# NIOSH Recommended Guidelines for Personal Respiratory Protection of Workers in Health-Care Facilities Potentially Exposed to Tuberculosis/IV. Considerations in the Selection of Respirators

​IV. Considerations in the Selection of Respirators
A. Nature of the Hazard to Workers—In considering appropriate personal respiratory protection for health-care-facility workers potentially exposed to tuberculosis, NIOSH considered multiple issues pertaining to the hazard presented to these workers by exposure to aerosolized droplet nuclei in the workplace. These issues included, but were not limited to, the following:
1. The risks of acquiring and medical consequences due to tuberculosis infection (e.g., risk of developing clinical tuberculosis) (e.g., 61,62).
2. The efficacy, benefits, and risks of chemoprophylaxis with isoniazid of those infected with tuberculosis (e.g., illness due to INH$^+$-induced hepatitis, death from hepatitis) (e.g., 61,62,63,64,65,66,67).
3. The risks and medical consequences of developing active tuberculosis (e.g., risk of death due to tuberculosis in treated and untreated infected persons, risk of transmitting tuberculosis to co-workers, family members, patients or clients, and the general public) (e.g., 61,62).
4. The nature of transmission and the relative risk of transmission due to the aerosolization of droplet nuclei from transmitters with differing generation rates of infectious tuberculosis particles. These were appraised for transmitters at varying locations and undergoing varying procedures in health- care facilities (e.g., 22,19,23,24,25,26,28,29,39,68).
After considering these issues, it was concluded that any tuberculosis infection in a health-care-facility worker due to occupational transmission should be considered unacceptable. Infection of health-care-facility workers with tuberculosis, whether with or without clinical disease, constitutes a preventable impairment of the health of these workers. Additionally, chemoprophylaxis of tuberculosis-infected workers with isoniazid (INH$^+$) poses further significant risks due to isoniazid-related hepatitis and other potential side effects.
The rationale for isoniazid chemoprophylaxis for both those infected and not infected with tuberculosis is to reduce the probability that infected persons will develop active tuberculosis (75,76):
When taken as prescribed, isoniazid preventive therapy is highly effective in preventing latent tuberculous infection from progressing to clinically apparent disease. In controlled trials conducted by the Public Health Service in ordinary clinical and public health settings, isoniazid preventive therapy reduced the incidence of disease by 54%-88%. The main reason for the variation in efficacy appears to have been the amount of medication actually taken during the year in which isoniazid was prescribed.
Others have described the limitations of isoniazid prophylaxis as follows (64):
Aside from toxicity, which is infrequent but potentially serious, the inconvenience and the lack of motivation for an apparently healthy person to accept long-term medication [6 to 12 months] pose formidable obstacles to preventive therapy programs. . . .
Preventive therapy is inefficient. Among newly infected persons, only about 10% will develop disease during a lifetime, but there is currently no reliable way to distinguish the 10% who will develop disease from the 90% who will not. Thus, 10 or more persons must be given preventive therapy to prevent one future case of tuberculosis.
If isoniazid chemoprophylaxis reduces the incidence of clinical disease by only 54%-88% (76), then 11 to 18 persons must be given isoniazid to prevent one future case of active tuberculosis.
In 1992, Snider and Caras reviewed the most serious hazard of isoniazid chemoprophylaxis, death from isoniazid-associated hepatitis (62):
Despite the limitations of this survey, we believe the following tentative conclusions are warranted: (1) As suggested by Dash and colleagues, deaths due to INH-associated hepatitis are probably less frequent now than in the early 1970s, but they are still occurring. Efforts to carefully select and monitor patients on INH preventive therapy [prophylaxis] must be continued; (2) Women may be at increased risk of death from INH-associated hepatitis. Therefore, women taking INH should be carefully monitored for hepatotoxicity and preventive therapy recommendations for women should be reconsidered; (3) As suggested by Franks and colleagues, the postpartum period may represent a period when women are especially vulnerable to INH hepatotoxicity; it may be prudent to avoid INH during the postpartum period or at least to monitor postpartum women more carefully; (4) Additional research is needed to identify groups at risk of death from INH-associated hepatitis, to quantify this risk in relative and/or absolute terms, and to identify cofactors that may influence the risk; (5) Better surveillance for INH-associated hepatitis death is warranted.
NIOSH concludes that any use of isoniazid chemoprophylaxis as a substitute for implementing all administrative, engineering, and personal respiratory protection controls indicated for protecting workers in a health-care facility from infection with tuberculosis transmitted in the facility is inconsistent with the rights of workers and obligations of employers established by the Occupational Safety and Health Act of 1970.
