life, blocking the intrusion of bacteria and viruses for us. However, have you ever thought about how many bacteria will be in the mask when you wear it for a whole day on a hot summer day with high temperature and humidity? Researchers conducted experiments hereof with KF94 masks, ordinary masks, cotton masks and surgical masks. The results showed that bacteria were found in the four masks after wearing for one day. The cotton masks and KF94 masks with better airtightness had the largest number of bacteria.