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 (PM(10)), gaseous pollutants (SO(2), NO(2), 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 PM(10) levels and markedly reduced excess risks of daily mortality associated with an IQR increase in PM(10) concentrations in the years after haze-control measures were implemented (2009–2016). We found mixed results for gaseous pollutants: SO(2) showed no significant change in excess risk of daily mortality throughout the study period, while NO(2) 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 PM(10) concentrations, which has led to a decrease in short-term all-cause mortality.