In this study, an indoor/outdoor monitoring program was carried out in a gymnasium at the University of Leon, Spain. Themain goalwas a characterization of aerosol size distributions in a university gymnasiumunder different conditions and sports activities (with and withoutmagnesia alba) and the study of the mass fraction deposited in each of the parts of the respiratory tract. The aerosol particlesweremeasured in 31 discrete channels (size ranges) using a laser spectrometer probe. Aerosol size distributions were studied under different conditions: i) before sports activities, ii) activitieswithout usingmagnesia alba, iii) activities usingmagnesia alba, iv) cleaning procedures, and v) outdoors. The aerosol refractive index and density indoors were estimated from the aerosol composition: 1.577–0.003i and 2.055 g cm−3, respectively. Using the estimated density, the mass concentration was calculated, and the evolution of PM1, PM2.5 and PM10 for different activities was assessed. The quality of the air in the gymnasiumwas strongly influenced by the use of magnesia alba (MgCO3) and the number of gymnasts whowere training. Due to the climbing chalk and the constant process of resuspension, average PM10 concentrations of over 440 μg m−3 were reached. The maximum daily concentrations ranged from 500 to 900 μg m−3. Particle size determines the place in the respiratory tract where the deposition occurs. For this reason, the inhalable, thoracic, tracheobronchial and respirable fractions were assessed for healthy adults and high risk people, according to international standards. The estimations show that, for healthy adults, up to 300 μg m−3 can be retained by the trachea and bronchi, and 130 μgm−3 may reach the alveolar region. The different physical activities and the attendance rates in the sports facility have a significant influence on the concentration and size distributions observed.