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Respirator Fit Testing Methods: Controlled Negative Pressure (CNP) vs. Ambient Aerosol

By June 17, 2019October 5th, 2022Blog Post

I recently read a blog written in support of the Controlled Negative Pressure (CNP) method which demonstrated an almost complete lack of understanding of the Ambient Aerosol method in an attempt to promote the CNP method. The author seemed to believe that the Ambient Aerosol method tested the respirator only as it protected against the challenge particulates and not as it would protect against any other environmental insult. This is itself an insult to the readers’ intelligence. Let’s discuss what is actually happening in both types of fit tests.

First let’s consider the CNP method. The CNP method depends on the ability of the instrument to measure a small change in the pressure inside the mask which would be associated with a leak. However, this measurement must be so sensitive that absolutely no movement by the person being tested can be tolerated while the measurement is occurring. Anyone who has a passing knowledge of the gas laws (specifically Boyle’s law) will understand that any change in the volume of the container (in this case the in-mask breathing zone) will result in a change in the pressure. Hence, no movement while the pressure measurement is being conducted.

In the real world, persons wearing a respirator are moving and performing work. As a result, the respirator/face seal is being stressed as movement occurs. The CNP method cannot measure the ability of the respirator to seal while the person wearing it is moving. It’s that simple. I have no doubt that if the object of respirator fit testing were to measure the fit of a respirator on a corpse, the results would be accurate and satisfactory.

But let’s return to reality.

In the early 1970’s Los Alamos National Laboratory working with the Naval Research Laboratory designed a system to test the fit of respirators using an aerosol of dioctyl phthalate (DOP) measured by a forward light scattering photometer. They anticipated questions which would arise concerning the accuracy of the system, so they also built a second system which used as a challenge agent sulfur hexafluoride (SF6) which is a gas at ambient conditions. They used a purpose-built flame photometric detector for the measurement of SF6, and ran the two systems in parallel. Comparison of the results of the two methods gave a correlation coefficient very close to unity. The generated aerosol concentration method became the “Gold Standard” against which all other methods are measured.

DOP is ideal for creating a reproducible aerosol, but is not ideal as a challenge agent which can be ingested by the test subject. Concerns about possible carcinogenicity were raised, and several alternative aerosols were tested. These showed promise, but in 1980 Dr. Klaus Willeke at the University of Cincinnati demonstrated an ambient aerosol (i.e. naturally-occurring ultrafine dust particles) method which was compared in parallel with the “Gold Standard” DOP method and which showed a correlation coefficient of 0.967! This was quickly developed into a commercial fit tester (TSI Model 8010). The point of the above is to provide proof that the Ambient Aerosol Method can be traced back to and favorably correlated with a method which used a gas (SF6) as an undisputed completely dispersed challenge agent. The only experiment I have ever seen which compared the CNP method to the “Gold Standard” was performed in the UK several years ago and which showed a correlation coefficient of 0.38!

Respirable particles (mostly ultrafine particulates with a mean aerodynamic diameter of ~15 nm to ~ 2μM) act very much like a gas. They are suspended in air by energetic nitrogen and oxygen molecules banging into them (Brownian Motion), and will penetrate through the most minute spaces. They are absorbed by pulmonary tissue, which is why they are called “respirable Particles” and which gives us the ability to create a clean, particulate-free breathing zone by use of a HEPA filter. One of the best things about them is the fact that they are naturally-occurring and present no increased toxicity. Best of all, by using these naturally-occurring ultrafine particulates as a challenge agent, we have the ability to test the respirator’s fit in a dynamic mode, i.e., when the person being tested is actually moving and breathing. Unfortunately, the Ambient Aerosol method would not work if we attempted to fit test a corpse, because we would have no way to clear the ultrafine particulates from the (former) breathing zone.

In summary, it can be demonstrated that the Ambient Aerosol method as used in the AccuFIT9000 is traceable to and compares extremely favorably to the proven “Gold Standard”. The same cannot be said of the CNP method. Remember that the Ambient Aerosol method tests the FIT of the respirator in a dynamic mode. If the respirator fits, and the filter cartridges or supplied air used are appropriate for the environmental insult, we can be sure that the respirator will provide the required protection even while the person wearing it is moving and breathing.

For additional Information on the AccuFit9000 click here.