5 Reasons to Use Controlled Negative Pressure Fit Testing Over CNC

What is Controlled Negative Pressure Fit Testing and Condensation Nuclei Counter Fit Testing?

Controlled Negative Pressure (CNP) and Condensation Nuclei Counter (CNC) fit testing are both quantitative methods of fit testing respiratory protective equipment (RPE). Under WHS regulation 44, personal protective equipment including RPE must be of a suitable size and fit for the worker as well as a reasonably comfortable fit. Fit testing is also required for all tight fitting RPE, whether it is disposable, reusable, half face, or full face. CNP fit testers such as the QuantiFit2 Respirator Fit Tester create a negative pressure and measure airflow while the person tested holds their breath to determine if there is a leak into the respirator. With CNC fit testers such as the PortaCount Respirator Fit Tester, the leakage of ambient aerosols into the mask is measured using a probe. When the particles counted inside the respirator are low enough, the mask is deemed safe. Both methods have been approved by RESP-FIT, an AIOH program for respirator fit testing training and accreditation.

While both fit testing methodologies are suitable for adhering with regulation, they each possess their own set of advantages and disadvantages, with the benefits of the newer CNP technology often being underestimated. Here are five reasons why we recommend using CNP over CNC for fit testing full face and half face respirators.

1 – CNP Fit Testers are Exceptionally Accurate and More Sensitive than CNC Fit Testing Devices

CNP measures the actual leakage of air into a respirator, whereas CNC measures the concentration of particles travelling through the air. This means that CNC is reliant on a variety of factors such as whether there are enough particles to count, what path these particles take into the mask and lungs, whether the probe is positioned along this path, what size the particles are and if there are particles coming from the wearer’s lungs. Compared to this, CNP uses air directly as a challenge agent, making these factors irrelevant. Given that particles travel through the air, it is more accurate to directly measure the flow of air rather than the particles that are carried by that air. In CNP fit testing, it is important that the person tested holds their breath and remains stationary for the duration of the 10 second tests to ensure the results are valid.

There is a misconception with CNP fit testing that it can be less reliable due to it requiring the wearer to avoid breathing and moving during the test, however the wearer is only required to hold their breath for 10 seconds, and CNP fit testing protocols require them to perform various exercises to ensure a proper fit is achieved.

In one particular case, the NSW police had their respirators passed by CNC, but fail in a CNP test. Out of the two masks that failed, one was found to have a blade of grass in the drinking tube, whereas the other had a small leak where there was a failed seal. Sometimes cracks and leaks in the respirator can be so small that particles cannot fit through, meaning that CNC cannot detect these leaks. CNP, however, uses air, meaning that aside from the fit of the mask, it can also determine the integrity of the respirator with exceptional accuracy and reveal any compromises before a worker wears it into the field, relying on its protection.

2 – CNP Allows for Real-Time Adjustments to the Fit of the Respirator

With the OSHA approved CNP REDON Protocol, the wearer is required to follow three test exercises and redon their respirator twice. By redonning the respirator between tests, this protocol ensures that the respirator will fit the wearer properly when used in the field, which is vital given that regardless of how effective the respirator is, its protection will become redundant if the mask is not worn correctly. Because the fit test is performed in real-time, any necessary adjustments to the respirator’s fit can be made immediately. For example, if the test indicates a leak around the nose area, the individual can adjust the nosepiece or try a different size respirator to achieve a better seal. The test can be repeated after each adjustment to ensure that the fit has been improved.

In comparison, to conduct a fit test correctly with a CNC fit test device, the subject cannot remove the respirator mid-test and is required to wear their mask for 5 minutes before testing and 30 minutes after eating as stated in the user manual. This is because CNC measures particles, and removing the mask would let particles in and ruin the test. This is also why 5 minutes is needed to purge the mask of particles from outside. Fit testing with CNC technology requires a 5 minute comfort assessment period under the following standards:

• ISO 16975-3 Section 6.6.2
• OSHA 1910.134 Appendix A Part I.A.5
• ANSI Z88.10-2010 Section 6.6.2

This period allows for a proper, more comfortable fit to be achieved. The reason for this is that even if a respirator is passed, if the mask is too tight or uncomfortable for the wearer, they are likely to remove and adjust the respirator, after which the fit test would not necessarily be reliable anymore. This is why a 5 minute comfort assessment period is not necessary when following the CNP REDON protocol, which already involves having the wearer redonning their mask multiples times during testing to gauge a proper and realistic fit.

3 – CNP is More Consistent and Less Affected by Environmental Conditions than CNC

Since CNC counts ambient particles to determine respirator leakage, environmental conditions such as temperature, humidity and anything that can affect the presence of particles in the air can impact the accuracy of results. This means that tests generally cannot be conducted in the field, as they need to be carried out in a controlled environment. Since CNP uses air, these conditions do not affect its measurements. Regardless of whether fit testing is conducted indoors or outdoors, it can produce consistently accurate results, making it easier to fit test in any location.

4 – CNP Fit Testing is Faster and Easier to Conduct than CNC

Depending on the protocol followed, CNC fit testing can be conducted in approximately 2 minutes when following the OSHA Standard, or 7 minutes when following the ISO Standard. CNP fit tests can similarly be completed in around 2 minutes, meaning that both methodologies can be conducted in approximately the same amount of time.

Although the fit testing time between the two methods is comparable, CNC still requires a 5-minute comfort assessment period and 30 minute gap after consuming food, whereas testing with CNP technology does not. Unlike CNC testing, CNP tests only need an adapter attachment to the respirator, while CNC requires the operator to insert a probe through the mask into the breathing zone. This process is more complex, time-consuming, and challenging, especially for intricate masks, making CNC fit testing generally take longer to conduct.

In addition to these protocols, the US military has developed their own CNP fit testing method called the Military Protocol. This protocol only takes 1 minute to complete, making it convenient for frequent testing with minimal disruption.

5 – CNP Fit Testing: A More Cost-Effective Fit Test Methodology Compared to CNC Fit Tests

When conducting CNC fit tests, consumables such as aerosols and wicks are required to ensure that there are enough particles to conduct a reliable test, along with probes to count these particles inside the mask. In comparison to this, CNP fit testing only uses air, and hence does not require any consumables. This makes CNP the more cost-effective option, especially in the long term where consumables would need to be purchased several times. Aside from this, other indirect costs incurred can include time or training, with CNC tests generally taking longer and having more requirements and conditions that need to be fulfilled to be conducted correctly.

Does This Mean That I Should be Using CNP for Fit Testing?

While CNP technology can offer numerous benefits over CNC fit tests, it's important to note that both methods are compliant for fit testing. One benefit CNC does have over CNP fit tests is that it is suitable for testing disposable masks, while CNP fit testing is currently unable to. Therefore, it is advisable for companies that need to fit test full or half face respirators and disposable masks to utilise both methods, further maximising efficiency and worker safety. On the other hand, if fit testing is only needed for reusable respirators, CNP can be more beneficial and may be the only technology required.