The limitations of qualitative respirator fit testing
Answered by Jeff Weed, product manager, TSI Inc., St. Paul, MN.
All existing qualitative fit test methods such as Saccharin, Bitrex, Isoamyl Acetate (banana oil) and Irritant Smoke share the limitation that they can only be used when a fit factor of 100 or less is considered to be an acceptable pass level. This means that whenever a passing fit factor higher than 100 is needed, you must use a quantitative fit test method.
Most respirator fit test standards and regulations currently in use around the world, including OSHA 1910.134, allow a fit factor of 100 for half-face respirators, but require a much higher level of fit such as 500, 1,000 or even 2,000 for full-face respirators. This means that you must use the quantitative method for full-face masks. One notable exception is often made for positive-pressure full-face masks, allowing them to be fit tested to only 100 while in negative-pressure mode.
The reason that qualitative fit test methods are limited to a fit factor of 100 is because 100 is the pass level these methods were designed for in the laboratory. All legitimate qualitative methods were carefully developed in a laboratory and validated against quantitative method instrumentation.
Why doesn't somebody develop and validate a qualitative fit test method for higher pass levels? The reason is related to human sensitivity to the chemical stimulants used in this method. For a qualitative fit test method to be validated for 100, the test subjects must be able to detect reliably a challenge agent concentration inside the respirator that is 100 times less than the challenge agent concentration outside the respirator.
Properly designed qualitative protocols include a threshold sensitivity test that each person must pass prior to being fit tested to ensure that he or she can detect the very low in-mask concentration that results when the fit factor is just below 100. If a person cannot pass the threshold test, that qualitative fit test method cannot be used.
Most people have the ability to sense 1/100 concentrations, but the percentage of people who can reliably sense lower concentrations such as 1/500 or less drops off quickly. Because of this, it is unlikely that a qualitative fit test method will ever be developed for a pass level of 500 or higher.
Quantitative fit test systems do not have the fit factor limitation of 100. True quantitative method instruments measure the challenge agent concentration outside the respirator as well as the concentration inside the respirator. The ratio of those two measurements is the fit factor. Measurement of the fit factor results in a fit test that is not affected by the chemical sensitivity of the respirator wearer. Quantitative fit test systems are very sensitive. They are typically capable of measuring fit factors of 10,000 and higher, making them suitable for fit testing any type of tight-fitting respirator.
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