When to issue the alert of air pollution in Chinese cities? Evidence on the threshold effect of heavy air pollution from Chengdu

Since 2012, the issuing of the alert for heavy air pollution has become a standard practice in major Chinese cities. An air quality index (AQI) of 200 is adopted in many cases as the triggering line of the alert system. Based on citylevel time series data on hospital admissions for respiratory system disease in Chengdu, between January 1, 2016 and May 1, 2017, we assessed the relevance of AQI 200 as the trigger of the alert system. Specifically, applying the Autoregressive Distributed Lag (ARDL) model, particularly its nonlinear form that accommodates the threshold effect, we searched through alternative levels of air pollution to determine whether and at what level a threshold effect of air pollution may exist. We found that while the linear form of the ARDL model showed a significant effect of air pollution similar to existing findings, the threshold form of ARDL showed that air pollution affects hospital admissions mainly when the AQI is above a certain threshold. This threshold lies in an AQI range no higher than 130, and associated with it is a sharply elevated effect on hospital admissions. Based on this finding, if an alert is triggered only when AQI is above 200, 53.2% of the hospital admissions due to air pollution will not be alerted, implying that a much lower level of AQI is needed to define a more relevant trigger of the air pollution alert system in China.