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. 2021 Feb 8;276:116682. doi: 10.1016/j.envpol.2021.116682

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Fig. 1 — Air pollution exposure, coronavirus disease 2019 (COVID-19) cases, and travel movements in 324 cities of China during Spring Festival 2020. (A) Distribution of cities with data on nitrogen dioxide (NO₂) concentrations and COVID-19 cases. Shading from light red to dark red represents cumulative number of confirmed COVID-19 cases in each city, from low to high, respectively, from December 31, 2019 to March 6, 2020; white area represents no data, and grey area represents no cases. Points colored from blue to red represent historic mean annual NO₂ concentrations (μg/m³), from low to high, respectively, during January 2015 and December 2019, prior to the COVID-19 epidemic. (B) Association between the cumulative number of confirmed cases, the number of human movements from Wuhan to each city, and historic mean annual NO₂ concentration. The area of the circles represents the cumulative number of cases reported by March 6, 2020. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)