Assigned Protection Factors (APFs) are the metric used for determining respirator performance on the worker. APF’s can be thought of as multiples of the occupational exposure limits (OEL) or fractions of the contaminated atmosphere. For example, with an APF of 10, the respirator would protect against 10 times the OEL or expose the worker to 1/10 of the contaminant in the atmosphere. APFs assume that the worker will have a fit factor 10 times the APF to allow for variations in donning. An APF is determined based on the leakage of the face seal, penetration through the filter, penetration or leakage from other sources, and a statistical determination to protect 95% of the end users. The leakage from the face seal is an additive to penetration from the filter. OSHA determines the assigned protection factors for all respirators.