APC Vs CNP Quantitative Testing: Choosing The Right Method For You

Two methods of quantitative Face Fit testing are commonly used to ensure RPE fits securely:

1. Ambient Particle Counting (APC) (sometimes known as CNC – Condensed Nuclei Counting),
   and
2. Controlled Negative Pressure (CNP).

In this article, we’ll discuss the key differences between these two to help you to choose the right one for your needs.

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 workplace. 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's 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 predicted inspiratory flow rate and the leakage flow rate.

APC Vs CNP Quantitative Testing: Which Should You Choose?

APC testing tends to be much more widely used, possibly because this is closely associated with the Reference Method of testing mask fit, first established by the US Airforce following anthropometric surveys of air force personnel in 1967 and 1968. This reference method was updated and expanded through a new anthropometric survey of a representative panel of American civilian workers in 2003,  updating it and expanding its relevancy to individuals outside of the Armed Forces.

The reference method used a series of 10 facial dimensions (adjustable by age and demographic background) associated with poor respiratory fit and leakage to create a standardised mask fit testing method expected to accurately accommodate up to 95% of the civilian workforce.

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 in mask performance in the workplace. 

CNP testing, however, because it is less dynamic is usually a much 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 moves at all (or wiggles their tongue) during the measurement phase of the test process, the measurement might not be valid. 

As CNP testing requires a pressure difference between the air inside the mask compared with outside it is not possible to test disposable type respirator masks (FFP/N95) using this method.  It is only suitable for elastomeric half and full face masks where the filter can be removed and replaced or for masks used with breathing air. 

The APC method can be used to test all types of respirator mask but as this measures particulate, it is important to ensure that the mask is fitted with a particulate type filter.  There are different types of particle filter (some filters capture more particles than others, see our blog on filter efficiency and how FFP masks work) and if the filter fitted is not 100% efficient (P3/P100) then it is likely that particles might enter the mask through the filter;  this being the case then it will be necessary to use a more specialist APC/CNC test device fitted with a particle classifier, often known as N95 technology, such as the AccuFIT Pro or Portacount 8038 or 8048.

Being quantifiable and therefore less subjective and data driven, the quantitative fit testing (QNFT) method is definitely the preferred method over qualitative testing methods by most however it is not as straightforward as pressing a button on a machine.  It is vital that the operator is properly trained in its application and principals of operation.

Find Out More

At Fire Safe International, our experienced Fit2Fit accredited instructors provide professional APC fit test training and training in the use of all types of fit test equipment. To find out more, please call us on 01743 761000.

Image Source: Fit2Fit