China Surgical Mask Manufacturers introduces how to correctly distinguish the front and back of masks:

Many people may think that the pros and cons are the same, as long as you wear them tightly. In fact, this is not the case. The functions of the front and back of the mask are different. Many masks use a three-layer design type. The filter layer for filtering germs is located in the middle, the outer layer is a waterproof layer, and the inner layer is a water-absorbing layer. If you wear it backward, the absorbent layer will become the waterproof layer, and the waterproof layer will become the absorbent layer, so

This will cause the saliva to adhere to the surface of the mask, and the accumulation of more and more may wet the mask and make the mask lose its filtering function.

First of all, it can be interpreted by "color": "dark color is outside, light color is inside"; secondly, it can be judged by "print", "print is generally outside, and the words are positive".

If you buy a pure white unprinted mask, it can be judged by the "fold direction". If there are three folds, then the direction of the folds downward is the outside; if it is four-folds, then the middle piece is convex after being stretched, which is the outside. In addition, after opening, the upper two folds are directed upward, and the lower two folds are directed downward.

Finally, we can also distinguish the front and back through the "metal strip". The protruding side of the metal strip is the outside, and the flat side of the metal strip is the inside.