Low concentrations of fine particle air pollution and mortality in the Canadian Community Health Survey cohort

BACKGROUND: Approximately 2.9 million deaths are attributed to ambient fine particle air pollution around the world each year (PM(2.5)). In general, cohort studies of mortality and outdoor PM(2.5) concentrations have limited information on individuals exposed to low levels of PM(2.5) as well as covariates such as smoking behaviours, alcohol consumption, and diet which may confound relationships with mortality. This study provides an updated and extended analysis of the Canadian Community Health Survey-Mortality cohort: a population-based cohort with detailed PM(2.5) exposure data and information on a number of important individual-level behavioural risk factors. We also used this rich dataset to provide insight into the shape of the concentration-response curve for mortality at low levels of PM(2.5). METHODS: Respondents to the Canadian Community Health Survey from 2000 to 2012 were linked by postal code history from 1981 to 2016 to high resolution PM(2.5) exposure estimates, and mortality incidence to 2016. Cox proportional hazard models were used to estimate the relationship between non-accidental mortality and ambient PM(2.5) concentrations (measured as a three-year average with a one-year lag) adjusted for socio-economic, behavioural, and time-varying contextual covariates. RESULTS: In total, 50,700 deaths from non-accidental causes occurred in the cohort over the follow-up period. Annual average ambient PM(2.5) concentrations were low (i.e. 5.9 μg/m(3), s.d. 2.0) and each 10 μg/m(3) increase in exposure was associated with an increase in non-accidental mortality (HR = 1.11; 95% CI 1.04–1.18). Adjustment for behavioural covariates did not materially change this relationship. We estimated a supra-linear concentration-response curve extending to concentrations below 2 μg/m(3) using a shape constrained health impact function. Mortality risks associated with exposure to PM(2.5) were increased for males, those under age 65, and non-immigrants. Hazard ratios for PM(2.5) and mortality were attenuated when gaseous pollutants were included in models. CONCLUSIONS: Outdoor PM(2.5) concentrations were associated with non-accidental mortality and adjusting for individual-level behavioural covariates did not materially change this relationship. The concentration-response curve was supra-linear with increased mortality risks extending to low outdoor PM(2.5) concentrations.