BackgroundRecent evidence identified exposure to particulate matter of size ≤2.5 µm (PM_2.5) as a risk factor for high prevalence of small airway dysfunction (SAD). We assessed the prevalence of SAD in a European region with low air pollution levels.MethodsSAD was defined as a maximum mid-expiratory flow (MMEF) <65% of predicted value (PV) or MMEF <lower limit of normal (LLN) measured by spirometry in the Swiss PneumoLaus cohort. We performed bivariate and multivariable analysis with MMEF criteria, age, sex, body mass index, respiratory symptoms and smoking status. Mean PM_2.5values were obtained from a Swiss national database.ResultsAmong 3351 participants (97.6% Caucasian, 55.7% female sex, mean age 62.7 years), we observed MMEF <65% PV in 425 (12.7%) and MMEF <LLN in 167 (5.0%) individuals. None of the participants had both MMEF <LLN and ≥65% PV. MMEF <65% PV and MMEF <LLN were significantly associated with age, smoking status, cough, sputum and dyspnoea, whereas a positive association with MMEF <65% PV was observed for individuals aged >65 years only. In an area where ambient PM_2.5concentration was <15 µg·m⁻³during the observation period (2010 and 2020), ≥72% of participants with SAD were ever-smokers.ConclusionsThe observed low prevalence of SAD of 5.0–12.7% depending on criteria employed may be related to lower PM_2.5exposure. Smoking was the main factor associated with SAD in an area with low PM_2.5exposure. Employing a MMEF threshold <65% PV carries a risk of SAD overdiagnosis in elderly individuals.