OSHA 1910.134 Appendix A: Fit testing procedures (mandatory)
A respirator can't protect you if it doesn't fit your face. Certain respirators, known as tight-fitting respirators, must form a tight seal with your face or neck to work properly, according to OSHA. If your respirator doesn't fit your face properly, contaminated air can leak into your respirator facepiece, and you could breathe in hazardous substances.
Before you wear a tight-fitting respirator, your employer must ensure that your respirator fits you. This is done by performing a fit test on you while you wear the same make, model, and size of respirator that you will be using on the job.