QUANTITATIVE RESPIRATOR FIT TESTING WITH
CONTROLLED NEGATIVE PRESSURE

(Reprinted with permission from: Respiratory Protection Update 6(1):9-12. ISSN 1048-6658, Respirator Support Services, 2028 Virts Lane, Jefferson, MD 21755, 1995.)

Clifton D. Crutchfield, Ph.D., 
CIHcdcrutch@u.arizona.edu
Environmental and Occupational Health
University of Arizona
1435 N. Fremont Avenue
Tucson, AZ 85719

The decision to use quantitative fit testing (QNFT) as a fundamental component of a comprehensive respiratory protection program can result in some substantial benefits to program users. QNFT can provide a quantitative basis for selecting the best fitting respirator for a given worker. It can also increase the likelihood that the worker will achieve a desired level of protection in the workplace. Perhaps equally as important, a well designed QNFT system can be a powerful training tool for demonstrating to respirator users the consequences of proper vs. improper respirator donning and wear techniques. From a regulatory perspective, the use of QNFT is currently mandated for certain potential exposures, and required use of QNFT is expected to increase when OSHA's Respiratory Protection Standard is updated.

The above listed benefits are not realized without cost, which implies that decisions about incorporating QNFT into a respiratory protection program will involve some level of cost-benefit analysis. Three distinctly different QNFT methods are currently available. Each has a different set of capabilities to factor into the cost-benefit equation. The purpose of this paper is to present some basic information about the newest of the three QNFT methods, which is based on the use of controlled negative pressure (CNP).

Description of CNP Method

CNP technology is based on exhausting air from a temporarily sealed respirator facepiece to generate and then maintain a constant negative pressure inside the facepiece. The rate of air exhaust is controlled so that a constant negative pressure is maintained in the respirator during the fit test. The level of pressure is selected to replicate the mean inspiratory pressure that causes leakage into the respirator under normal use conditions. With pressure held constant, air flow out of the respirator is exactly equal to air flow into the respirator. Therefore, measurement of the exhaust stream that is required to hold the pressure in the temporarily sealed respirator constant yields a direct measure of leakage air flow into the respirator. The Fit Tester 3000 (Dynatech Nevada) represents the currently available application of CNP technology.

A list of the major advantages of the controlled negative pressure method relative to currently available aerosol methods is provided in Table I.

Table I. Comparative Advantages of the Controlled Negative Pressure (CNP) Method

1. The direct measurement of respirator leakage, which is the basic parameter that defines respirator fit. Aerosol-based methods produce indirect measures of aerosol penetration into the respirator.

2. The use of ambient air as a standard, non-varying, gaseous challenge agent. Air molecules provide a more rigorous test of mask fit than aerosol challenge agents. Air is the medium that carries contaminants into a respirator through a leak site. If air can leak into a respirator, a particle or vapor might leak into the mask; if air cannot leak into the mask, a particle of vapor cannot leak into the mask. The preceding statement cannot be applied with certainty to aerosol-based systems.

3. The elimination of differential mask leak penetration losses that can occur when different sizes of aerosol challenge agents are used.

4. The elimination of in-mask sampling biases, since negative pressure inside the mask equilibrates at sonic velocity and does not rely on the quality of in-mask mixing of penetrating particles.

5. The elimination of aerosol lung deposition losses and the effects of subject-generated particles, which can substantially alter aerosol fit test results.

6. Greatly enhanced fit test precision. The Fit Tester 3000 has a demonstrated flow measurement capability of + 2%. CNP leak measurement is independent of a variety of factors (e.g. probe location, leak location, streamlining, lung deposition, and mask dependent flow dynamics) than can substantially affect aerosol-based measurements of respirator leakage. Therefore, the selection of the best fitting respirator for a worker based on CNP test results can be made with more confidence.

7. Greatly increased test speed. The time required to conduct a single CNP fit test is approximately 10 sec. The almost instantaneous feedback provided by the CNP method makes it a powerful training tool.

8. The ability to precisely set and control the test challenge pressure, which allows respirator fit to be tested at inspiratory pressures that are more representative of actual use conditions. A variety of challenge pressures can be used to simulate a variety of work and breathing rates ranging from rest to high levels of exertion.

9. The ability to measure the fit of a worker's issued respirator, since the CNP method does not require an invasive probe through the respirator facepiece.

Table I. Comparative Advantages of the Controlled Negative Pressure (CNP) Method (cont.)

10. The ability to calibrate the CNP system with generally available primary calibration systems. The ability of aerosol-based fit test systems to measure respirator leakage cannot be traced to primary calibration standards, and has never been validated.

11. The Fit Tester 3000 enjoys substantial cost savings over currently available aerosol-based systems.

Results of Comparison Studies

CNP systems have consistently generated more conservative, and therefore more health protective, fit test results during comparison studies with aerosol systems. Such findings are fully consistent with the facts that a) the CNP system is based upon the use of a more rigorous test challenge agent (i.e. air), and b) the CNP system is independent of in-mask sampling biases that have been shown by a number of researchers to substantially affect aerosol fit test results.

During comparisons of a CNP system against a generated-aerosol standard system, the CNP system detected up to 10 times as much respirator leakage as the standard aerosol system.⁽¹⁾ CNP measurements of fixed leak sources were also much more accurate and precise than those made by the standard method. In a subsequent validation study,⁽²⁾ a CNP system detected unacceptable respirator leakage in all 50 cases in which leakage in excess of 530 ml/min was introduced into respirators worn by human subjects. The generated-aerosol system passed 30 of those same 50 respirators with the same level of induced leakage.

