Quantitative fit testing (QNFT) is the most accurate fit testing method - using data to calculate the fit factor. Passing a real-time quantitative respirator fit test proves that the facepiece is sized and donned correctly. Selecting properly fitting respirators and checking daily donning protects workers against insufficient oxygen environments, harmful dusts, fogs, smokes, mists, gases, vapors, and sprays. These hazards may cause cancer, lung impairment, diseases, or death.