a b s t r a c tIndoor air quality in mosques during prayers may be of concern for sensitive/susceptible sub-groups of the population. However, no indoor air pollutant levels of potentially toxic agents in mosques have been reported in the literature. This study measured PM concentrations in a mosque on Friday when the mid-day prayer always receives high attendance. Particle number and CO 2 concentrations were measured on nine sampling days in three different campaigns before, during, and after prayer under three different cleaning schedules: vacuuming a week before, a day before, and on the morning of the prayer. In addition, daily PM 2.5 concentrations were measured. Number concentrations in 0.5e1.0, 1.0e5.0, and > 5.0 mm diameter size ranges were monitored. In all campaigns the maximum number concentrations were observed on the most crowded days. The lowest number concentrations occurred when vacuuming was performed a day before the prayer day in two of the three size ranges considered. PM 2.5 concentrations (four-hour samples that integrated before, during, and after the prayer) were comparable to the other indoor environments reported in the literature. CO 2 concentrations suggested that ventilation was not sufficient in the mosque during the prayers. The results showed that better ventilation, a preventive cleaning strategy, and a more detailed study are needed.