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Respirator fit testing

What are the different types of respirator fit testing? What are their limitations and benefits?


Photo: OHD

Responding is Stephanie Lynch, Ph.D., CSP, senior technology and research manager, OHD, Hoover, AL.

There are two overarching types of respirator fit testing:
Qualitative fit testing involves the use of a challenge agent that can be sensed in some way by the respirator wearer. The wearer dons their respirator and is then exposed to an agent they can taste, smell or react to. The tester will ensure the wearer can sense the challenge agent, and then the wearer will go through a series of exercises, or protocol, while the space around their face (often contained within a hood) is filled with the challenge agent. If the wearer doesn’t sense the challenge agent during the series of exercises, then the test is passed. If the wearer does sense the agent during the exercises, then the test is failed.
The benefits of qualitative fit testing are that it’s relatively inexpensive and requires very little maintenance. The limitations are that it’s a subjective test that only provides you with pass/fail results and has no automated documentation. The test is also somewhat long.

Quantitative fit testing uses a machine to quantify the fit of a respirator with what’s called a fit factor. There are two common, but very different, technologies that are available and approved globally:

  • Condensation nuclei counting. CNC uses ambient particles in the air to challenge the fit of a respirator. The fit factor is calculated by taking the concentration of ambient particles in the air outside of the mask and dividing it by the concentration of particles inside the mask. Any particles found inside the mask are presumed to be the result of leakage.
  • Controlled negative pressure. CNP uses air itself as the challenge agent by creating a controlled negative pressure within the respirator and then measuring the air that leaks in. The fit factor is calculated by putting the breathing rate that’s associated with the challenge pressure over the leakage measured in cubic centimeters per minute.

Both methods have technology-specific protocols and take the fit factors from each exercise in their protocol and average them together using a harmonic mean – a special kind of average that places more weight on the lower numbers. If the overall fit factor is greater than or equal to a reference passing fit factor, determined by the regulation and type of respirator tested, then the test is passed. If not, then the test is failed.

CNC has specific environmental requirements (a minimum number of particles must be present in the air – particles can be generated for testing) and has some consumables associated with use (isopropyl alcohol, wicks, probes, etc.), but can be used with any tight-fitting respirator to include filtering facepiece respirators. CNP has no consumables or environmental requirements but can only be used with elastomeric respirators. These machines are expensive compared with qualitative fit testing, but they’re objective and provide automated documentation and storage of fit test results. They’re also much faster.

The protocols for all fit tests and the reference passing fit factors are outlined in CFR 1910.134 appendix A.

Editor's note: This article represents the independent views of the author and should not be considered a National Safety Council endorsement.

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