The Circuit Breaker (CB) policy in Singapore which took effect on April 7, 2020 required all business, social, and other activities to be conducted through telecommuting. Besides effectively suppressing the transmission of COVID-19, this mobility restriction has also unintentionally contributed to the mitigation of air pollution. Based on data from Google mobility reports and the World Air Quality Index project, this week’s Chart of the Week depicts the correlation between workplace mobility and air pollution in Singapore before and during the circuit breaker period.
- Workplace mobility in Singapore was at about 93% of pre-pandemic times. It immediately dropped to approximately 39% once the circuit breaker policy was put into effect. The peaks and troughs in mobility coincide with the weekends and public holidays, respectively.
- During the circuit breaker period, PM2.5 AQI in Singapore was visibly lower than before the policy.
- The average PM2.5 AQI after the commencement of the circuit breaker reduced to a level 12% lower than before the measure was put into place. Since workplace mobility is directly linked to road traffic, it can be hypothesised that there is a causal effect of mobility on air quality.
- Empirical research with the circuit breaker serving as an instrumental variable (IV) confirms the hypothesis. The positive coefficient of workplace mobility indicates that a 10 percentage point decrease in mobility will result in a 0.99-1.48 decrease in PM2.5 AQI. Given the low pollution level in Singapore, this is about a 1% to 2% reduction in air pollution. 
 Refer to the US EPA standards for conversion between AQI and concentration: https://www.airnow.gov/publications/air-quality-index/technical-assistance-document-for-reporting-the-daily-aqi/
By ZHU Yan