Disposable medical surgical masks, as a rule, are single-use and not reusable. The longest time limit for medical surgical masks is 4 hours, and they must be replaced after 4 hours. If the mask is contaminated by spatter, or some wet liquid, it needs to be replaced at any time. This is because the mask will gradually lose its filtering function as well as its moisture and splash-proof function as time goes on. So we observe the mask with the naked eye, and there is no special difference. But in fact, we have filtered and adsorbed a large number of pathogenic microorganisms and viruses through the mask during the process of breathing. If the mask is not replaced for a long time, the protection effectiveness of the mask will be reduced, or even become zero. Therefore, in the national guidelines for the use of masks, it is emphasized that the maximum time should not exceed 4 hours, and the mask should be replaced at any time if it is sprayed or contaminated with liquid.