The assigned protection factor according to ANSI.

The American National Standards Institute (ANSI) standard for respiratory protection (ANSI Z88.2 1992) lists assigned protection factors (APFs) for various respirators. The committee that developed the APFs based its decisions on a review of available studies of respirator performance. If workplace studies were available, these formed the basis for the number assigned. If no such studies were available, then laboratory studies, design analogies, and other information were used to decide what value to assign. For half-mask air purifying respirators, four workplace protection factor studies were consulted to arrive at an APF. For loose-fitting facepiece powered air purifying respirators (PAPRs), five workplace studies and two laboratory studies were reviewed. For full-face air purifying, helmet/hooded PAPRs, and most supplied air respirators no workplace studies were available. The APF was based on laboratory studies or decided by analogy to other equipment. For the remaining respirators only single workplace protection factor studies were available, and these were used by the committee to assign an APF. The database available to the ANSI committee was limited. Most of the studies available for review were not published. The committee in a sense was required to perform peer review on the information to use the data. Studies completed after the ANSI committee finished its deliberations, when added to the data used, continue to support the APFs assigned by ANSI.