OSHA requires annual respirator fit testing (29 CFR 1910.134) for all employees who must wear tight-fitting respirators. Two methods exist: qualitative (pass/fail using taste or smell agents like Bitrex or saccharin) and quantitative (using instruments to measure leakage). Employees must be fit-tested before initial use, whenever a different respirator facepiece is used, and at least annually thereafter. Conditions like significant weight change, dental work, or facial scarring require re-testing.