The Science Behind Fit Testing

Checking that a respirator 'with a tight-fitting facepiece' will provide an adequate seal to the wearer’s face has long been considered best practice as part of a general respiratory protection programme; indeed, in the UK, it is a requirement.[1]

Respirators with tight-fitting facepieces include disposable respirators, half-face masks, and full-face masks, including those that form part of a powered or air-fed respirator sysytem. This article will discuss methods of fit testing and some of the solutions available to support a fit testing regime.

When to Fit Test

Best practice suggests that fit testing should be carried out:

  • Before the respirator is issued
  • On all wearers of respirators with tight-fitting facepieces where fit testing has previously not been performed


Fit testing should be repeated at appropriate times such as:

  • If the wearer significantly loses or gains weight, has major dental work or sustains a major facial injury
  • If a different size or model of PPE is specified
  • At regular intervals as defined by national guidance or company guidelines


It should be noted that fit testing is in addition to the requirement to perform a seal check.

Fit Test Methods

There are two main methods of fit testing available:

  • Qualitative fit testing
  • Quantitative fit testing


During both methods of testing, it is essential that the subject is free from facial hair and other items that may interfere with the face seal. Wearers with facial hair or stubble (even a few days’ growth) that interferes with this seal must not wear a respirator which has a tight-fitting facepiece.

"All tests require a series of exercises to be performed during the fit test."

Manufacturers’ instructions for the test equipment should be followed at all times. A record of the result of the test should be generated and retained.

Qualitative Fit Testing

Qualitative fit tests provide a pass or fail result based upon the wearer detecting a test agent. The test agents are detected by taste and they provide a subjective measure of the quality of the seal of the facepiece to the wearer’s face. The test introduces a controlled aerosol concentration into a hood fitted over the wearer's head and the method is suitable for disposable and half-mask respirators. It should be noted that while half-mask respirators may be used with gas and vapour filters, they should only be fit tested using particle filters (or particulate filters worn in combination with gas and vapour filters). Some national authorities, such as those in the UK, do not allow the fit testing of full-face respirators with higher protection factors using qualitative methods.


  • Inexpensive
  • Simple to use
  • No calibration of equipment required
  • No modification of facepiece required
  • Detection makes the wearer feel more involved in the test



  • Subjective-relies on wearer's response
  • Unable to test full-face masks


Quantitative Fit Testing

Quantitative fit tests give an objective measure of the quality of the seal between the wearer’s face and the facepiece, as quantified by the calculation of a numerical 'Fit Factor'. However, these tests do require specialized equipment such as particle counting devices.

Ambient Particle Devices

These devices usually depend on naturally occurring particles circulating within the ambient atmosphere. The test involves connecting a probe-fitted facepiece to the counting device via plastic tubing. Particles of a certain size identified within the facepiece are counted and this number is compared to the number of particles counted outside the respirator in the ambient atmosphere. In certain cases, it may be necessary to increase the ambient atmospheric particle concentration by means of a particle generator. The result of this test is expressed as the ratio between the particle counts inside and outside of the respirator (the aforementioned 'fit factor'). These fit factors can often be high and should not be confused or used as the basis to select a respirator.


  • Obtains a numerical result
  • Result is objective, independent of wearer perception
  • Computer compatible software allows print-out of records



  • Expensive
  • Knowledge of how to probe fit a facepiece required
  • Test operator training is recommended
  • Requires an additional adaptor to test FFP1 and FFP2 respirators, or those with P1 or P2 filters


Controlled Negative Pressure Devices

This test method uses a device to remove air from the facepiece and then maintain a constant negative pressure inside of it while the wearer holds their breath and remains motionless. The airflow out of the facepiece needed to maintain the constant negative pressure is equivalent to the airflow into it (the degree of face seal leakage) and the ratio of 'inspiratory flow rate' and 'leakage flow rate' can be used to generate a fit factor. However, such systems can only be used with facepieces that feature detachable filters or supplied air connections.


  • Does not require a specific ambient particle concentration
  • Does not require in mask probes



  • Cannot fit test disposable respirators
  • Wearer requires good breath-holding technique


Fit Factors

Minimum required fit factors are defined by national standards where those exist. The minimum fit factors as defined in the UK as shown in Table 1.[1]

Facepiece Type
Quantitative Fit Test Methods
Ambient Particle Counting
Controlled Negative Pressure
Disposable Half-Mask
Half-Face Mask
Full-Face Mask
Table 1. Required minimum fit factors for quantitative fit testing


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1 Guidance on respiratory protective equipment (RPE) fit testing (INDG479, rev1). HSE 2019.

Worker wearing a face mask

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