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Honesty in Fit Testing

When it comes to respirator fit testing, deciding what technology to use is a critical decision. Unfortunately, not all technologies are equal - or advertised accurately. 

Time needed:

It has been stated by OHD that the CNP REDON fit testing protocol, as used by the OHD Quantifit and Fit Tester 3000 is significantly faster than the Aerosol fit testing method. In fact OHD advertises “1-3 minute respirator fit testing” - but just how accurate is this statement? In our opinion, it is not even close. Below are the OSHA approved procedures for the REDON protocol taken directly from 29CR1910.134 Appendix A: 

Exercises  Exercise Procedure  Measurement Procedure  
Facing Forward    Stand and breathe normally, without talking, for 30 seconds. Face forward, while holding breath for 10 seconds. 
Bending Over  Bend at the waist, as if going to touch his or her toes, for 30 seconds. Face parallel to the floor, while holding breath for 10 seconds 
Head Shaking  For about three seconds, shake head back and forth vigorously several times while shouting.  Face forward, while holding breath for 10 seconds. 
REDON 1  Remove the respirator mask, loosen all facepiece straps, and then redon the respirator mask.  Face forward, while holding breath for 10 seconds. 
REDON 2  Remove the respirator mask, loosen all facepiece straps, and then redon the respirator mask again.  Face forward, while holding breath for 10 seconds. 
Table A-1. – CNP REDON Quantitative Fit Testing Protocol, as found in OSHA 1910.134 Appendix A.


When you consider that the first two exercises in the fit test take a minimum of 1 minute and 20 seconds to complete, and the last 2 exercises require the fit test subject to remove the mask completely, loosen all the mask straps fully, and then redon properly, it’s plain to see that it would be impossible to complete a fit test in the 1-3 minutes, as advertised by OHD. The problem is that many CNP users are taught to cheat their way through a fit test by not performing the first two 30 second exercise procedures and by not removing the mask completely and loosening all the straps as instructed by OSHA. 

When the actual time needed to perform a CNP REDON fit test according to an OSHA-approved procedure is compared with that of the Aerosol fit test, it is clear that any difference is negligible. For more detailed information on the true time needed to perform a fit test refer to the TSI application note RFT-008.

1. Occupational Health Dynamics, LLC. 2009. 
2. Occupational Safety and Health Administration (OSHA). U.S. Department of Labor. 29 CFR 1910.134, 2004. 
3. Occupational Health Dynamics, LLC. 


The CNP method, specifically as utilized by the OHD Quantifit, advertises the ability to simulate actual work conditions by changing the assumed breath rate depending on the characteristic of the fit test subject. However these selectable challenge pressures are static pressures and, as is obvious to all, a living person breathes, which means the in-mask pressure varies continuously, and this cannot be accounted for in the CNP method. In addition to this, there is no way to really know which condition to select for each individual on the OHD, so the only thing you know for sure is that the selected in-mask pressure is wrong

Another consideration is when the actual testing, or measurement, of the fit factor occurs. The CNP method requires the person to perform the fit test exercise, and then remain completely motionless, holding their breath while the leak rate is being measured. In fact, here are the specific instructions the fit test subject is required to be trained on and to follow in order to complete the fit testing process:
  • Take a breath and hold it, keeping mouth closed
  • Keep mouth closed (do not swallow or move mouth or tongue)
  • Do not exhale any air through nose
  • Do not make head or facial movements. Sit or stand as still as possible in the position as instructed.
Anyone who has donned a full face respirator for any length of time will easily understand the amount of pressure and strain such requirements will put on a person’s ability to breath, and on their pulmonary system.

The aerosol technology does not need to “simulate” such conditions, as the fit factor measurement is done under conditions that really exist, such as while the fit test subject is moving and breathing. The aerosol technology will measure fit factors as these variations occur, making it a dynamic method of measurement. The CNP method only measures during a static pose, so unless the person being fit tested doesn’t breathe or move during their work hours, you must ask yourself how can this be an accurate simulation?

