Assigned protection factor (APF)

The OSHA respirator rule, 29 CFR 1910.134, lists APFs in Table 1. A half-mask air-purifying respirator has an APF of 10, a full facepiece air-purifying respirator has 50, and an SCBA operated in pressure-demand mode has 10,000. The higher the number, the lower the fraction of outside contaminant that is expected to reach the wearer.

The APF is used to set a maximum use concentration. Roughly, the highest air concentration at which a respirator can be used is its APF multiplied by the occupational exposure limit for the contaminant, such as the Permissible exposure limit (PEL). If a task could produce air levels above that product, the chosen respirator is not adequate and a higher-rated one is needed.

APFs describe performance only when everything else is in place: an approved respirator, a successful Respirator fit test where required, training, medical evaluation and correct maintenance. They are not guarantees for any single wearer on any single day, and they do not apply if the filter is wrong for the chemical or if the user has a poor seal.

For immediately dangerous conditions, the rules are stricter. Where the concentration is at or above Immediately dangerous to life or health (IDLH) levels or the atmosphere lacks oxygen, only certain supplied-air types are permitted, regardless of simple APF calculations.