Face the facts

FACE FIT testing plays a crucial role in ensuring that Respiratory Protective Equipment (RPE) is effective and provides an adequate seal when worn by the user. Respirable hazards, such as construction dust and some cleaning chemicals, pose a risk to human health if they are accidentally inhaled, and a poorly fitting mask can expose a worker to danger by allowing the contaminated air to breach the barrier and enter the user’s lungs.

Two methods of quantitative face fit testing are commonly used to ensure RPE provides effective protection: 
1. Qualitative Fit Testing
or
2. Quantitative Fit Testing
Quantitative Fit Testing can in turn be split into two methods
a. Ambient Particle Counting (APC) (sometimes known as CNC – Condensed Nuclei Counting),
and 
b. Controlled Negative Pressure (CNP) testing. 

Let's look at the differences between these to help you to choose the right one for your needs.

Qualitative testing

Qualitative testing involves subjecting the wearer, while wearing the mask, to an agent that can be detected by the wearer but also to which is challenged by the respirator mask and filter. If the wearer fails to detect the agent, then it is assumed that is because the mask (and therefore also the seal) is effective.  

As the name suggests, this assesses the quality of the seal, it does not quantify its performance. Performance of the seal is a concept that is not clearly defined in this test method as we will review later. The test results in a pass or fail result based on the subjective opinion of the wearer but it is important for the tester to be aware of the many reasons why the wearer may not respond to the test agent, regardless of the mask performance.

APC/CNC Testing

In Ambient Particle Counting testing, the concentration of particles inside and outside the facepiece is measured. Often the user will perform a set of specific exercises at the same time so that the performance of the seal can be ascertained during movements simulating the work place. This is sometimes known as dynamic testing. By comparing the two measurements (inside the mask with outside the mask), the Fit Factor can be calculated. If P3 filters are installed in the mask and the fit test is performed correctly, any particles detected inside the mask can be attributed to leakage around the seal.

However, APC testing relies on there being a suitable quantity of ambient particles to challenge the mask; if there are no particles outside the mask, none will leak in and therefore it is impossible to measure the leakage. It is also assumed that any particle detected inside the mask has leaked in, however it is not unusual for the user’s exhaled breath to contain some particulate which is impossible to discount from the sample making a true reading difficult to confirm. To ensure accurate results, the wearer should refrain from eating or drinking for approximately 30-60 minutes before the test is conducted.

CNP Testing

Controlled Negative Pressure tests generate and maintain a constant negative pressure inside the wearer's mask. If the pressure changes, this may be as a result of leakage and so to maintain a constant pressure, a constant flow of air will need to be sucked out of the mask. If the pressure is maintained, then the flow of air out of the mask will be equal to the flow of air into the mask and so the rate of leakage can be measured. The fit factor can then be calculated using the ratio of the inspiratory flow rate and the leakage flow rate.

Which Should You Choose?

QLT testing is very widely used as the equipment required is relatively inexpensive, is readily available and it looks like it is very easy to do; it is, but it is also even easier to do it wrong! Incorrect application of this test method nearly always will result in a “false pass” – this is clearly unacceptable and for this reason it is imperative that despite the apparent ease of use the tester must be properly trained and understand all of the potential causes of a false pass to ensure that the only reason the wearer does not respond to the test agent is because of the respirator. This test method is also not suitable for testing full face pieces. 

APC testing is also widely used, possibly because this is closely associated to the reference method of testing mask fit typically used in a laboratory environment. For many, simply understanding the concept of measuring particles (which in many cases is in fact the workplace hazard) is much easier to do. It also allows for dynamic testing so for many this provides a good level of confidence of mask performance in the workplace. 

CNP testing however, because it is less dynamic can be a faster means of completing a test and it does not rely on ambient particles.  The lack of movement however is sometimes perceived as the main challenge – to measure such small amounts of leakage (and there is always some) the pressure device has to be incredibly sensitive.  The wearer must stand perfectly still and hold their breath for at least 8 seconds for a measurement to be taken; if the wearer so much as wiggles their tongue during the measurement phase of the test process, the measurement might be invalid.  This means proving a high fit factor (low leakage rate) is very challenging so this method might be considered more suitable for masks not needing to demonstrate extremely low levels of leakage, especially in areas where the ambient particle count cannot be maintained.

The good news is that there is a choice of methods available and likely more will come on line in the near future meaning access to fit testing will become even easier improving our overall understanding and application of RPE in the workplace and so worker overall health and well being. The most important message is that which ever method you choose, ensure that it is executed by a competent person; a person who understands the principals and technology being applied to conduct the fit test and overall understands all of the reasons why the test might fail – a false pass is still a fail!

Paul Salisbury is a respiratory protection specialist at Fire Safe International. For more information, visit www.firesafeinternational.com