Background: Understanding the shape of the relationship between long-term exposure to ambient fine particulate
matter (PM₂.₅) concentrations and health risks is critical for health impact and risk assessment. Studies evaluating
the health risks of exposure to low concentrations of PM₂.₅ are limited. Further, many existing studies lack
individual-level information on potentially important behavioural confounding factors.
Methods: A prospective cohort study was conducted among a subset of participants in a cohort that linked
respondents of the Canadian Community Health Survey to mortality (n = 299,500) with satellite-derived ambient
PM₂.₅ estimates. Participants enrolled between 2000 and 2008 were followed to date of death or December 31,
2011. Cox proportional hazards models were used to estimate hazard ratios (HRs) for mortality attributed to PM₂.₅
exposure, adjusted for individual-level and contextual covariates, including smoking behaviour and body mass
index (BMI).
Results: Approximately 26,300 non-accidental deaths, of which 32.5 % were due to circulatory disease and 9.1 %
were due to respiratory disease, occurred during the follow-up period. Ambient PM₂.₅ exposures were relatively low
(mean = 6.3 μg/m³), yet each 10 μg/m³ increase in exposure was associated with increased risks of non-accidental
(HR = 1.26; 95 % CI: 1.19-1.34), circulatory disease (HR = 1.19; 95 % CI: 1.07–1.31), and respiratory disease mortality
(HR = 1.52; 95 % CI: 1.26–1.84) in fully adjusted models. Higher hazard ratios were observed for respiratory mortality
among respondents who never smoked (HR = 1.97; 95 % CI: 1.24–3.13 vs. HR = 1.45; 95 % CI: 1.17–1.79 for ever
smokers), and among obese (BMI ≥ 30) respondents (HR = 1.76; 95 % CI: 1.15-2.69 vs. HR = 1.41; 95 % CI: 1.04–1.91
for normal weight respondents), though differences between groups were not statistically significant. A threshold
analysis for non-accidental mortality estimated a threshold concentration of 0 μg/m³ (+95 % CI = 4.5 μg/m³).
Conclusions: Increased risks of non-accidental, circulatory, and respiratory mortality were observed even at very
low concentrations of ambient PM₂.₅. HRs were generally greater than most literature values, and adjusting for
behavioural covariates served to reduce HR estimates slightly.Medicine, Faculty ofNon UBCPopulation and Public Health (SPPH), School ofReviewedFacultyResearcherOthe