyses. Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) in regression models that included up to six criteria pollutants; four spatial decompositions of PM; and two- and five-year lagged mean PM exposures in the temporally-decomposed cohorts. Meta-analytic fixed-effect estimates were calculated using results from temporally-decomposed analyses and compared with time-independent results using 17- and 28-year exposure windows.

RESULTS

In multiple-pollutant analyses, PM demonstrated the most robust pollutant-mortality association. Coarse fraction particulate matter (PM) and sulfur dioxide (SO) were also associated with excess mortality risk. The PM-mortality association was observed across all four spatial scales of PM, with higher but less precisely estimated HRs observed for local (< 1 km) and neighborhood (1-10 km) variations. In temporally-decomposed analyses, the PM-mortality HRs were stable across yearly cohorts. The meta-analytic HR using two-year lagged PM equaled 1.10 (95% CI 1.07, 1.13) per 10 μg/m. Comparable results were observed in time-independent analyses using a 17-year (HR 1.13, CI 1.09, 1.16) or 28-year (HR 1.09, CI 1.07, 1.12) exposure window.

CONCLUSIONS

Long-term exposures to PM, PM, and SO were associated with increased risk of all-cause and cardiopulmonary mortality. Each spatial decomposition of PM was associated with mortality risk, and PM-mortality associations were consistent over time.