One of the frustrations associated with conducting comparison studies of different fit test methods has involved the lack of a validated standard fit test method. A means of introducing known amounts of leakage into respirators has been developed by inserting fixed leak sources (hypodermic needles) into respirators worn by human subjects. A quantitative fit test conducted with the leak needle capped generates a measure of baseline respirator leakage. Repeating an identical test with the same fit test system after the leak needle is uncapped generates a measure of respirator plus needle leakage. The difference between these two measures represents the leakage introduced through the fixed leak needle.

Two separate studies have employed the above technique to assess the leak measurement capabilities of a CNP and an ambient aerosol fit test system. In one study, known amounts of respirator leakage were introduced into half-mask respirators worn by human subjects.⁽³⁾ The CNP system (Dynatech Nevada Fit Tester 3000) measured an average of 105.2% of the known leakage, while the ambient aerosol system (TSI Portacount Plus) measured an average of only 20.8% of the known leakage. CNP fit tests were completed in approximately one-fifth of the time required to complete the ambient aerosol tests.

A follow-on study of the same two QNFT systems was conducted using a series of fixed leaks introduced into both half-mask and full-face respirators mounted on a breathing machine-headform system.⁽⁴⁾ The Fit Tester 3000 system detected an average of 98.4% of the known leakage introduced into the respirators during 96 separate fit tests. The coefficient of variation (COV) was 4.3%. An analysis of variance revealed that CNP measurements were not affected by leak location. Portacount Plus measurements of the same leaks averaged 40.3% of the known leak rates, with a COV of 46.9%. An analysis of variance detected significant differences in the ambient aerosol system's measurements of leakage as a function of leak location. The absence of lung deposition in the bellows of the breathing machine accounted for the increase in Portacount leak detection from 20.8% to 40.3% of the known leakage.

Fit Test Exercises

CNP systems have consistently measured more leakage into respirators than aerosol systems during studies that included dynamic exercise protocols. The assumption that dynamic exercises significantly increase respirator leakage during fit tests has not been proven. The assumption that including exercises in fit tests make them more representative of the level of protection achieved with the respirator in the work place has also not been demonstrated, although a number of workplace protection factor studies have addressed the issue. The basis for and effect of exercise protocols currently specified by OSHA and ANSI have not been described nor validated.

Given the current lack of information on the value and effect of fit test exercises, it seems that currently specified protocols focus primarily on form at the expense of function. A primary cost associated with currently specified fit test exercise protocols is the time that is required to complete them. Current requirements contained in OSHA’s substance-specific standards can easily push the time required to conduct an aerosol-based fit test for one worker to more than 45 minutes. A critical question involves the level of benefit that is derived from that level of effort and cost.

A thorough examination of exercise cost-benefit is needed. The basic questions we try to answer with QNFT provide a starting point for such an examination. The first question involves the fundamental fit of the respirator. For example, does a medium or a large respirator fit this particular worker better? Fundamental fit represents the most important information developed during a fit test, and is currently represented by the normal breathing exercise.

Beyond information about fundamental fit, we need to know whether movements normally associated with work will cause a significant shift in the fundamental fit of the respirator. The effects of transient breaks in the facepiece seal (which become leaks only during inhalation and typically apply for only a small fraction the total time that the respirator is worn) are minor compared to a substantial shift to a lower respirator fit that does not return to our measured fundamental fit. In that case, the wearer may spend a majority of the time in the workplace with the lower fit. Most of the current OSHA exercises seem to focus more on trying to detect transient leaks than permanent shifts in fundamental fit.

The third piece of information that seems vital to the determination of respirator fit involves respirator donning. If the fit achieved when the respirator is donned for the fit test does not closely parallel the fit achieved each time the respirator is donned in the workplace, then the fit test does not seem to have much value. We can feel much more confident about measured respirator fit if a worker can achieve the same relative level of fit while removing and redonning a respirator several times during a fit test. Instead of devoting countless minutes to superficial exercises that have little defined meaning, it seems that a more cost-beneficial fit test could be accomplished in much less time by simply measuring fundamental fit several times before and after a vigorous challenge of the facepiece seal and between a series of redonnings of the respirator. The time available for fit testing workers is a precious commodity; we should make the best possible use of it with our test protocols and procedures.

References

1. Crutchfield, C., R. Murphy, and M. Van Ert: "A Comparison of Controlled Negative Pressure and Aerosol Quantitative Respirator Fit Test Systems Using Human Subjects." Am. Ind. Hyg. Assoc. J. 54(1):10-14, 1993.

2. Crutchfield, C., Ruiz, A., and Van Ert, M.:"A Validation Study of Respirator Fit Testing by Controlled Negative Pressure". Appl. Occ. Environ. Hyg. 9(5):362-366, 1994.

3. Crutchfield, C., Park, D. Hensel, J., et al.: "Determinations of Known Respirator Leakage Using Controlled Negative Pressure and Ambient Aerosol QNFT Systems." Am. Ind. Hyg. Assoc. J. 56(1):16-23, 1995.

4. Crutchfield, C., and D. Park: "Effect of Leak Location on Measured Respirator Fit." Am. Ind. Hyg. Assoc. J. 58(6):413-417, 1997.