1. Occupational Health Dynamics, LLC. User’s Manual Quantifit. 2008. 


It’s obviously very important to make sure you are utilizing an accurate method of measurement when determining Fit Factors. The Aerosol technology measures an actual Fit Factor with a high degree of accuracy and repeatability, while the CNP does NOT measure a fit factor at all, it only extrapolates or assumes one. The CNP method extrapolates a fit factor by measuring the leak rate, and then assuming a breath flow rate for each individual, the problem with that is that the breath rate of the fit test subject cannot be accurately guessed. The assumption made by the CNP method to determine an estimated fit factor introduces errors in the range of (-39%) to (+93%) for men and (-21%) to (+58%) for women. There is nothing you can do mathematically to improve an inherently uncertain guess. 

It has also been stated that the CNP method can be calibrated back to a NIST primary standard. This is not wholly accurate. While the measurement of the leak rate can be traced to a NIST primary standard, the Fit Factor, that which is critical to the health of the worker, cannot. The inherent inaccuracy of the extrapolated fit factor mentioned above still applies.

Aerosol-based fit testers, like the TSI PortaCount® Pro/Pro+ measure fit factor directly by making a concentration measurement both inside and outside the mask while the test subject simultaneously moves and breathes. The dynamic in-mask pressure and breathing rate are “as occurs” for each individual. With the PortaCount, any errors that affect both the Cout and Cin measurements automatically cancel out when the fit factor is calculated, thus making them irrelevant. There are no artificial conditions, assumptions or estimations involved in the calculation of a PortaCount fit factor.

To further elaborate on the accuracy and reliability of the aerosol method let’s take a look at what the National Institute for Occupational Safety (NIOSH) says. In 1998 NIOSH undertook a huge research project to study different quantitative respirator fit test (QNFT) methods. The objective of the study was to find out if the fit factors measured by any of the existing methods had what really counts; a relationship to actual respirator protection levels. In other words, which methods work, and which do not?

Comparison of the different fit test methods was done by calculating a correlation coefficient. A coefficient of 1.0 means that there is a perfect match and coefficients above 0.8 are usually considered to be very good while those below 0.5 are considered poor. Here is what NIOSH found in their study with half face masks;


QNFT Method Correlation Coefficient (R2)
Generated Aerosol (CHD) 0.81
PortaCount Fit Tester (AA1) 0.79
Controlled Negative Pressure (CNP) 0.36

It’s important to note, that according to NIOSH the actual “Gold Standard” for quantitative fit testing methods is the Generated Aerosol (CHD) method, not the CNP method as OHD has advertised. As you can clearly see in these NIOSH findings, the aerosol method of the PortaCount correlated extremely well, while the CNP method did not. More information on this study can be found in the TSI application note RFT-009.

1. Wallaart, J.C.: A Study of the Relationship between Heart Rate and Minute Breathing Volume at Various Levels of Work Demonstrating the Spread between Individuals in a Group and the Implications in Industry. Presented at ISRP Conf. Amsterdam (1997).
2. Berndtsson G.: Peak Inhalation Air Flow and Minute Volumes Measured in a Bicycle Ergometer Test,
J. Int. Soc. Resp. Prot., 21:21-30 (2004)
3. Kaufman, J., Hastings, S.: Respiratory Demand During Rigorous Physical Work in a Chemical Protective Ensemble,
Journal of Occupational and Environmental Hygiene, Vol 2, No 2, (Feb 2005)
4. Janssen, L.L.: Interpretation of Inhalation Airflow Measurements for Respirator Design and Testing,
J. Int. Soc. Resp. Prot., 22 : 122- 141 (2005)
5. Coffey, C.C., D.L. Campbell, W.R.Myers, Z. Zhuang, S. Das: ―Comparison of six respirator fit-test methods with an actual measurement of exposure in a simulated health care environment: Part I—Protocol development.‖ Am. Ind. Hyg. Assoc. J. 59:852–861 (1998).
6. Coffey, C.C., D.L. Campbell, W.R. Myers, and Z. Zhuang. ―Comparison of Six Respirator Fit Test Methods with an Actual Measurement of Exposure in a Simulated Health-Care Environment: Part II—Method Comparison Testing.‖ Am. Ind. Hyg. Assoc. J. 59:862-870. (1998).



