How often a mask should be changed to prevent viral transmission is determined by the type of mask. Ordinary medical masks should be changed once every 4~8 hours, and N95 masks should be changed once every 2~8 hours depending on the situation. 1. Ordinary medical masks: Usually, ordinary medical masks are replaced at least once every 4 hours. As the material of ordinary medical mask is light and thin, it is easy to be wet by saliva, and the anti-virus ability of the mask will be reduced after wetting, so the ordinary medical mask should be replaced every 4 hours. During the new crown epidemic, wearing ordinary medical masks for a continuous period of time is recommended to be no more than 8 hours. 2.N95 masks: N95 masks are generally worn by health care workers or epidemic prevention staff, the use of time depends on the specific circumstances, generally 4 ~ 8 hours for a change. If you are taking care of patients with viral infections, you should change it once in 2~4 hours. If the mask is not changed for a long period of time, it may lead to a decrease in the protective ability of the mask and increase the chance of infection among healthcare workers. It is recommended to follow your doctor’s instructions and wear the mask wisely to prevent infection with the new coronavirus.