C. Hazardous Face-Seal Leakage—A proper seal between a respirator's sealing surface and a wearer's face is absolutely essential for effective and reliable performance of any respirator with negative pressure inside the facepiece. It is much less critical, but still important, for a positive-pressure respirator. Hazardous face-seal leakage can result from factors such as incorrect facepiece size or shape, incorrect or defective facepiece sealing-lip, beard growth on a wearer, perspiration or facial oils that can result in facepiece slippage, user failure to use all the headstraps, incorrect positioning of a facepiece on a wearer's face, incorrect headstrap tension or position, improper mask maintenance, and mask damage.
To assure an adequate seal, quantitative fit tests must be performed periodically and accurately to detect face-seal leakage. Fit tests help ensure that a respirator can provide adequate protection on each wearer and that it is fitted properly to each wearer's face. However, fit tests can detect only the hazardous face-seal leakage that exists at the time of the fit testing. Also, fit tests do not detect hazardous leakage through the filter.
An additional benefit of quantitative fit tests is that the screening cutoff value in these fit tests can be adjusted to assure very low face-seal leakage considerably less than 2% (81,82). For example, when quantitatively fit testing, NIOSH uses a screening value of 0.2% leakage for the non-powered operational mode of powered, HEPA-filter, halfmask respirators to assure less than 2% leakage in the powered mode of these respirators.
Because point-of-use factors can create a considerable risk of undetected hazardous leakage past a face seal when a respirator is worn in a hazardous environment, each wearer must have the capability of effectively and reliably fit checking his or her respirator for proper fit before every respirator use. This is the purpose of fit checks that must be performed by users each time they don their respirator. The rationale for and the essential nature of both fit tests and fit checks are summarized in Table 1 on page 20.
[Table 1]
Table 1—Requirements for and Essential Roles of Fit Tests and Fit Checks
* 1—A qualified representative of an employer must decide for which workers personal respiratory protection is indicated using the guidance given in Table 3 starting on page 40.
* 2—A qualified representative of the employer must identify the "best-fitting" make and size respirator from several different brands and sizes (generally three different sizes are necessary for each brand of respirator). This selection should be done using quantitative fit test(s) (QNFT). Powered masks should be tested and selected while operating in the nonpowered mode.
* 3—A qualified representative of an employer must then accurately fit-test screen, with the same QNFT from step #2, the face-seal protection afforded to each prospective wearer by the face seal of the respirator identified in the previous element as "best fitting." This screening must accurately detect ("diagnose") those respirator-wearer combinations that will not yield substantial protection on the prospective wearers. Powered respirators should be tested and selected while operating in the nonpowered mode. No filter testing is performed, since it is reliably assumed that the HEPA filters to be worn on the facepieces will have essentially zero leakage. The qualified representative of the employer must also periodically retest the fit of each assigned respirator on its wearer with the QNFT.
* 4—A qualified representative of an employer then provides and assigns a respirator make and size to those prospective wearers that passed the preceding QNFT screening.
* 5—Each worker must then (1) decide to wear their respirator, (2) take action to don their respirator, and (3) must properly adjust their assigned respirator on their head and face before each and every entry into any location or before performing any procedure as indicated in Table 3 starting on page 40.
* 6—Each worker must then perform an accurate fit check at the point of use before each use of their assigned respirator. Fit checks are very simple tests compared to the QNFT performed by a qualified person in steps #2 and #3. The fit check must be done to identify ("diagnose") those respirator "fittings" (respirator facepiece position and headstrap adjustments) not providing substantial protection due to point-of-use factors that are preventing a proper fit (e.g., incorrect respirator position on the user's face, incorrect headstrap tension, incorrect headstrap position on and behind the user's head, user failure to use all the headstraps, changes in a user's facial surface such as facial stubble and perspiration, respirator damage, improper respirator maintenance, or beard stubble).
* 7—Each worker seeking personal protection must properly wear their assigned respirator in any location or before performing any procedure indicated in Table 3 starting on page 40. They must not wear their respirator when conditions prevent a proper seal of the facepiece to the wearers skin. For respirator-related causes (e.g., respirator malfunction, detection of room-air leakage at their face seal into the respirator), they must (1) decide to leave the location or procedure and then (2) take action and leave.
