The association between long-term exposure to low-level PM(2.5) 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.