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

Abstract

Background: To date, few studies have investigated the causal relationship between mortality and long-term exposure to low level of fine particulate matter (PM2.5) concentrations. Methods and findings: We studied 242,320 registered deaths in Queensland between Jan. 1, 1998 to Dec. 31, 2013 with satellite-retrieved annual average PM2.5 concentrations to each postcode. A variant of difference-in-differences (DID) approach was used to investigate the association of long-term PM2.5 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 PM2.5 concentrations ranged from 1.6 to 9.0 μg/m3, which were well below the current World Health Organization (WHO) annual standard (10 μg/m3). Long-term exposure to PM2.5 was associated with increased total mortality and cause-specific mortality. For each 1 μg/m3 increase in annual PM2.5, we found 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 PM2.5 exposure. However, if this assumption is violated (e.g., socioeconomic status and outdoor physical activities), the DID design is still subject to confounding.Conclusions: Long-term exposure to PM2.5 was associated with total, non-accidental, cardiovascular, and respiratory mortality in Queensland, Australia, where PM2.5 levels were measured well below the WHO air-quality standard.