Acute effects of air pollution on all-cause mortality: a natural experiment from haze control measures in Chiang Mai Province, Thailand

Background. Serious haze episodes have been a seasonal event in Chiang Mai province for more than a decade. In 2008, local government agencies introduced comprehensive measures to control haze and limit its impacts on public health. This study assessed the acute effects of ambient air pollutants on all-cause mortality before and after the introduction of those haze control measures. Methods. We obtained daily mortality counts and data on mass concentrations of particulate matter <10 micron in aerodynamic diameter (PM10), gaseous pollutants (SO2, NO2, O-3, and CO), and meteorology in Chiang Mai Province between January 2002 and December 2016. We analyzed the data using a case-crossover approach adjusting for temperature, relative humidity, seasonality, and day-of-week. We assessed change in the excess risks of all-cause mortality associated with an increase in interquartile range (IQR) of pollutant concentration before and after control measures came into force. Results. We found decreased PM10 levels and markedly reduced excess risks of daily mortality associated with an IQR increase in PM10 concentrations in the years after haze-control measures were implemented (2009-2016). We found mixed results for gaseous pollutants: SO2 showed no significant change in excess risk of daily mortality throughout the study period, while NO2 and CO showed significant excess risks only in the period 2012-2016, and 8-h maximum O-3 showed a decrease in excess risk despite an increase in its atmospheric levels after the introduction of haze control measures in 2008. Conclusions. The findings indicate that the government haze control measures first introduced in Chiang Mai province in 2008 have successfully reduced episodic PM10 concentrations, which has led to a decrease in short-term all-cause mortality.