2. The user feels that the breathing resistance becomes greater.
3. The mask is damaged.
4. Dust-proof filter cotton. When the mask is in good contact with the user's face, when the user feels that the breathing resistance is great, it means that the filter cotton is already covered with dust particles and should be replaced with a new mask.
5. The anti-virus filter box, when the mask is in good contact with the user's face, when the user smells toxic substances, it should be replaced with a new one.
6. Wearing ordinary masks: activated carbon masks can generally be used for 2 days, n95 can be used for one week. If the mask has not been worn for a long time after cleaning, it is better to re-clean it before wearing it.