OBJECTIVES: Physical exertion is a high-risk activity for aerosol emission of respiratory pathogens. We aimed to determine the safety and tolerability of healthy young adults wearing different types of face mask during moderate-to-high intensity exercise. DESIGN: Cross-over randomised controlled study, completed between June 2021 and January 2022. PARTICIPANTS: Volunteers aged 18-35 years, who exercised regularly and had no significant pre-existing health conditions. INTERVENTIONS: Comparison of wearing a surgical, cloth and filtering face piece (FFP3) mask to no mask during 4×15 min bouts of exercise. Exercise was running outdoors or indoor rowing at moderate-to-high intensity, with consistency of distance travelled between bouts confirmed using a smartphone application (Strava). Each participant completed each bout in random order. OUTCOMES: The primary outcome was change in oxygen saturations. Secondary outcomes were change in heart rate, perceived impact of face mask wearing during exercise and willingness to wear a face mask for future exercise. RESULTS: All 72 volunteers (mean age 23.9) completed the study. Changes in oxygen saturations did not exceed the prespecified non-inferiority margin (2% difference) with any mask type compared with no mask. At the end of exercise, the estimated average difference in oxygen saturations for cloth mask was -0.07% (95% CI -0.39% to 0.25%), for surgical 0.28% (-0.04% to 0.60%) and for FFP3 -0.21% (-0.53% to 0.11%). The corresponding estimated average difference in heart rate for cloth mask was -1.20 bpm (95% CI -4.56 to 2.15), for surgical 0.36 bpm (95% CI -3.01 to 3.73) and for FFP3 0.52 bpm (95% CI -2.85 to 3.89). Wearing a face mask caused additional symptoms such as breathlessness (n=13, 18%) and dizziness (n=7, 10%). 33 participants broadly supported face mask wearing during exercise, particularly indoors, but 22 were opposed. CONCLUSION: This study adds to previous findings (mostly from non-randomised studies) that exercising at moderate-to-high intensity wearing a face mask appears to be safe in healthy, young adults. TRIAL REGISTRATION NUMBER: NCT04932226.