With regard to hazardous face-seal leakage, all non-powered filter masks (e.g, surgical masks and disposable PRs including disposable HEPA-filter respirators) have an inherent deficiency that markedly reduces the level and reliability of personal protection these devices can deliver even when correctly used under ideal conditions. During each inhalation by a wearer, a negative pressure (relative to the workplace air) is created inside the facepiece of this type of respirator. Due to this negative pressure, air containing aerosolized droplet nuclei can take a path of least resistance into the respirator—through leaks at the face-seal interface—thus avoiding the higher-resistance filter material. Additionally, the filter material creates a resistance to the wearer’s breathing, which results in physical discomfort, perceptible increases in the work of breathing, and impaired verbal and nonverbal communications (83).
In contrast to non-powered filter respirators, powered respirators (with HEPA filters) have a design advantage that markedly increases the level and reliability of personal respiratory protection these respirators can deliver under real-world conditions. A powered filter respirator produces a positive pressure inside the facepiece under most conditions of use. These respirators deliver a forced airstream to the facepiece using a battery-powered blower. The blower forcibly draws ambient air through HEPA filters, then delivers the filtered air to the facepiece. This air is blown into the facepiece at volumetric flow rates ranging from 115 to 170 L/min (4.1 to 6.0 cubic feet per minute). These flow rates exceed the vast majority of inhalation flow rates expected in workers needing personal protection against droplet nuclei. The small positive pressure inside the facepiece reduces face-seal leakage to very low levels, particularly during the relatively low inhalation rates expected in healthcare settings. NIOSH conservatively estimates that these respirators have less than 2% face-seal leakage under routine conditions (57). Thus, a powered filter respirator offers
D. Hazardous Leakage Through Filters—Aerosol leakage through filter media is dependent on at least five types of independent variables (84):
Respirator filter media other than HEPA filters (e.g., surgical masks, dust and mist filters, or fume filters) have widely varying efficiencies against aerosols less than about 2 to 4 µm (85,86,87,88). Only HEPA filtersare certified to provide to provide the highest possible efficacy against aerosols smaller than 2 to 4 μm. For HEPA respirator filters, the NIOSH certification performance standard requires these filters be at least 99.97% efficient (i.e., leakage must be less than or equal to 0.03%) against the most filter-penetrating aerosol size (approximately 0.3 µm) (80). NIOSH certifications for dust and mist filters, and fume filters, do not permit their use for protection against highly toxic substances (i.e., those substances with exposure limits less than 50 micrograms per cubic meter) (80). In contrast, HEPA filters have been previously recommended for general ventilation air that is recirculated from the rooms of known or potential tuberculosis transmitters (15) and general-use areas in health-care facilities (10).
When HEPA filters are used on an air-purifying respirator, filter efficiency can be reliably assumed to be effectively 100% and hazardous filter leakage is not a consideration. Hence, for all HEPA-filter respirators, the potential for inward hazardous leakage of droplet nuclei ​is essentially that which occurs at a mask's face seal. In marked contrast, with both surgical masks and NIOSH-certified, disposable, particulate-filter respirators (PRs), one must accept the likelihood of some hazardous leakage through the filter that adds to the hazardous leakage at the face seal.
E. Powered, HEPA-Filter Halfmask and Positive-Pressure, Air-Line, Halfmask Respirators—Available NIOSH-certified, powered, HEPA-filter respirators can supply a constant flow of HEPA-purified air under positive pressure for a period of 8 hours with a fully-charged battery pack. This type of filter respirator is also known by the general term powered, air-purifying respirator or PAPR. The specific type of PAPR discussed in these recommendations can be referred to as a "halfmask HEPA PAPR." Two examples of this respirator type are shown in Figures 1 and 2 on pages 34 and 35. NIOSH conservatively estimates that these respirators have less than 2% face-seal leakage under routine conditions (57).
The tight-fitting, elastomeric facepieces and breathing-hose assemblies of these respirators are small and relatively lightweight. The total weight of these devices can go to 5 to 6 pounds, most of which is in the belt-mounted battery, blower, and HEPA-filters assembly. These respirators are designed for continuous use in temperatures ranging from 40°F to 120°F.
The forced, HEPA-filtered airstream flowing into the facepiece of a powered HEPA-filter respirator offers the advantage of a cooling effect in conditions of warm temperatures (this can be a disadvantage for use in cold temperatures). More important, because minimal inhalation effort is needed by the wearer to draw air across the HEPA filters, breathing in a powered respirator is substantially more comfortable than in a non-powered filter respirator (e.g., NIOSH-certified, dust, fume, and mist (DFM) filter respirators).
