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Assigned Protection Factors The assigned protection factor (APF) of a respirator reflects the level of protection that a properly functioning respirator would be expected to provide to a population of properly fitted and trained users.  For example, an APF of 10 for a respirator means that a user could expect to inhale no more than one tenth of  the airborne contaminant present. See Table 1. -- Assigned Protection Factors, 29 CFR 1910.134(d)(3)(i)(A). Keep In Mind Various groups such as the National Institute for Occupational Safety and Health (NIOSH), the Occupational Safety and Health Administration (OSHA), and the American National Standards Institute (ANSI) have proposed factors for the different types of respirators available. OSHA will enforce the APFs listed in its standards unless an alternative APF has been granted by a specific OSHA interpretation.