A study by environmental health scientists suggests that inexpensive cloth masks worn by people in the hope of reducing their exposure to air pollution vary widely in effectiveness. Their use could be giving these users a false sense of security, especially in highly polluted areas. In a series of experiments with an experimental mannequin, the scientists tested four masks: one, a pleated surgical type, two cloth ones and the last, a cone-shaped cloth with exhalation flaps. Among the cloth masks, the one with exhaust valves performed fairly well, removing 80-90 per cent of synthetic particles and about 57 per cent of diesel exhaust. Plain cloth masks were “only marginally beneficial” in protecting people from particles smaller than 2.5 micrometers, often considered more harmful than larger particles because they can penetrate the lungs more deeply