The respiratory protective devices (RPD) can protect workers only if their protective properties are adequate to the conditions in the workplace. Therefore, specialists have developed criteria for the selection of proper, adequate respirators, including the Assigned Protection Factors (APF) - the decrease of the concentration of harmful substances in the inhaled air, which (is expected) to be provided with timely and proper use of a certified respirator of certain types (design) by taught and trained workers (after individual selection with a tight-fitting mask and fit testing), when the employer performs an effective respiratory protective device programme.
Background
The different methods of protection from air pollution and their effectiveness The imperfection of technological processes, machines and other equipment can lead to air contamination with harmful substances in the workplace. Protecting of the workers' health in this situation may be achieved by different means, listed below in order of decreasing of their effectiveness: Hierarchy of hazard control 1. The use of alternative substances which are less hazardous. 2. The substitution of a given substance in a form that is less hazardous, e.g. replacing a fine powder by a coarser powder, pellets, or by a solution 3. The substitution of a process by an alternative process likely to generate lower airborne concentrations of substances 4. Total or partially enclosed process and handling systems 5. Partial enclosure with local exhaust ventilation 6. Local exhaust ventilation 7. General ventilation 8. Reducing period of exposure 9. The introduction of appropriate working practices and systems of work, e.g. to close and store containers securely when not in use 10. Use of monitors and warning devices to give a clear indication when unsafe airborne concentrations are present 11. Good housekeeping 12. Provision of a respiratory protective device program
If the use of these methods is impossible, or if their use did not reduce the concentration of harmful substances to a safe value, workers must use respirators. These respirators must be sufficiently effective, and they should correspond to known or expected conditions at the workplace. However, the sole reliance on personal protective equipment for personnel is considered the least effective means of controlling hazards, for reasons including: non-usage of the respirators in the contaminated atmosphere; leakage of unfiltered air through the gaps between the mask and face; and delayed replacement of gas cartridges.
The effectiveness of respirators with different designs Different terms may be used to describe the protective properties of respirators: Penetration = (the concentration of harmful substances under mask) / (concentration outside the mask); Efficiency = ( (concentration outside the mask) - (the concentration of harmful substances under mask) ) / (concentration outside the mask) = 1 - Penetration; Protection Factor (PF) = (the concentration of harmful substances on the outside of the mask) / (concentration under mask) = 1 / Penetration. The term "Protection Factor PF" has been used in the U.S., and the term "Penetration" was used in the soviet literature from the 1960s.
In the first half of the 20th century, experts measured protective properties of respirators in the laboratories. They used different control substances (argon, halogenated hydrocarbon vapour, aerosols of sodium chloride and oil mist, fluorophores, dioctyl phthalate, and others, and they measured their concentrations under the mask, and outside masks (simultaneously). The ratio of the measured concentrations is an indicator of the protective properties of different types of respirators. These measurements showed that if the efficiency of filters is sufficiently high, then the gaps between the mask and the face become the main way of penetration of air contamination under the mask, in the same way that on a cold day, a person wearing a warm jacket and pants will lose most of their heat through the head and extremities.
The shape and size of these gaps is not constant, and depends on many factors (the degree of fit the mask to the face - by shape and size; the correct donning the mask; the mask slippage on the face during the work due to execution of different movements; the design of the mask). The respirator's PF may change dozens of times over several minutes; and the two average PF (that were measured for the same worker in one day; for example - before and after the lunch break) can differ by more than 12 000 times.
Experts believed that the measurement of protection factors in the laboratory allows them to correctly evaluate, predict the RPD efficiency in the workplace conditions. But after the detection of cases of excessive harmful exposure on employees who used high quality respirators with HEPA particle filters in the nuclear industry of the US, the experts changed their opinion. Studies have been carried out to measure the protection factors for the various types of respirators - not only in the laboratories, but also at the workplaces. Dozens of such field studies have shown that the performance of serviceable respiratory protective equipment at the workplaces may be significantly less than in laboratory conditions. Therefore, the usage of laboratory results to assess the real efficiency is incorrect; and can lead to a wrong choice of such respirators that can not reliably protect workers.
