Introduction Studies on air pollution and COVID-19 are limited to the first pandemic wave (April/June 2020) and by their ecological design.

Objectives To investigate the association between airborne pollutants and SARS-CoV-2 incidence up to March 2021 in the Varese city (Lombardy region), with individual-level data on exposures, disease and confounders.

Methods Varese citizens aged 18+ years as of Dec31st,2019 were linked by residential address to 2018 average annual exposure to outdoor concentrations of PM2.5, PM10, NO2, NO and O3 modelled using FARM chemical-transport model (linkage coverage: 97.4%). Citizens were linked to Regional datasets for COVID-19 case ascertainment (positive nasopharyngeal swab specimens) and to define age, sex, residential care home living, population density and comorbidities. We estimated rate ratios and additional number of COVID-19 cases for 1 µg/m3 increase in air pollutants, from single- and bi-pollutant Poisson regression models.

Results Among the 62.848 residents, we observed 4408 COVID-19 cases. Yearly average PM2.5 exposure was 12.5 µg/m3. Cumulative incidence curves suggest an increased risk for PM2.5>13.5 µg/m3 in correspondence of downtrend periods in the pandemic curve. Age, residential care home living, history of stroke, medications for diabetes, hypertension and obstructive airway disease were independently associated with COVID-19 rate. In single-pollutant multivariate model, 1 µg/m3 increase in PM2.5 was associated with 5.1% increase in COVID-19 rate (95%CI: 2.7%-7.5%), corresponding to 294 additional cases per 100.000 person-years. These figures were confirmed in bi-pollutant models and after excluding subjects in residential care homes. Similar findings were observed for PM10, NO2 and NO. O3 was associated with a 2% decrease in disease rate, the association being reversed in bi-pollutant models.

Conclusions In our study, long term exposure to low-levels of air pollutants, especially PM2.5, positively affected COVID-19 incidence. Causality warrants confirmation in future studies; meanwhile, governmental efforts to further reduce air pollution should continue.