A Systematic Literature Review of the Impact of COVID-19 Lockdowns on Air Quality in China

This literature review systematically examines the effect of COVID-19 lockdowns on pollutant concentrations in China by synthesising the reported evidence. Following PRISMA guidelines, we used predefined eligibility criteria to search the databases of PubMed, Scopus, Web of Science and EBSCO Host for peer-reviewed published literature that investigated the nexus between COVID-19 and air quality in China. After screening the titles, abstracts and full texts of the retrieved results, two reviewers independently evaluated the relevant data. 35 of 508 studies met our criteria. The majority of the eligible studies reported data from central China (e.g., Wuhan and Hubei Province), and the most frequently measured air pollutant was nitrogen dioxide (NO2; 51 values in 28 studies), followed by fine particulate matter (PM2.5; 49 values in 26 studies). We found evidence of a substantial reduction in air pollution immediately after lockdown measures were implemented, with traffic-related NO2 exhibiting the largest decrease. The reported reductions in air pollution varied by region and period. Specifically, urban, industrial and highly populated areas of China experienced greater improvements in air quality than rural, residential and less populated areas. Additionally, owing to meteorological factors, the effects differed between inland and coastal regions. However, despite the changes, the pollutant concentrations in many regions (e.g., Beijing, where PM2.5 and PM10 levels remained above 100 mu g m(-3)) still exceeded the World Health Organization (WHO)'s 24-hour mean guidelines (e.g., 25 mu g m(-3) and 50 mu g m(-3) for PM2.5 and PM10, respectively). Without the support of adaptive environmental strategies, the recent gains in air quality will be unsustainable.