Effect of ambient PM 2.5 on healthcare utilisation for acute respiratory illness, Melbourne, Victoria, 2014-2019.

Ambient particulate matter (PM 2.5 ) is an important component of natural and human-generated air pollution and a major contributor to the global burden of disease. Short-term effects of PM 2.5 exposure on respiratory illness have been described but most evidence arises from high pollution settings. We used case-crossover methods to estimate effects of outdoor PM 2.5 levels on emergency department (ED) presentations and hospital admissions for a range of acute respiratory illnesses and age groups in Melbourne, Australia from 2014-2019, with and without adjustment for other pollutants and weather conditions, using daily and one-week averaged lags. We estimated incidence rate ratios for a 10 μg/m 3 increase in 7-day average ambient PM 2.5 of 1.043 (95% confidence interval (CI): 1.000 - 1.089) on ED presentation and 1.013 (95% CI: 0.971 - 1.056) on hospital admissions for acute respiratory illnesses for patients of any age. We observed distinct temporal patterns in daily lag effect by disease. The largest effects on acute lower respiratory tract infection and asthma were observed in children. Ambient PM 2.5 levels rarely exceeded standards in place at the time. Although uncertainty around most point estimates was relatively wide, these findings are most compatible with adverse health effects of ambient PM 2.5 at levels below currently established Australian national standards.