2.1. Air pollution exposure

We collected the hourly or daily monitoring data of ambient air pollutants (i.e., NO₂, O₃, PM_2.5, PM₁₀, and SO₂) in Lombardy and Veneto in Italy from January 2013 to May 2020 from the air quality database of the European Environment Agency (EEA) (https://discomap.eea.europa.eu/map/fme/AirQualityExport.htm), as reported by member states. In this study, we calculated the short- and long-term exposure to air pollutants (i.e., STAP and LTAP) in Lombardy and Veneto to estimate the acute and chronic contributions of air pollution to the COVID-19 pandemic. The daily average concentration of air pollutants in different regions in an individual day was calculated as STAP in the corresponding location and day. For LTAP, it was defined as the mean of air pollution measurements in the individual region in the coming 8 years. The monitoring data were validated, and the measurements below detection limits were replaced using 0.5 times the detection limit.