Changes in exposure to ambient fine particulate matter after relocating and long term survival in Canada: quasi-experimental study.

Objective: To investigate the association between changes in long term residential exposure to ambient fine particulate matter (PM _2.5 ) and premature mortality in Canada.
Design: Population based quasi-experimental study.
Setting: Canada.
Participants: 663 100 respondents to the 1996, 2001, and 2006 Canadian censuses aged 25-89 years who had consistently lived in areas with either high or low PM _2.5 levels over five years preceding census day and moved during the ensuing five years.
Interventions: Changes in long term exposure to PM _2.5 arising from residential mobility.
Main Outcome Measures: The primary outcome was deaths from natural causes. Secondary outcomes were deaths from any cardiometabolic cause, any respiratory cause, and any cancer cause. All outcomes were obtained from the national vital statistics database.
Results: Using a propensity score matching technique with numerous personal, socioeconomic, health, and environment related covariates, each participant who moved to a different PM _2.5 area was matched with up to three participants who moved within the same PM _2.5 area. In the matched groups that moved from high to intermediate or low PM _2.5 areas, residential mobility was associated with a decline in annual PM _2.5 exposure from 10.6 μg/m ³ to 7.4 and 5.0 μg/m ³ , respectively. Conversely, in the matched groups that moved from low to intermediate or high PM _2.5 areas, annual PM _2.5 increased from 4.6 μg/m ³ to 6.7 and 9.2 μg/m ³ . Five years after moving, individuals who experienced a reduction in exposure to PM _2.5 from high to intermediate levels showed a 6.8% (95% confidence interval 1.7% to 11.7%) reduction in mortality (2510 deaths in 56 025 v 4925 deaths in 101 960). A greater decline in mortality occurred among those exposed to a larger reduction in PM _2.5 . Increased mortality was found with exposure to PM _2.5 from low to high levels, and to a lesser degree from low to intermediate levels. Furthermore, the decreases in PM _2.5 exposure were most strongly associated with reductions in cardiometabolic deaths, whereas the increases in PM _2.5 exposure were mostly related to respiratory deaths. No strong evidence was found for the changes in PM _2.5 exposure with cancer related deaths.
Conclusions: In Canada, decreases in PM _2.5 were associated with lower mortality, whereas increases in PM _2.5 were associated with higher mortality. These results were observed at PM _2.5 levels considerably lower than many other countries, providing support for continuously improving air quality.
Competing Interests: Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/disclosure-of-interest/ and declare: support from funded by Health Canada for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.
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