Theoretically KN95 masks can be used for 2-3 days, but for the current prevention and control of the new coronavirus, experts recommend using them for no more than 4 hours. For professional health care workers once the protection is taken off, it needs to be replaced if they re-enter a high-risk environment; it needs to be replaced immediately when the mask is contaminated with patient blood, respiratory secretions or other body fluids; and it also needs to be replaced after a highly suspected patient is seen. However, for the general healthy population without symptoms, considering the actual situation of mask shortage, medical surgical masks or general disposable masks can be chosen if there is no particularly high risk of exposure. For the healthy public who have used KN95 masks, it is possible to extend or repeat the limited use appropriately, provided that one must ensure that the donning and doffing of the mask is free from contamination and that the mask is always placed in a sealed bag and then placed in a clean, dry and contamination-free place.