It has been wrongly assumed by OHD that the aerosol method of fit testing has potential “sampling biases”. This assumption is commonly based on the following three erroneous ideas, so let’s review and debunk each:

  1. “Penetration losses” - The assumption that there is a discrepancy between Aerosol Particulate penetration into the mask, and Vapor/Gas penetration into the mask. The fact is, that Vapors penetrate the mask the same as Aerosol particulate do, and studies have shown that when comparing the two challenge agents, aerosol particulate versus vapor/gas, they compare very well. Refer to the references provided below for further details.
  2. “Particle streamlining” – The theory that particles which leak into the mask might be breathed in by the person rather than sampled by the PortaCount, this is because the inhaled breath has a larger flow rate than the flow rate of the PortaCount. However this is simply not true, the flow rates involved with breathing will not adversely affect the dispersion of aerosol concentration within the mask. Whether the fit test subject is inhaling or not, the PortaCount still pulls the same amount of air to be sampled, and the air being sampled will have the same particle concentration as the air being breathed in.
  3. “Lung deposition” – The belief that particles which are inhaled by the fit test subject would be deposited in the lungs, and therefore not be counted by the PortaCount. But the penetration efficiency for a typical human lung is a complex function of particle size (ICRP Publication 66). Ambient air is polydispersed aerosol with a wide variety of particle sizes ranging anywhere from just 1 nanometer (nm) up to several microns (μm) in diameter. As a result, there may be particles that get deposited in the lung if inhaled but it is the really small particles, less than 10 nm, and the large particles, greater than 1 μm, that have high rates of deposition in the lungs. The particles found in between 10nm and 1μm, the same particles which are counted by the PortaCount, have a very high probability of being exhaled. In ‘real life’ terms, if this assumption were correct, one would barely see cigarette smoke being exhaled by a smoker.
It has been further assumed by OHD that air is the best challenge agent, but studies have shown that this simply isn’t accurate. In fact the CNP method has been openly questioned by various safety and regulatory boards because it measures the exhalation valve leak during the measurement. This measurement is unrealistic as compared to actual respirator usage. Refer to the NIOSH study titled “Comparison of Six Quantitative Fit-Test Methods to a Measurement of Actual Exposure...” which is discussed further in the TSI application note RFT-009.

OHD has never claimed to be the experts at particle or aerosol measurement techniques, and for good reason. They are not the experts... TSI is. TSI has over 4 decades of experience in particle physics and ambient aerosol research, working and collaborating with Universities and Laboratories worldwide using our particle measurement devices in a variety of applications. 
Furthermore, a quick reference to the NIOSH study mentioned previously will clear up any confusion as to whether or not these so-called biases truly occur. If these biases do occur, then what explanation do we have for NIOSH continuing to use the aerosol method for much of their fit testing research and certification of respirators?

1. Gardner, P., Hofacre, K., Richardson, A.: Comparison of Simulated Respirator Fit Factors Using Aerosol and Vapor Challenges, Journal of Occupational and Environmental Hygiene, Vol 1, No 29-38, (Jan 2004)
2. Occupational Health Dynamics, LLC.
3. Coffey, C., Jensen, P., Lawrence, R., Zhuang, Z.: Comparison of Six Quantitative Fit-Test Methods with an Actual Measurement of Exposure in a Simulated HealthCare Environment: Part II – Method Comparison Testing. American Industrial Hygiene Association. J. 59:862,870 (1998).


View Studies

Several studies have been conducted, evaluating and comparing CNP and Aerosol Quantitative Fit Test Methods.  

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In the end, you need to select a respirator fit tester that will provide the highest level of assurance and protection.  In the following testimonial, you will see why PortaCount® Fit Testers are the most trusted quantitative fit testers on the market today. 

Independent Review of Quantitative SCBA Fit Testing Machines »

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Only one quantitative respirator fit tester family can deliver all the features you need for respirator training and fit testing - TSI PortaCount® Pro and Pro+ Fit Testers.  

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