The association between long-term exposure to low-level PM2.5 and mortality in the state of Queensland, Australia: A modelling study with the difference-in-differences approach

Background To date, few studies have investigated the causal relationship between mortality and long-term exposure to a low level of fine particulate matter (PM 2.5 ) concentrations. Methods and findings We studied 242,320 registered deaths in Queensland between January 1, 1998, and December 31, 2013, with satellite-retrieved annual average PM 2.5 concentrations to each postcode. A variant of difference-in-differences (DID) approach was used to investigate the association of long-term PM 2.5 exposure with total mortality and cause-specific (cardiovascular, respiratory, and non-accidental) mortality. We observed 217,510 non-accidental deaths, 133,661 cardiovascular deaths, and 30,748 respiratory deaths in Queensland during the study period. The annual average PM 2.5 concentrations ranged from 1.6 to 9.0 μg/m 3 , which were well below the current World Health Organization (WHO) annual standard (10 μg/m 3 ). Long-term exposure to PM 2.5 was associated with increased total mortality and cause-specific mortality. For each 1 μg/m 3 increase in annual PM 2.5 , we found a 2.02% (95% CI 1.41%–2.63%; p < 0.01) increase in total mortality. Higher effect estimates were observed in Brisbane than those in Queensland for all types of mortality. A major limitation of our study is that the DID design is under the assumption that no predictors other than seasonal temperature exhibit different spatial-temporal variations in relation to PM 2.5 exposure. However, if this assumption is violated (e.g., socioeconomic status [SES] and outdoor physical activities), the DID design is still subject to confounding. Conclusions Long-term exposure to PM 2.5 was associated with total, non-accidental, cardiovascular, and respiratory mortality in Queensland, Australia, where PM 2.5 levels were measured well below the WHO air quality standard.