The following conditions need to be replaced in a timely manner: 1, high-risk people need to be replaced again after taking off their masks at the end of work, midway through a meal, drinking, etc.; 2, masks are contaminated by patients’ blood, respiratory secretions, nasal secretions and other body fluids, and need to be replaced immediately; 3, higher-risk personnel need to replace their masks in a timely manner after receiving highly suspected patients; 4, masks worn by exposed personnel in other risk categories can be used repeatedly for several times, but need to be replaced in a timely manner if they are dirty, deformed, damaged or have a bad odor.