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 $\le 2.5\mu m$ (${\mathrm{PM}}_{2.5}$) 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 ${\mathrm{PM}}_{2.5}$ 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 ${\mathrm{PM}}_{2.5}$ at a $1\times 1\mathrm{km}$ spatial resolution from 1988 to 2015. We ran Cox proportional hazards models for ${\mathrm{PM}}_{2.5}$ 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 ${\mathrm{PM}}_{2.5}$ and nonaccidental mortality.

Results: The mean 3-y annual average estimate of ${\mathrm{PM}}_{2.5}$ exposure ranged from 6.7 to $8.0{\mu g/m}^3$ over the three cohorts. We estimated a hazard ratio (HR) of 1.053 [95% confidence interval (CI): 1.041, 1.065] per $10{\text{-}\mu g/m}^3$ change in ${\mathrm{PM}}_{2.5}$ 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 $2{\mu g/m}^3$ in the 1996 cohort, and above $5{\mu g/m}^3$ in the 2001 cohort.

Discussion: In a very large population-based cohort with up to 25 y of follow-up, ${\mathrm{PM}}_{2.5}$ was associated with nonaccidental mortality at concentrations as low as $5{\mu g/m}^3$. https://doi.org/10.1289/EHP5204.