Low-concentration air pollution and mortality in American older adults: A national cohort analysis (2001-2017)

Abstract

BACKGROUND AND AIM: Mounting epidemiological evidence has documented the associations between air pollution and mortality. There is a pressing need to determine whether risks persist at low concentrations including below current national standards. Better understanding of the health effects of low-level air pollution is essential for the amendment of National Ambient Air Quality Standards. METHODS: A nationwide, population-based, open cohort study was conducted to estimate the association between long-term exposure to low-level PM2.5, NO2, O3 and mortality. The study population included all Medicare enrollees in the contiguous United States from 2001-2017. We further defined three low-exposure sub-cohorts comprised of Medicare enrollees who were always exposed to low-level PM2.5 (annual mean ≤12 µg/m3), NO2 (annual mean ≤53 ppb), and O3 (warm-season mean ≤50 ppb), respectively, over the study period. We applied multiple statistical methods and compared causal modeling methods to standard methods, to provide strong epidemiologic evidence. RESULTS:Of the 68.7-million Medicare enrollees, 33.1%, 93.8%, 65.0% were always exposed to low-level annual PM2.5, annual NO2, and warm-season O3 over the study period, respectively. Among the low-exposure cohorts, a 10-µg/m3 increase in PM2.5, 10-ppb increase in NO2, and 10-ppb increase in warm-season O3, was respectively associated with an increase in mortality rate ranging between 10-13%, 2-4%, and 12-14% in single-pollutant models, and between 6-8%, 1-3%, and 9-11% in tri-pollutant models, using three statistical approaches. There was strong evidence of linearity in concentration-response relationships for PM2.5 and NO2 at levels below the current NAAQS, suggesting that no safe threshold exists for health-harmful pollution levels. For O3, the concentration-response relationship shows an increasingly positive association at levels above 30-ppb. CONCLUSIONS:Long-term exposures to PM2.5, NO2, and O3 were significantly associated with an increased risk of all-cause mortality, particularly at levels below the current NAAQS standards, suggesting that implementing more stringent regulations in air quality may yield substantial health benefits. KEYWORDS: low concentration, air pollution, NAAQS, survival analysis, causal modeling