In use against non-biological aerosols, HEPA filters are routinely replaced only when: (A) airborne materials load them to a point that the flow to the facepiece is not adequate to provide positive pressure or (B) physical damage occurs to a filter. However, in health-care ​settings as compared to the dusty industrial environments for which these respirators were originally produced, there should be minimal "loading-size particulates" in the air. Thus, in theory, the HEPA respirator filters could provide a useful life of weeks to months. These respirator filters would normally have to be replaced on about the same frequency as the HEPA filters in the ventilation systems. Before each use, the outside of each HEPA filter should be inspected for physical damage. Biological contamination of HEPA respirator filters should not be a concern, since once any bioaerosols impact on the filter media they are not readily reaerosolized.
Positive-pressure, air-line, halfmask respirators are recommended in Table 3 starting on page 40 as the minimal acceptable devices for a limited number of procedures where the potential for aerosolization of droplet nuclei containing tubercle bacilli is high (e.g., bronchoscopy). These devices are also referred to as pressure-demand, air-line, halfmask respirators. An example of this respirator type is given in Figure 3 on page 36. NIOSH conservatively estimates that these respirators have less than 2% face-seal leakage under routine conditions (57). Additionally, the protective reliability of these respirators is substantially higher than that of powered, HEPA-filter, halfmask respirators because these devices can deliver a higher air flow to a facepiece at a higher positive pressure. Additionally, these respirators do not depend on a battery-powered blower to force clean air into their facepiece. Since no filters are used with these respirators, there is no potential for hazardous filter leakage through the rare occurrence of a damaged or improperly manufactured filters.
F. Practical Disadvantages of the Recommended Respirators—NIOSH recognizes that the respirators discussed in the preceding section IV.F have some practical disadvantages when compared to the disposable particulate respirators discussed in the next section (IV.G starting on page 27). Due to the use of powered air forced into their facepieces, these devices generate some background noise and impair voice communication to some degree, and affect a wearer's range of motion. Also, initially these respirators may present an "intimidating" appearance to patients accustomed to the surgical masks currently used in health-care facilities. These characteristics may affect patient care.
Other drawbacks of powered HEPA-filter respirators include the fact that the battery assembly must be recharged for at least 8 hours after each 8 hours of use. However, this problem can be minimized by purchasing multiple battery packs, dual-rate chargers, and establishing a charging station near the locations and procedures that require respirator usage. Additionally, several types of periodic maintenance are required for a powered respirator. The elastomeric facepieces must be periodically cleaned and disinfected, since these facepieces are not discarded after each use. However, extra halfmask facepieces (available in up to two halfmask models, three facial sizes, two types of elastomeric materials, and two headband types) can be purchased at less than $20 each for assignment to individual workers. Thus, one blower-filter unit can be used for numerous workers at different times. It is not necessary to purchase one complete respirator for each health-care-facility worker.
Also, the breathing hose and facepiece assembly must be periodically inspected for damage or malfunction. The blower must be inspected for adequate delivery of air to the facepiece. Other possible disadvantages are the weight and encumbrance of the battery, blower, and filter assembly, which must be worn on a belt at waist level with a 30-inchlong, corrugated breathing tube connected to the facepiece.
Positive-pressure (a.k.a. pressure-demand), air-line, halfmask respirators require an air supply from an uncontaminated compressed-air source as stipulated by OSHA (89). The ​air must conform to at least Grade D of ANSI Standard Z86.1. For mobile use of these respirators, the air supply can come from a large laboratory-type cylinder (up to about 4 hours of use) or a much smaller, lighter, 45-cubic-foot cylinder (up to about 30 minutes of use). The latter cylinder is less than 2 feet long and about 8 inches in diameter.
However, any disadvantages of the respirators recommended in these guidelines should be evaluated in the context of the aggregate of other isolation precautions already accepted and in use for potential tuberculosis transmitters. The disadvantages cited must be balance against the hazard of tuberculosis infection that tuberculosis transmitters pose to healthcare-facility workers. The medical community has consistently proved willing to accept the burdens of isolation precautions previously recommended for tuberculosis to assure protection of patients and health-care workers.
It has been argued that hospital personnel will refuse to wear the respirators recommended in these guidelines. This attitude is not a new problem in health-care. Other well-established isolation precautions in health-care settings were initially viewed as inconvenient, burdensome, and deleterious to good patient care. Garner and Simmons have addressed this issue as follows (52):
All personnel-physicians, nurses, technicians, students, and others—are responsible for complying with isolation precautions and for tactfully calling observed infractions to the attention of offenders. Physicians should observe the proper isolation precautions at all times; they must teach by example. The responsibilities of hospital personnel for carrying out isolation precautions cannot be effectively dictated but must arise from a personal sense of responsibility.
