Fit Factor

By Tabitha Mishra
Last updated: January 11, 2022

What Does Fit Factor Mean?

Fit factor is a quantitative estimate derived from a respirator fit test. It represents the ratio of the concentration of a substance in the environment to the concentration of that same substance in the respirator while it is worn.

The fit factor is derived from a quantitative fit test (QNFT).

Safeopedia Explains Fit Factor

Respirators are personal protective equipment (PPE) that reduce exposure to inhalation hazards. To function optimally, the respirator must form a tight seal against the wearer's face. Fit testing is a procedure used to identify any leakages in the respirator's seal to determine whether it is fitted properly.

Fit testing is done on an individual basis – each user must be fit tested with the respirators they will be using. The tests fall into two categories: qualitative and quantitative fit tests. Quantitative fit tests are the ones that yield a fit factor.

Fit Factor Pass Criteria

According to OSHA 1910.134 (f) (7), a properly fitted respirator will have a fit factor of 100 or greater for half-face respirators and 500 or greater for full-face respirators.

OSHA Acceptable QNFT Tests

Quantitative fit tests are performed by a machine or computer that is attached to the respirator via a hose and probe.

According to OSHA, there are five acceptable quantitative fit tests:

  • Ambient aerosol condensation nuclei counter (CNC)
  • Controlled negative pressure
  • Generated aerosol
  • Modified ambient aerosol condensation nuclei counter (for full and half face-piece elastomeric respirators)
  • Modified ambient aerosol CNC (for filtering face-piece respirators)

The Fit Test Procedure

Except for the CNP and CNP REDON protocols, the following test exercises are performed for all fit tests. Each exercise lasts one minute, with the exception of the grimace exercise which only lasts 15 seconds.

  1. Normal breathing
  2. Deep breathing
  3. Turning the head slowly from side to side
  4. Moving the head up and down, inhaling when the head is lifted up
  5. Talking out loud or reading the Rainbow Passage
  6. Grimacing (smiling or frowning)
  7. Bending at the waist, reaching for the toes
  8. Normal breathing (a repeat of the first exercise)

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