AbstractObjectiveTo investigate the association between changes in long term residential exposure to ambient fine particulate matter (PM_2.5) and premature mortality in Canada.DesignPopulation based quasi-experimental study.SettingCanada.Participants663 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_2.5levels over five years preceding census day and moved during the ensuing five years.InterventionsChanges in long term exposure to PM_2.5arising from residential mobility.Main outcome measuresThe 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.ResultsUsing a propensity score matching technique with numerous personal, socioeconomic, health, and environment related covariates, each participant who moved to a different PM_2.5area was matched with up to three participants who moved within the same PM_2.5area. In the matched groups that moved from high to intermediate or low PM_2.5areas, residential mobility was associated with a decline in annual PM_2.5exposure from 10.6 μg/m³to 7.4 and 5.0 μg/m³, respectively. Conversely, in the matched groups that moved from low to intermediate or high PM_2.5areas, annual PM_2.5increased from 4.6 μg/m³to 6.7 and 9.2 μg/m³. Five years after moving, individuals who experienced a reduction in exposure to PM_2.5from high to intermediate levels showed a 6.8% (95% confidence interval 1.7% to 11.7%) reduction in mortality (2510 deaths in 56 025v4925 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_2.5from low to high levels, and to a lesser degree from low to intermediate levels. Furthermore, the decreases in PM_2.5exposure were most strongly associated with reductions in cardiometabolic deaths, whereas the increases in PM_2.5exposure were mostly related to respiratory deaths. No strong evidence was found for the changes in PM_2.5exposure with cancer related deaths.ConclusionsIn Canada, decreases in PM_2.5were associated with lower mortality, whereas increases in PM_2.5were associated with higher mortality. These results were observed at PM_2.5levels considerably lower than many other countries, providing support for continuously improving air quality.