CE, FFP2, FDA etc.
The anti-side leakage design of the mask is to prevent air from being sucked in through the gap between the mask and the human face without filtering, considering technical requirements.
Air is like water flow, where the resistance is small, it flows first. When the shape of the mask is not close to the human face, dangerous substances in the air will leak from the close to the human's respiratory tract.
Well, even if you choose a filter with the best mask. Nor can it protect your health. Many foreign regulations and standards stipulate that workers should regularly perform mask tightness tests. The purpose is to ensure that workers choose appropriate masks and wear them in the correct steps.