Background. Rheumatoid arthritis (RA) flare is related to an increased joint damage, disability and healthcare use. The impact of short-term air pollution exposure on RA activity is still a matter of debate. We investigated in a cross-sectional study whether short-term exposure to particulate matter (PM)10, PM2.5, nitrogen dioxide (NO2) and ozone (O3) affected RA disease activity. Methods. 422 consecutive RA patients resident in Lombardy, North of Italy were studied. Air pollutant concentrations, estimated by Regional Environmental Protection Agency (Lombardy – Italy) at municipality resolution, were used to assign short-term exposure from the day of visit back to 14 days. Results. Sparse punctual significant negative associations emerged between PM10, PM2.5, NO2 and RA disease activity, whereas positive associations were observed for O3. Moreover, patients were stratified according to their ongoing Disease Modifying anti-Rheumatic Drugs (DMARDs) treatment: no DMARDs (n=25), conventional synthetic - csDMARDs (n=108), and biological or targeted synthetic b/tsDMARDs (n=289). At visit, an inverse association was observed in the b/tsDMARDs group between PM2.5 and Disease Activity Score on 28 joints (DAS28) (-0.047±0.020, p=0.023), a positive tendency in the no-DMARD group (0.125±0.070, p=0.075) and no association for csDMARDs group (0.042±0.039, p=0.288). The association between O3 the day before the visit and DAS28 was positive only in the b/tsDMARDs group (0.038±0.009, p<0.001). Conclusion. The impact of air pollution short-term exposure seems minimally clinical relevant, as the scattered significant differences were observed. Further evidence is needed to elucidate determinants of RA flare and the implications for management.