Road traffic and air pollution: Evidence from a nationwide traffic control during coronavirus disease 2019 outbreak.

Existing estimations of air pollution from automobile sources are based on either experiments or small-scale governmental interventions. China's nationwide traffic control during the coronavirus disease 2019 outbreak provided us a unique opportunity to assess the direct dose-effect relationship between vehicle density and air pollution. We found that, during the coronavirus disease 2019 outbreak, the nationwide reduced air pollution (except for O) could be largely explained by traffic control measures. During the traffic control period, every doubling of vehicle density was associated with a decrease of 4.2 (2.0, 6.4) μg/m in PM, 5.5 (2.9, 8.1) μg/m in PM, 1.5 (0.9, 2.0) μg/m in NO, and 0.04 (0.02, 0.07) mg/m in CO comparing cities with different vehicle densities. Similarly, for every 10% increase in the truck proportion, PM decreased by 12.3 (4.1, 20.6) μg/m, PM decreased by 14.3 (4.6, 23.9) μg/m, and CO decreased by 0.14 (0.05, 0.23) mg/m. Moreover, the associations between vehicle density and reduction in PM, PM, and CO during the traffic control period were stronger and showed near-complete linearity in cities with low green coverage rate (All P < 0.05 for interaction). According to our estimation, PM emissions from every doubling of vehicle density can lead to over 8000 excess deaths per year, 66% of which were caused by cardiopulmonary diseases. This natural experiment study is the first to observe the dose-effect relationship between on-road traffic and traffic-generated air pollution, as well as the mitigating effect of urban greening. Findings provide key evidence to the assessment and control of traffic-generated air pollution and its public health impact.