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The Difference Between CNC & CNP Fit Testing

By March 17, 2020December 7th, 2022Blog Post

Two Quantitative Fit Testing Methods

Respirator fit testing is becoming more important all the time. There are two methods for performing quantitative fit testing, Condensation Nuclei Counting (CNC) and Controlled Negative Pressure (CNP). So, what’s the difference? 

There are essentially two methods currently in use to perform quantitative fit testing of tight fitting respirators:

CNC Fit Testing

The most widely used method is the ambient aerosol method using CNC. This test is done by challenging the seal of the respirator with naturally occurring, ultra fine particulates. The extremely low mass characteristics of these particulates mimic gas molecules. 

CNP Fit Testing

The CNP method pulls a slight negative pressure inside the mask and then measures the exhaust to determine how much more air must be pumped out in order to maintain that same low pressure. 

While both methods are approved by OSHA and are in use today, they are not created equal. So which method is better for your respirator fit testing needs?

CNC vs. CNP: Pros and Cons?

When comparing these methods, it’s best to examine the pros and cons of how each method is carried out, what factors may impact the results, and how accurate and repeatable those results are likely to be.

CNC Fit Testing Method Pros

The CNC testing system measures the concentration of ultra fine particulates in the ambient atmosphere and then measures the concentration of these ultra fines, a.k.a. respirable particulates in the breathing zone of the respirator. While the test subject is actually moving and breathing. Then this system measures the ambient concentration, again, computes an average of these two values and compares that value to the concentration inside the breathing zone.

That number becomes the fit factor, which is a real measure of how well the respirator actually fits. And because these particulates are respirable – can be breathed safely – using these ambient aerosols are perfect for use as the challenge agent in these tests. 

If the respirator being tested is set up with a HEPA filter in the inlet in about a minute, the concentration ACE approaches zero then any new particulates observed in the breathing zone penetrated past the respirator seal. 

Finally, and perhaps most importantly, the CNC method can be used with disposable filtering facemask respirators like the N95, whereas the CNC method cannot.

CNC Fit Testing Method Cons

Historically, CNC testing methods took significantly longer than CNP methods. This made it difficult and expensive for larger organizations to schedule mass testing. As a result, the CNP method quickly became the standard for quantitative respirator fit testing.

With the newest protocols approved by OSHA, however, CNC testing methods can now be done in less than 3 minutes. 

CNP Fit Testing Method Pros

The way the CNP system measures the pressure inside the respirator is by using a very sensitive pressure transducer. When the pressure rises, the system runs the vacuum pump and measures the amount of air in the exhaust. Theoretically, that measure is equal to the amount of air which is leaked past the respirator seal.

The sensitivity of the transducer and the clearly measurable results are both major positives and give this method an advantage over qualitative fit testing methods.

CNP Fit Testing Method Cons

While theoretically sound, there is a major flaw to this testing method. That is the human element. More specifically, it is the test subject’s breathing. The subject’s breathing affects the pressure in the mask zone and can throw off the pressure measurements, rendering the results null and void. 

So the test subject has to hold his or her breath while the leak rate is measured. Furthermore, the test subject cannot move at all or the pressure will change slightly and the system will interpret that change as a leak. 

Since people move and breathe when using their respirators on the job, this testing method is not a good correlation to actual use. Even if a test subject manages not to throw off the measurements, this testing method is hard to trust as the seal on a respirator is challenged much more when the wearer is active than when they are still.

So Which Quantitative Fit Testing Method is Best?

Since OSHA adopted the newest Fast Fit Testing Protocols, the biggest con for CNC testing has been eliminated.

With more reliable results, better correlation to real-world application, and fewer ways to skew the results, CNC fit testing methods have become the best quantitative fit testing method available. 

Keep your employees safe using the newest and best CNC quantitative fit testing methods and equipment from Accutec IHS.