Association between short-term exposure to air pollution and COVID-19: a case-crossover cohort study.

OBJECTIVE: To investigate associations between short-term fluctuations in urban background air pollution and the incidence of COVID-19 across three Swedish cities using detailed individual-level data. METHODS: A case-crossover analysis was conducted using individual-level data from a prospective cohort of adults across three Swedish cities (Stockholm, Gothenburg and Uppsala), including 3999 confirmed COVID-19 cases. Subjects were classified as cases on the date of their first positive PCR test, which occurred between 7 March 2020 and 26 January 2022. Exposure data for daily levels of PM2.5, PM10 and NO2 were obtained from urban background monitoring stations. Lag-specific and cumulative associations were examined using distributed lag models and distributed lag non-linear models, adjusting for meteorological variables. RESULTS: A significant association was observed between elevated PM2.5 levels and testing positive for COVID-19 on the same day (lag 0), with an OR of 1.084 (95% CI 1.011 to 1.162) per IQR (3.3 µg/m³) increase. Similar lag 0 effects were found across the three cities. For PM10, the most consistent associations were observed at lag 3. For NO2, significant effects at lag 0 emerged only after excluding the final COVID-19 wave. Subgroup analyses showed similar effects among individuals with pre-existing respiratory conditions, though results were less consistent. CONCLUSIONS: Our findings suggest that short-term fluctuations in air pollution, particularly for PM2.5, are associated with the timing of confirmed COVID-19 cases. A possible explanation for the short lag association is that air pollution exacerbates symptoms, prompting individuals to seek testing, rather than directly facilitating new infections.