How to distinguish the front and back of a white flat mask?

How to distinguish the front and back of a white flat mask?
The front and back of the white flat mask can be distinguished by metal strips, creases, thread ends, colors and fabrics.

1. Metal strip distinction
Distinguished by metal strips, the color of medical masks is usually the same, which makes it difficult for people to distinguish. Generally, the metal strip is above and the side protruding outward is the outside, and the opposite side is inside.

2. Classification of creases
It is distinguished by creases. The upward side of the crease is the inner surface, and vice versa is the outer surface.

3. Distinguish between thread ends

The side with thread ends is outside, otherwise, the side without thread ends is opposite.