Long term exposure to air pollution and COVID-19 incidence: a prospective population-based study in northern Italy

Background and aim: Most of studies on air pollution and COVID-19 are limited to the first pandemic wave and by their ecological design. We investigate the association between long-term exposure to airborne pollutants and SARS-CoV-2 incidence up to March 2021 in Varese city, Lombardy Region, one of the most polluted areas of Europe. Methods: A prospective study of Varese adult citizens as of Dec31st,2019, linked by residential address to 2018 average annual exposure to outdoor concentrations of PM2.5, PM10, NO2, NO and O3 (FARM chemical-transport model), and to Regional datasets for age, sex, residential care home living, population density, comorbidities, and COVID-19 case ascertainment (positive nasopharyngeal swab specimens) until March 2021. We estimate rate ratios and additional number of cases for 1µg/m3 increase in air pollutants from single- and bi-pollutant Poisson regression models, reporting how sensible our estimates are to residual confounding through the E-value. Results: The 62848 residents generated 4408 cases. Yearly average PM2.5 exposure was 12.5µg/m3. In single-pollutant models adjusting for age, residential care home living, history of stroke, medications for diabetes, hypertension and obstructive airway diseases, PM2.5 was associated with 5.1% increase in COVID-19 rate (95%CI:2.7%-7.5%), corresponding to 294 additional cases per 100000 person-years. Further adjustment for area-based indicators of social deprivation and use of public transportation slightly attenuated the risk excess to 3.6% (95%CI:0.9%-6.4%). To explain away this estimated effect, an unmeasured confounder would need a rate ratio of 1.23 (E-value) with both exposure and outcome besides measured covariates. Bi-pollutant models confirmed the association. Results for PM10, NO2 and NO were similar. O3 was associated with a 2% decrease in disease rate, the association being reversed in bi-pollutant models. Conclusions: Our prospective study provides solid evidence to the association between long-term exposure to low-levels of PM2.5 and increased COVID-19 incidence. Keywords: air pollution, COVID-19, prospective study