Changes in residential exposure to ambient fine particulate matter due to relocation and long-term survival in Canada: a quasi-experimental study

Abstract

BACKGROUND AND AIM: Despite significant advances in understanding the health burden of hypothetical changes in ambient fine particulate matter (PM2.5), less is known how actual changes in PM2.5 levels may influence its long-term adverse effects, especially on mortality. We aimed to conduct a quasi-experiment to evaluate the association between mortality and changes in PM2.5 in Canada. METHODS: We identified movers from a national cohort of Canadian census respondents (10 million) who were aged 25-89 years, had a history of either high or low exposure to PM2.5 before census day, and moved within the following five years, yielding two cohorts. Exposures were ascertained using satellite-derived PM2.5 measurements based on movers’ postal-code addresses since five years before the census day. To assess the relationship between changes in PM2.5 and mortality, we conducted a propensity score matching analysis with Cox proportional hazards model including various covariates. We did various sensitivity analyses, such as considering multiple imputation. RESULTS:Residential mobility yielded a decline in annual PM2.5 exposure from ~10 μg/m3 to 7.4 μg/m3 and to 5.0 μg/m3 in the high to intermediate (or low) movers. Conversely, annual PM2.5 increased from ~4.6 μg/m3 to 6.7 μg/m3 and to 9.2 μg/m3 in the low to intermediate (or high) movers. Over five years after the move, we observed a 6.8% reduction in mortality among individuals whose PM2.5 exposures decreased from high to intermediate levels (95% confidence interval: 1.7%-11.7%). A greater decline in mortality was observed among individuals with a larger reduction in exposures. These results were consistent in sensitivity analyses. Additionally, we found an increase in mortality with elevated PM2.5 exposure. CONCLUSIONS:Decreases in PM2.5 were associated with lowered mortality whereas increases in PM2.5 were associated with elevated mortality in Canada. These results were found at the PM2.5 levels considerably lower than many other countries, providing support for continuously improving air quality. KEYWORDS: quasi-experiment, fine particulate matter, mortality