Examining the Shape of the Association between Low Levels of Fine Particulate Matter and Mortality across Three Cycles of the Canadian Census Health and Environment Cohort

BACKGROUND: Ambient fine particulate air pollution with aerodynamic diameter [Formula: see text] ([Formula: see text]) is an important contributor to the global burden of disease. Information on the shape of the concentration–response relationship at low concentrations is critical for estimating this burden, setting air quality standards, and in benefits assessments. OBJECTIVES: We examined the concentration–response relationship between [Formula: see text] and nonaccidental mortality in three Canadian Census Health and Environment Cohorts (CanCHECs) based on the 1991, 1996, and 2001 census cycles linked to mobility and mortality data. METHODS: Census respondents were linked with death records through 2016, resulting in 8.5 million adults, 150 million years of follow-up, and 1.5 million deaths. Using annual mailing address, we assigned time-varying contextual variables and 3-y moving-average ambient [Formula: see text] at a [Formula: see text] spatial resolution from 1988 to 2015. We ran Cox proportional hazards models for [Formula: see text] adjusted for eight subject-level indicators of socioeconomic status, seven contextual covariates, ozone, nitrogen dioxide, and combined oxidative potential. We used three statistical methods to examine the shape of the concentration–response relationship between [Formula: see text] and nonaccidental mortality. RESULTS: The mean 3-y annual average estimate of [Formula: see text] exposure ranged from 6.7 to [Formula: see text] over the three cohorts. We estimated a hazard ratio (HR) of 1.053 [95% confidence interval (CI): 1.041, 1.065] per [Formula: see text] change in [Formula: see text] after pooling the three cohort-specific hazard ratios, with some variation between cohorts (1.041 for the 1991 and 1996 cohorts and 1.084 for the 2001 cohort). We observed a supralinear association in all three cohorts. The lower bound of the 95% CIs exceeded unity for all concentrations in the 1991 cohort, for concentrations above [Formula: see text] in the 1996 cohort, and above [Formula: see text] in the 2001 cohort. DISCUSSION: In a very large population-based cohort with up to 25 y of follow-up, [Formula: see text] was associated with nonaccidental mortality at concentrations as low as [Formula: see text]. https://doi.org/10.1289/EHP5204