Reference

A user seal check is the fast self-test you run every time you put on a tight-fitting respirator to confirm the facepiece is sealed to your skin. This guide walks through the positive and negative pressure methods required by OSHA 1910.134 Appendix B-1, explains why a seal check is not a fit test, and shows a half mask example.

What Happens If a Respirator Doesn't Fit: A respirator that does not seal properly against the face stops working as rated. Fit failure collapses the assigned protection factor (APF) - a half-face respirator with an APF of 10 can deliver as little as 2× protection if the face seal leaks.