The Effects of Temperature and Humidity on Electrostatic Changes in Respirators and Their Filtration Efficiency

The filtering mechanism of respirators involves static electricity and physical mechanisms, such as inertial impaction, interception, and diffusion. Static electricity is used to reduce pressure resistance, but electrostatic effects and the environmental variables that affect them are not well studied. Therefore, this study evaluated the contribution of static electricity to the filtration efficiency of respirators and the effects of changes in temperature and humidity on static electricity and the resulting filtration efficiency. Eight respirators from four manufacturers—four for workers (1^st class respirators) and four for citizens (KF-94 respirators)—were selected, all of which met legal standards in Korea. The filtration efficiency and pressure drop were evaluated at baseline and after removing static electricity with isopropanol. To study the effects of humidity and temperature on the efficiency of the respirators, the respirators were exposed to 30, 50, and 98% humidity while the temperature was fixed at 25°C and to temperatures of −30, 25, and 70°C while the humidity was fixed at 50%. Static electricity was measured using a surface-potential meter, and the filtration efficiency was measured using NaCl and paraffin oil. When static electricity was removed from the 1^st class and KF-94 respirators, the filtration efficiency decreased by 21.7% and 19.5%, respectively. Humidity affected the change in static electricity slightly more than temperature, but since not all static electricity was removed, the filtration efficiency was not affected greatly. The changes in static electricity and filtration efficiency due to humidity were significant, but this was because the standard deviation was small. Even under 98% humidity, the filtration efficiency was only 1–2% lower than that at 30% humidity and still met the 94% filtration efficiency criterion for respirators.