RPE fit testing: choosing the correct method

Face fit testing plays a crucial role in ensuring that respiratory protective equipment (RPE) is effective and provides an adequate seal between the mask and the face when worn by the user. Respirable hazards, such as construction dust and cleaning chemicals, pose a real 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.

Health and Safety Executive (HSE) figures show that every year in the UK, around 17,000 people develop incurable lung diseases as a result of exposure to inhalable substances at work, and there are around 12,000 occupational lung disease deaths. That’s seven times more than the number killed in road traffic accidents and 90 times more than the number of fatal workplace accidents.

We can and must do better at protecting our workers from respiratory hazards and facemask fit testing is an important element in any robust respiratory protection programme. So how can – and should it – be done?

Two methods of face fit testing are commonly used to ensure RPE provides effective protection:
1. Qualitative fit testing – QLT
2. Quantitative fit testing – QNT.

Quantitative fit testing can in turn be split into two sub-methods:
a.  Ambient particle counting (APC) (sometimes known as CNC – condensed nuclei counting)
b. Controlled negative pressure (CNP).
This article will explain the key differences between these two methods to help duty holders choose the most appropriate one for their needs.

Qualitative testing

Qualitative testing involves subjecting the wearer, whilst wearing the mask, to an aerosol as a test agent that can be detected by the wearer but cannot penetrate through the filter of the mask. If the wearer detects the agent, then it is clear the mask is not effectively protecting them from inhalation exposure. However, if the wearer fails to detect the agent, then it can be assumed this is because the mask (and therefore also the seal to the face) is effective.

Every year in the UK, around 17,000 people develop incurable lung diseases as a result of exposure to inhalable substances at work. Photograph: iStock

As the name suggests, qualitative testing assesses the quality of the seal, but it does not quantify its performance. The performance of the face seal is a concept that is not clearly defined in the qualitative test method, and this is explained in more detail further below. A qualitative fit test results in a pass or fail based on the subjective opinion of the wearer (i.e. whether or not they report tasting the test agent).

However, the person conducting the test needs to have a thorough understanding of the various reasons why the wearer may not respond to the test agent, regardless of whether the mask is performing as intended – i.e. whether it fits correctly and is providing an adequate face seal.

The validity of a qualitative test relies heavily on the competence of the tester and their diligence in following the required process and protocols accurately and effectively. For example, if the tester fails to correctly generate and apply the test aerosol then this may be the reason why the wearer does not report any taste sensation.

Quantitative fit testing – APC/CNC testing

When conducting an ambient particle counting (APC) test, the concentration of naturally occurring or generated particles in ambient air (i.e. outside the mask) is measured and then compared with the concentration of particles inside the mask.

Clearly, if these two measurements are the same then the test has demonstrated the mask is very ineffective. If the mask is fitted with a suitable filter and the mask is in good condition, then any particles found inside the mask can be attributed to face seal leakage. Therefore, the ratio between the concentration of particles in ambient air (outside the mask) and concentration of particles inside the mask provides a ‘quantified’ measure of face seal leakage or fit.

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.

If any particles are found inside the mask, it is important the tester does not automatically assume these have leaked in, since it is not unusual for the user’s exhaled breath to contain some particulate. It is impossible to discount these particles from the in-mask sample, making it difficult to achieve a true reading during the test. As a result, to ensure accurate results, the wearer should refrain from certain activities – such as eating or drinking – for approximately 30–60 minutes before the test is conducted.

It is not unusual for the mask user’s exhaled breath to contain some particulate. Photograph: iStock

Depending upon the test protocol being followed, the tester will also ask the user to perform a set of physical exercises during the fit test so the performance of the mask’s seal can be assessed during movements that simulate actions the individual is likely to carry out in the workplace. This is sometimes known as dynamic testing.

CNP testing

During a controlled negative pressure (CNP) test, a device is used to generate and maintain a constant negative pressure inside the mask whilst it is being worn. If the pressure changes, it can be assumed this is due to leakage into the mask. To maintain a constant negative pressure inside the mask, the test machine continuously ‘sucks’ air out of the mask – if the pressure remains the same then the volume of air sucked out of the mask must equal the volume of air that is leaking into it.

By measuring the volume of air that leaks into the mask over time, a fit factor can be calculated using the ratio of the predicted inspiratory flow rate and the measured leakage flow rate.

QLT vs APC vs CNP testing: which method should you choose?

QLT testing is very widely used because the equipment required is relatively inexpensive and readily available, and for those carrying out tests, it appears the test method is very easy to carry out. However, while a QLT test is quite a simple process, it is equally easy to carry it out incorrectly.

Incorrect application of a QLT test will nearly always will result in a ‘false pass’, and this is clearly unacceptable, because the user will be at risk of inhaling harmful contaminants that leak into the mask due to the poor fit and the poor seal between the mask and their face. As a result, it is essential the person conducting the test is properly trained and understands all the potential reasons why a false pass may occur. In essence, the tester needs to have the necessary skills and knowledge to ensure that if the wearer fails to respond to the introduction of the test agent into the mask area, this is solely because the respirator fits correctly and therefore is capable of providing the level of protection expected.

The QLT test method is suitable for disposable and reusable half masks, but not suitable for testing full-face pieces (i.e. full-face masks). Full-face pieces are generally considered to offer a higher level of protection from the inhalation of hazardous substances and this is mostly because they provide a better quality of fit to the wearer’s face. In short, full-face pieces leak less but the QLT test method is not sensitive enough to effectively measure very small amounts of leakage. As a result, fit testing of full-face masks should only be carried out using one of the much more sensitive QNT methods.

APC (CNC) testing is a method of QNT testing and is widely used in the UK, possibly because this method is very similar to the reference method of testing mask fit typically used in a laboratory environment. For many fit test personnel, simply understanding the concept of measuring particles as a means of testing the performance of a mask designed to protect the wearer from particles is a much easier process.

APC testing also means the tester can employ ‘dynamic testing’ methods – where the wearer conducts physical exercises that simulate their likely physical movements at work when exposed to the respiratory hazard. In turn, this provides the tester with a perceived good level of confidence about how the mask will perform when used in the actual workplace.

CNP testing is considered to be a much faster way of carrying out a fit test because it involves less ‘dynamic testing’. This is because during a CNP test the measurement of any leakage can only be taken whilst the wearer remains still and holds their breath. If the protocol being followed calls for any movements, then these are carried out for a period of time before the measurement is taken, or the measurements are taken whilst the wearer holds a static position, such as looking up or down. In contrast, during APC testing, in-facepiece samples can be tested while the wearer undertakes exercises like talking and turning their head from side-to-side.

Because a CNP test does not measure the facepiece fit while the wearer carries out physical movements, there are questions about its effectiveness in predicting the workplace performance of a facepiece. To measure small amounts of leakage into the mask (and there is always some), the pressure device used for a CNP test has to be incredibly sensitive and if the wearer moves during a test measurement, the inner mask pressure can be affected and this can influence the test result.

Paul Salisbury is director & respiratory protection specialist at Fire Safe International Ltd