Terminology to describe the different PF, and methods for APF development The experts used the results of measurements at the laboratories and at the workplaces to develop more completely terminology for description of the respirators' performance; and this terminology has been applied officially, and in the preparation of research results for publication. Specialists began to use different terms to describe the protection factors, which were measured at workplaces with continuous use of respirators; and measured in the workplace when the workers used of respirators intermittently; measured not in the workplace while fit testing; measured in the laboratories under the simulation workplace's conditions; and for the protection factors, that can be expected (in most cases) when the workers properly used the respirators at the workplace.
The modern terminology for RPD performance description (pp. 22–26) Protection Factor A description of the term Assigned Protection Factor (APF) The minimum anticipated protection provided by a properly functioning respirator or class of respirators to a given percentage of properly fitted and trained users. Fit Factor A quantitative measure of the fit of a specific respirator facepiece to a particular individual (FF are measured during fit test). Simulated Workplace Protection Factor (SWPF) A surrogate measure of the workplace protection provided by a respirator (SWPF are measured in laboratory conditions, that imitate real workplace conditions during "work") Workplace Protection Factor (WPF) A measure of the protection provided in the workplace only while the respirator was properly worn and used during normal work activities (WPF are measured at the workplaces, after fit testing; and without mask doffing during measurement).
A significant difference between respirator performance in the laboratories compared to the efficiency at the workplaces not allowed to use the laboratory results to predict the degree of protection offered in practice. And instability of respirators' protective properties (for the same RPD design, and in the same usage conditions) prevented evaluate their efficiency. For solving these problems, scientists Donald Campbell and Steven Lenhart suggested to use the results of measurements of Workplace PF values for development of Assigned (expected in practice) PF values (APF) - as the lower 95% confidence interval of WPF values. The results of measurements of WPF has been used in the development of APF by ANSI (for the recommended standard, that is not mandatory). The same was made during the development of the APF by OSHA (in the development of the standard, that is mandatory for the employer).
Development of APF values for the various respirator types Results of measurements of WPF in the US and the UK became the basis for the development of APF for UK standard and for English version of EU standard. In some cases, there was no information on the effectiveness for respirators of specific design (type) in the workplace. This is due to the fact that the measurement of workplace PF is very difficult, time-consuming, and expensive work, which was carried out not very often. For these types of respirators experts used the results of WPF measurements of other types of respirators, which are similar. For example, the effectiveness of the Supplied Air Respirators (SARs, with hose) was considered similar to the efficiency of Powered Air Purifying Respirators (PAPRs), if they have the same facepieces and the same air supply mode. Finally, in the absence of this information, specialists could use the results of Simulated WPF measurements; or estimates of competent experts.
Correction of Assigned PF values Measurement of workplace protection factors surprisingly revealed the low efficiency of some designs of respirators, and that results have led to a sharp tightening of the requirements for application limits for respirators of such designs.
PAPR with helmets or hoods In a 1984 study by Myers et al., the WPF measurements for Powered Air Purifying Respirators (PAPRs) with helmets (that is not tight-fitted to the face) showed that the ingress of harmful substances in the inhaled air can be very high (PF = 28 and 42 for two models). This came as a surprise, since earlier studies in the laboratory showed that the flow of clean filtered air from the inside to the outside of the helmet prevents ingress of harmful substances under the helmet (PF > 1000). Additional studies, from 1986 and 1981, agreed with the result from the Myers et al. 1986 study: the minimum values of the workplace protection factors of two models of respirators were 31 and 23; and leakage of unfiltered air attained 16% in some cases in wind tunnel at 2 m/s air velocity
Therefore, the use of such RPD types was limited 25 PEL in the United States, and 40 OEL in the UK.
Negative pressure full face masks Measurement of protection factors of negative pressure full face masks with high-efficiency filters in the laboratory revealed a risk of decrease in protective properties to a small values. Therefore, the use of such respirators has been limited to the values 50 or 100 PEL in the United States. However, the experts in the UK believed that the quality of their masks is higher than American masks, and were allowed to use up to 900 OEL. But the study showed that the value of the protection factor of > 900 has been achieved in practice infrequently. Minimum protection factors of 3 different models of full facepiece respirators were 11, 18 and 26. So, the new standards limit usage of these respirators up to 40 OEL in UK (after this study).
Negative pressure half mask respirators (after fit testing)