Impact of a 10-year shift in ambient air quality on mortality in Canada: a causal analysis of multiple pollutants

Background: The impact of past air quality improvements on health and equity at low pollution levels near the revised WHO air quality guidelines remains largely unknown. Less is known about the influence of simultaneous reductions in multiple major pollutants. Leveraging real-world improvements in air quality across Canada, we sought to directly evaluate their health benefits by quantifying the impact of a joint shift in three criteria pollutants on mortality in a national cohort. Methods: In this population-based cohort study, we assembled a cohort of 2·7 million adults living in Canada in 2007 who were followed up through 2016. Annual mean concentrations of fine particulate matter (PM_2·5), nitrogen dioxide (NO₂), and ozone (O₃) were assigned to participants’ residential locations. For each pollutant individually and combined, we conducted a causal analysis of the impact of the decadal shift in annual exposure from the pre-baseline level (2004–06) on the risk of non-accidental mortality using the parametric g-formula, a structural causal model. To check the robustness of our results, we conducted multiple sensitivity analyses, including exploring alternative exposure scenarios. We also evaluated differential benefits across regions and socio-demographic subgroups. Findings: Between 2007 and 2016, annual mean exposures to PM_2·5 and NO₂ decreased (from 7·1 μg/m³ [SD 2·3] to 5·5 μg/m³ [1·9] for PM_2·5 and from 11·1 ppb [SD 6·6] to 8·0 ppb [4·9] for NO₂), whereas O₃ declined initially and then rebounded (from 38·6 [SD 8·3] ppb to 36·0 [6·0] ppb and then 38·1 [5·4] ppb). Compared to pre-baseline (2004–06) levels, the joint change in the pollution exposures beginning in 2007 resulted in, per million population, 70 (95% CI 29–111) fewer deaths by 2009, 416 (283–549) fewer deaths by 2012, and 609 (276–941) fewer deaths by 2016, corresponding to a –0·7% change in mortality risk over the decade. Stratified analyses showed greater beneficial impacts in men, adults aged 50 years and older, low income-earners, and residents in regions undergoing substantial air quality improvements. Had all regions experienced pollution reductions similar to the most improved region, approximately three times as many deaths would have been averted (2191 fewer deaths per million). Conversely, if the observed air quality improvements had been delayed in all regions by 3 years, there would have been 429 more deaths per million by 2016. Interpretation: In Canada, substantial health gains were associated with air quality improvements at levels near the revised WHO guidelines between 2007 and 2016, with notable heterogeneity observed across socio-demographic subgroups and regions. These findings indicate that modest declines in air pollution can considerably improve health and equity, even in low-exposure environments. Funding: Health Canada.