ABSTRACT The present study numerically and experimentally investigates, for the first time in the literature, the thermo-fluid dynamics and ultrafine particle (UFP) concentration in surgical smoke, with diameters smaller than 100 nm, inside an actual operating room (OR) equipped with a turbulent circular diffuser system. The velocity values and temperature fields measured during the experimental analysis at different points in the OR are in reasonable agreement with the numerical results. For UFP diameters smaller than 70 nm, the particle concentration obtained from the numerical analysis agrees with the experimental data. The numerical results have demonstrated that at the sterile zone, UFP concentration caused by candles smoke is around particles/cm3, which are exposed by the occupants in OR. UFP decay time of 25 and 20 minutes are obtained in current analysis, respectively, in order to evaluate the time needed by the ventilation system to evacuate the particles from the OR.