Disposable masks have a limited period of use, and it is usually necessary to pay attention to the labeled effective time. If the mask has exceeded the specified effective time, the material of the mask itself has not changed significantly, i.e. the effect of the mask in preventing viruses and bacteria has not been impaired. However, because the mask has been in use for too long, there may be a problem with the cleanliness of the mask. For example, if there is bacterial reproduction inside the mask, the mask itself may be the source of contamination, and there may be unexplained bacterial contamination. So if you are choosing to use an expired mask, you should be careful to find a way to sterilize it, and it can still be used normally after sterilization. Source: Dr. Yurai