Professional Trust Should Be Earned
CNP systems have demonstrated the ability to accurately and consistently measure known levels of respirator leakage.
- By Luke Allen
- May 01, 2015
The featured article in the November 2014 issue of OH&S, titled "Fit Testing Industry Professionals Can Trust," presented a compelling case for an effective Respiratory Protection Program to protect workers from a broad array of toxic exposures encountered in a wide variety of occupational settings. The article also described the two basic types of respirators (supplied air and air purifying) that are available to help protect workers from such exposures.
A strong emphasis of the article was directed toward the need to fit test tight-fitting respirators to help ensure that desired levels of protection are achieved. It described the two basic methods of respirator fit testing that have been approved by OSHA--Qualitative Fit Testing (QLFT) and Quantitative Fit Testing (QNFT). The article cited several significant limitations associated with the four QLFT methods described, including 1) potential exposure to test agents, 2) the ease with which test protocols can be skipped or compromised in the interest of saving time, and 3) QLFT's reliance on subjective responses from test subjects, which introduces the potential for deceitful responses from the person being tested.
A much stronger case was made for QNFT, which is based on objective measures of respirator fit that are made with computerized instruments that can precisely control all aspects of the fit test protocol. The article then went on to state that "There are two aerosol-based OSHA-accepted QNFT methods: Generated Aerosol (complex and lacks portability) or Ambient Aerosol (simple and portable)." Both methods measure aerosol challenge agent concentration outside and inside the test respirator. The numerical ratio of those measurements is defined as a fit factor, which describes the level of respirator fit achieved during the fit test. The article stated that the Ambient Aerosol QNFT method, being simpler to operate and portable, is the most economical QNFT method and is industry preferred. The article seemed to imply that, when it comes to QNFT, the two OSHA-approved available choices are Ambient Aerosol (preferred) and Generated Aerosol (complex and expensive).
It was quite surprising that an article describing respirator fit testing that industry professionals can trust made no mention of the newest QNFT technology, controlled negative pressure (CNP). Perhaps even more surprising is the fact that, of the three QNFT methods accepted by OSHA, only CNP technology has met the QNFT validation criteria established by the American National Standards Institute. The aerosol-based QNFT methods were essentially grandfathered into OSHA acceptance and ANSI’s Respirator Fit Test Methods Standard, Z-88.10, without validation studies. The ability of aerosol-based fit test systems to effectively measure respirator leakage has not been validated against known respirator leakage.
How does CNP technology differ from the older aerosol-based technology? CNP measures respirator leakage directly, whereas aerosol-based systems produce an indirect measure of leakage represented by measured aerosol penetration, which is the reciprocal of the fit factor. How do the measurements of respirator leakage or penetration made by the different methods compare? Respirator fit test systems based on controlled negative pressure (CNP) technology have consistently measured significantly more leakage than aerosol-based systems during a substantial number of comparative studies. A meta-analysis of the combined results of seven comparative studies conducted in a variety of settings showed a greater than tenfold difference between CNP and aerosol-based measurements of respirator leakage and fit.
What factors could explain such large differences in respirator leakage measurements? CNP systems have demonstrated the ability to accurately and consistently measure known levels of respirator leakage. Those measurements can be directly related to primary calibration devices. Aerosol-based measurements of respirator leakage cannot be related to any such known value. The scientific literature contains a substantial number of studies that have identified and investigated a number of factors that tend to bias both in-mask and outside measurements of QNFT aerosol concentrations. Some of the primary bias factors include differences between breathing flow rates and mask sample flow rates, leak location, sample probe location, aerosol streamlining past the sampling probe to the lungs, and differential aerosol deposition and retention in the lungs. The effect of most of these bias factors is to reduce the in-mask aerosol measurement, which results in a higher fit factor and less leakage being reported by the fit test system.
Differences Between CNP and Ambient Aerosol Fit Test Systems
The results of CNP vs. ambient aerosol fit tests conducted consecutively on the same person wearing the same respirator, without removal, are not surprising. First of all, consider the test challenge agent. For the most common ambient aerosol system, a 40-second mask sample has a minimum measurable concentration of 0.004 particles per cm3. The in-mask measurement would be based on sampling only 0.65 percent of the air a worker would inspire at a low-moderate work rate. A majority of ambient-aerosol fit factors are based on measured in-mask concentrations of less than 1 particle per cm3. By contrast, CNP systems use air molecules to measure mask leakage. In-mask concentrations of air molecules during a CNP fit test are more than a billion billion molecules per cm3. Having enough of what you are trying to measure makes a big difference.
Other substantial differences between CNP and ambient aerosol fit test systems include the time required to make a leak measurement (CNP = 8 seconds; ambient aerosol = 60 seconds) and the fit test exercise protocol employed by each system. CNP fit tests are done using static positions, while aerosol-based fit tests are done using dynamic exercise protocols. It has been claimed (though not demonstrated) that CNP systems therefore miss the respirator leakage associated with dynamic exercises. If that were the case, shouldn’t CNP fit factors generally be higher than aerosol-based fit factors, instead of the demonstrated mean average of 10 times lower?
A detailed analysis of fit test exercise results revealed that most of the exercises did not produce fit factors that were significantly different from the first normal breathing exercise, which is the best descriptor of basic respirator fit. Further analysis showed that respirator donning has by far the greatest effect on respirator fit. As a result of that analysis, the CNP REDON exercise protocol was developed, tested, and approved by OSHA in 2004.
In conclusion, it seems fair to state that with respirator fit testing, as with life, it is important to focus on what matters most (in this case of respirator fit testing, that would be respirator leakage) and try to find the best available technology for measuring it. At its core, earning professional trust involves developing technology that is designed to directly address the problem at hand and that can be held to measurable performance standards.
This article originally appeared in the May 2015 issue of Occupational Health & Safety.