In order to provide adequate motivation for respirator wear when it is indicated, personnel must be fully informed regarding the specific risks of tuberculosis infection for which personal respiratory protection is indicated. As noted in section V.D.3 starting on page 43, respirator wearers must receive training in the reasons for the need for wearing their respirator and the potential risks of not doing so. This training and written material would include a full disclosure of the nature, extent, and hazards of tuberculosis infection including ​a description of specific risks to each exposed individual due to infection, any subsequent treatment with isoniazid, and the possibility of active disease. Necessary topics are detailed in section V.D.3 starting on page 43.
G. Surgical Masks and Other Disposable Particulate Respirators—As noted, NIOSH has reexamined the hazards of aerosolized droplet nuclei containing tubercle bacilli and the role, reliability, and efficacy of various personal respiratory protective devices to protect healthcare-facility workers against transmission of airborne tuberculosis. Based upon this reexamination, NIOSH concluded that negative-pressure, non-elastomeric, cup-shaped, disposable, particulate-filter respirators ( PRS) without HEPA filters (e.g., surgical masks not certified by NIOSH; NIOSH-certified dust and mist filters; NIOSH-certified dust, fume, and mist filters) cannot be relied upon to protect workers exposed to infectious tuberculosis. These devices include those negative-pressure, non-elastomeric, cup-shaped, disposable masks that consist partly or entirely of filter media integrated into the facepiece. That is, certain "maintenance-free" masks with filtering facepieces for which it is difficult, if not impossible, for a wearer to cover the entire filter-surface area, but not cover the face seal between the respirator and the wearer's face.
This conclusion is based on a body of data indicating that these cup-shaped, disposable masks cannot provide effective and reliable personal respiratory protection due to: (1) unreliable face-seal efficacy, (2) inevitable and dangerous face-seal leakage, and (3) potentially excessive filter leakage (46,59,90,91,92,93).
Face-seal leakage has long been recognized by respirator specialists as compromising adequate personal protection from any air-purifying respirator, particularly negative-pressure halfmask respirators (55,94,95). As noted, existing standard performance tests for surgical masks have not addressed either face-seal leakage or the effects that prolonged use might have on this leakage (59). ​Respirator specialists, manufacturers, and OSHA recognize that cup-shaped, disposable masks have up to 10% (55,56) to 20% (57,58) face-seal leakage even after passing a fit test performed by a qualified individual. This inevitable leakage past face seals results from inherent limitations in the design and construction of these masks. This amount of leakage is unacceptable for effective and reliable respiratory protection against aerosolized droplet nuclei.
What is more relevant, the face-seal leakage for cup-shaped, disposable masks can be considerably higher than 10% to 20% if these masks are not properly fitted to each wearer's face, fit tested by a qualified individual, and then fit checked by each wearer before each respirator use. Both fit testing and fit checking are essential elements in any effective and reliable personal respiratory protection program (55,57,89) as summarized in Table 1 on page 20. At this time there are no NIOSH-recommended qualitative or quantitative fit tests for these masks (81,82). Cup-shaped, disposable masks cannot be reliably fit checked be wearers (58). Therefore, the efficacy and reliability of the face seals on cup-shaped, disposable masks are undependable because there are no proven reliable fit tests nor reliable fit checks. Such devices cannot be relied upon to assure protection of workers against exposure to aerosolized droplet nuclei containing tubercle bacilli.
Another major problem that can contribute to hazardous face-seal leakage of cup-shaped, disposable masks is that in almost all cases these masks are available in only one size. This contrasts with the elastomeric facepieces used for powered, HEPA-filter, halfmask respirators and positive-pressure, air-line, halfmask respirators, which are available in up to three different sizes to fit small, medium, and large facial sizes. The single size in which cup-shaped, disposable masks are available tends to produce higher leakage for wearers with small face sizes (e.g., women, Hispanics, Asians) (96).
Table 2 starting on page 29 summarizes the substantial differences in protection efficacy and reliability between three categories of respirators, which have been considered for protection of health-care-facility workers potentially exposed to tuberculosis.
