THOSE FANTASTIC FIT FACTORS

(Reprinted with permission from: Respiratory Protection Update 6(4):1,4-5. 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

Those fantastic fit factors we have learned to love may be too good to be true. During the recent International Society for Respiratory Protection Conference in Vancouver, I had an opportunity to chat with some folks from Europe about gas mask selection and performance. I inquired about the level of fit that is generally expected and achieved with the British-made mask that has been adopted by several European countries. I was told that a fit factor of 10,000 is considered acceptable, and that fit factors in excess of 40,000 are routinely measured with salt fog fit test systems. Those are pretty impressive numbers. U.S. Army personnel who have been involved in testing the gas masks currently used by the U.S. Department of Defense indicate that similar levels of fit are being recorded on this side of the Atlantic. A fit factor of 40,000 is certainly a comforting number when you stop to consider the kinds of toxic agents that gas masks are designed to protect against, but I have to wonder whether anyone has given much thought to the true meaning of such numbers.

The fit factor, which is defined as the ratio of challenge agent concentration outside a mask to the concentration measured inside the mask during a fit test, evolved from our first attempts to quantify respirator leakage. The outside/inside concentration ratio represents an expression of the level of contaminant reduction that is achieved by the respirator system. The reciprocal of the fit factor, the inside/outside concentration ratio that is defined as penetration, provides an indirect measure of mask leakage during the fit test. Although aerosol ratios have historically been the medium through which we expressed respirator leakage, a closer at the actual numbers show that such ratios can be very deceptive.

A 40,000-fold reduction in aerosol concentration seems feasible enough given the fact that a generated-aerosol fit te system can produce a concentration of more than 150 million aerosol particles in an air volume of 750 cm³, which is the approximate volume of air inhaled with each breath during a fit test. Even with a fit factor of 40,000, a test subject would still inhale more than 3,500 generated aerosol particles with each breath. That sounds like a pretty big number too, until it is contrasted with the number of ambient aerosol particles that are inhaled with a typical breath. We routinely breathe in more than three million particles in a typical breath. The purpose of these illustrations is to point out that, from a particle number perspective, fit factors of 40,000 or more are easy to produce and may seem fairly logical. The logic breaks down, however, when we view a fit factor of 40,000 from the perspective of actual respirator performance.

When we switch from generated aerosol fit test systems to ambient aerosol systems such as the Portacount, we generally see lower fit factors. The most obvious explanation lies in the fact that the challenge agent concentration is reduced from millions to thousands of particles per cm³. The greater sensitivity of the Portacount, which can report fractions of a particle per cubic centimeter, can still produce fit factors of over 100,000. It should be noted that the use of fractions of a particle per cubic centimeter as the in-mask concentration makes the resultant high fit factor extremely volatile.

Respirators preclude contaminants from a worker's breathing zone by preventing contaminated air from entering that zone. Any contaminant that does enter the respirator is carried into its interior by inspiratory air flow. Contaminant leakaage into the mask interior can occur by two paths. Either the contaminant penetrates through the respirator's filters or cartridges (the air-purifying path), or it is carried through a leakage path that exists in the mask's valves or seals, with the facepiece-to-face seal being the most probable leakage path. Contaminant leakage through the air-purifying path is primarily function of cartridge efficiency. During quantitative fit testing, penetration is generally assumed to be essentially zero when HEPA filters are used for aerosol challenge agents. The effect of any contaminant penetration through the air purifying path would be increased leakage and lower fit factors measured during the fit test.

Since air is the medium that carries contaminants or fit test challenge agents into the respirator, the leakage parameter that we are really concerned about when a respirator is worn is air leakage. If air cannot enter the respirator through leakage path, then a contaminant cannot enter the respirator through a leakage path. Given our assumption that contaminant penetration through the air-purifying path is minimal, the fit factor can be redefined as the ratio of total air flow to leakage air flow into the respirator. If 1% of the air entering a respirator enters through a leakage path, then 99% of the air would enter through the air-purifying path and the ratio of total flow (99% + 1%) to leakage flow (1%) would yield a fit factor of 100.

