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.