[Table 2]
Table 2—Summary Comparison of Three Respirator Categories Evaluated for Protection of Health-Care-Facility-Workers Potentially Exposed to Tuberculosis
| CONSIDERATION | Surgical Masks Not Certified by NIOSH as Dust and Mist Masks | Cup-Shaped, Disposable-Mask, Particulate Respirators (PRs) Certified by NIOSH | Powered, HEPA-Filter, Halfmask Respirators and Postitive-Pressure, Air-Line, Halfmask Respirators Certified by NIOSH |
| EFFICACY OF PROTECTION Face-Seal Leakage | Under optimal use conditions, up to 10% to 20% face-seal leakage is likely for any respirable aerosol (i.e., less than 10 micrometers aerodynamic diameter). Considerably more than 10% to 20% face-seal leakage is possible, since these masks cannot be properly fitted to each wearer's face with fit tests and fit checks. Face-seal efficacy with these masks is uncertain, since there are no proven fit tests for them and they cannot be fit checked.Current standard tests for these masks do not address either face-seal leakage or the effect that prolonged use might have on this hazardous leakage. Only one facepiece size is generally available that tends to produce higher leakages on small facial sizes (e.g., women, Hispanics, Asians). | Under optimal use conditions, up to 10% to 20% face-seal leakage is expected for any respirable aerosol (i.e. less than 10 micrometers aerodynamic diameter). Considerably more than 10% to 20% face-seal leakage is possible, since these masks cannot be properly fitted to each wearer's face with fit tests and fit checks. Face-seal efficacy with these masks is uncertain, since there are no proven fit tests for them and they cannot be fit checked. These NIOSH-certified masks are not NIOSH tested for hazardous face-seal leakage during the certification process. Only one facepiece size is generally available that tends to produce higher leakages on small facial sizes (e.g., women, Hispanics, Asians). | Up to 2% face seal leakage under routine use conditions for any respirable aerosol (i.e., less than 10 micrometers in aerodynamic diameter). Less than 2% seal leakage can be routinely expected, sine these respirators can be properly lined to each wearer's face with fit tests and fit checks. The screening cutoff value in quantitative fit tests can be adjusted to assure very low faceseal leakage considerably less than 2%. Face-seal efficacy with these respirators is predictable, since they can be both fit tested and fit checked. These NIOSH-certified respirators have been NIOSH tested for their face-seal efficacy during the certification process. Up to three halfmask sizes are generally available to fit a wider range of facial sizes including small faces. |
| Filter Leakage | Filter leakage of 25% to 85% has been reported at 30 L/min over the size range 1 to 5 micrometers aerodynamic diameter. Filter efficacy has not been adequately evaluated against droplet nuclei. | Filter leakage of 0% to 40% has been reported at 30 L/min over thes size range 1 to 5 micrometers aerodynamic diameter. Filter efficacy has not been adequately evaluated against droplet nuclei. | HEPA filters are NIOSH-certified to exhibit less than 0.03% filter leakage against any size aerosol. These filters do not need to be tested specifically against droplet nuclei. With air-line respirators, no filter leakage can occur, since no filters are used in these devices. |
| CONSIDERATION | Surgical Masks Not Certified by NIOSH as Dust and Mist Masks | Cup-Shaped, Disposable-Mask, Particulate Respirators (PRs) Certified by NIOSH | Powered, HEPA-Filter, Halfmask Respirators and Postitive-Pressure, Air-Line, Halfmask Respirators Certified by NIOSH |
| RELIABILITY OF PROTECTION Face-Seal Leakage | Negative-pressure operation markedly decreases the level and reliability of protection delivered by these masks. Users cannot assure themselves that they are receiving adequate protection because there are no proven fit tests for them and they cannot be fit checked by their users. Employers cannot assure that protection is being received b their employees. | Negative-pressure operation markedly decreases the level and reliability of protection delivered by these masks. Users cannot assure themselves that they are receiving adequate protection because there are no proven fit tests for them and they cannot be fit checked by their users. Employers cannot assure that protection is being received by their employees. | Positive-pressure operation markedly increases the level and reliability of protection delivered by these respirators. Users can assure themselves that they are receiving adequate protection because these respirators can be quantitatively fit tested and fit checked by their wearers. Also, users can readily detect whether or not air is being forced into their facepieces, which indicates protection is being received. Employers can assure that protection is being received by their employees. |
| Filter Leakage | Filter reliability has not been adequately evaluated against droplet nuclei. | Filter reliability has not been adequately evaluated against droplet nuclei. | HEPA filters are highly reliable against any size aerosol. Filter reliability is not an issue with air-line respirators, since no filters are used. |