The flow based definition of fit factor means that, for a fit factor of 40,000, the inspiratory flow rate through the respirator cartridges would be 40,000 times greater than the flow rate through all combined leakage paths into the respirator. The total or inspiratory flow rate into the respirator is primarily a function of work rate. For a subject taking a fit test at essentially a resting work rate, a total flow rate of 30 l/min is representative. In such a case, a fit factor of 40,000 would necessitate a leakage flow rate into the respirator of only 0.75 cm³/min. At such a leak rate, it would take more than 22 hours to achieve a single air exchange in a full-face respirator. Researchers attempting to seal respirators onto mannequin heads with glue and caulk have found it extremely difficult to achieve such extremely low air leakage rates. It is not reasonable to assume that workers or soldiers can routinely achieve a more airtight seal by merely donning a respirator or gas mask.

One other aspect of fantastic fit factors that needs to be better understood is their volatility. If a fit factor of 40,000 sounds good, then a fit factor of 80,000 must surely border on the incredible. In the example cited above, the difference between the fit factor of 40,000 and a fit factor of 80,000 would be a reduction in the leakage flow rate of only 0.38 cm³/min. That amount of difference, which would be extremely difficult to measure, is essentially no difference at all. For the Portacount used in a typical ambient aerosol concentration of 5,000 particles/cm³, a fit factor change from 40,000 to 80,000 would be recorded if the measured in-mask concentration changed from 0.13 to 0.06 particles/cm³. At a fit factor of 5,000, which a lot of people advocate as a good pass-fail level for Portacount ambient aerosol fit test device, the in-mask concentration would be 1.0 particle/cm³. Based on the above example a fit factor of 5,000 would result from a leakage flow rate of only 6.0 cm³/min.

Given the high fit factors that are routinely produced by aerosol-based fit test systems, the idea of raising the pass-fail point for fit tests to a higher level has been discussed. The historically used pass-fail value has been ten times the assigned protection factor for the respirator being tested. A number of studies have demonstrated that pressure-based fit test systems detect up to 5-10 times more respirator leakage than aerosol-based systems. Given the relatively low aerosol counts or leakage flow rates associated with fit factors of 5,000, it is not clear that changing the pass-fail value to this level is scientifically justifiable. After all, does achieving a real 5,000-fold reduction in contaminant concentration with a half-mask respirator seem like something that can be routinely achieved by the vast majority of respirator wearers most of the time? My experience of measuring respirator leakage directly with a controlled negative pressure system, whose test results can be traced to primary calibration systems, causes me to believe that the answer to that question is no.

ABOUT THE AUTHOR

Dr. Crutchfield received his BS degree in Engineering Management from the US Air Force Academy in 1969. Following active duty assignments as an F-4 weapons system officer in New Mexico, Germany, and Southeast Asia, he returned to graduate school and received MSPH (1976) and PhD (1978) degrees in Industrial Hygiene and Environmental Sciences from the University of North Carolina at Chapel Hill. Dr. Crutchfield's doctoral research involved the development of a spirometric transducer based on ion convection.

Dr. Crutchfield is currently Associate Professor and Director of the Graduate Industrial Hygiene Program at the University of Arizona. He is also a Chief Bioenvironmental Engineer in the US Air Force Reserves, and is certified by the American Board of Industrial Hygiene. His research has resulted in the publication of more than 20 scientific papers, as well as development of the controlled negative pressure respirator fit test method. Dr. Crutchfield was presented the prestigious John M. White Award by the Respiratory Protection Committee of the American Industrial Hygiene Association in 1994. He serves as a voting member on the recently reformed American National Standards Institute subcommittee helping to create a new national consensus standard, ANSI Z88.10, "Respirator Fit Test Methods".