Chapter 2 - Types of Respirators

1. Particulate Filtering Respirators

   Particulate filtering respirators are used for protection against dusts, fumes, and/or mists. A dust is a solid, mechanically produced particle. A fume is a solid condensation particulate, usually of a vaporized metal. A mist is a liquid condensation particle.

   Presently, all particulate filtering respirators use fibrous material (a filter) to remove the contaminant. As a particle is drawn onto or into the filter, it is trapped by the fibers. The probability that a single particle will be trapped depends on such factors as its size relative to the fiber size; its velocity; and, to some extent, the composition, shape, and electrical charge of both particle and fiber. With current filter media, any filter designed to be 100% efficient in removing particles would be unacceptably difficult to breathe through.

   Figure 2-10.  Typical "mouthpiece" respirator
   Figure 2-11.  Loose-fitting blouse
   Figure 2-12.  Typical abrasive blasting hood

   Manufacturers try to produce the most efficient filter with the lowest breathing resistance. As the particulate respirator is used particulate material collects on the filter and the openings between fibers become smaller. This results in an increase in the breathing resistance. The filter may also become more efficient.

   There are several designs of respirator filters. Each can be described by its filtration mechanism(s), production methods or type, the aerosol against which it is designed to provide protection, and the filtering efficiency.

1. Filtration Mechanisms
   

Particulate filters are of two types: absolute and non-absolute. Absolute filters use screening to remove particles from the air; that is, they exclude the particles which are larger than the pores. However, most respirator filters are non-absolute filters, which means they contain pores which are larger than the particles to be removed. They use combinations of interception capture, sedimentation capture, inertial impaction capture, diffusion capture, and electrostatic capture to remove the particles. The exact combination of filtration mechanisms which come into play depends upon the flowrate through the filter and the size of particle. Brief descriptions of these filtration mechanisms follow.

1. Interception Capture
   

As the air streams approach a fiber lying perpendicular to their path, they split and compress in order to flow around the fiber and rejoin on the other side (Figure 2-13). If the center of a particle in these airstreams comes within one particle radius of the fiber, it encounters the fiber surface and is captured. As particle size increases, the probability of interception capture increases. The particles do not deviate from their original streamline in this mechanism.

2. Sedimentation Capture
   

Only large particles (2µ and larger) are captured by sedimentation. Since this type of capture relies on gravity to pull particles from the airstream, flowrate through the filter must be low (Figure 2-14).

3. Inertial Impaction Capture
   

As the airstreams split and change direction suddenly to go around the fiber, particles with sufficient inertia cannot change direction sufficiently to avoid the fiber. Thus they impact on the surface of the fiber (Figure 2-15). A particle's size, density, speed and shape determine its inertia.

Figures 2-13, 2-14, 2-15.  Various capture mechanisms, including interception, sedimentation and impaction.¹

¹ Japuntich, Daniel A.  Respiratory Particulate Filtration.  J. Ind. Soc. Respir. Prot. 1984; (2)(1):137-169

4. Diffusion Capture
   

The motion of smaller particles is affected by air molecules colliding with them. The particles then can randomly cross the airstream and encounter the fiber as they pass (Figure 2-16). This random motion is dependent on particle size and the air temperature. As the particle size decreases and air temperature increases the diffusive activity of the particle increases. This increases the probability of capture. Lower flowrate through the filter also increases the probability of capture because the particle spends more time in the area of the fiber.

5. Electrostatic Capture
   

In electrostatic capture, the particle is charged and the filter fibers have the opposite charge. Therefore, the particles are attracted to the fibers (Figure 2-17). The electrostatic capture mechanism aids the other capture mechanisms, especially interception and diffusion.

As was mentioned previously, the exact combination of capture mechanisms taking place depends upon several factors. However, some generalizations can be made. Large heavy particles are usually removed by inertial impaction and interception. Large light particles are removed by diffusion and interception. Diffusion removes very small particles.

2. Types of Filters
   

Three types of particulate filter predominate. The most common type presently available is a machine-made flat disk of random laid non-woven fiber material which is carefully controlled to produce maximum filter efficiency and minimum resistance.

Another type (Figure 2-18) is a flat disk of compressed natural wool or synthetic fiber felt, or a blend, to which an electrostatic charge is imparted during manufacture by impregnating the material with a resin and mechanically beating or "needling" it. This charge increases the filter efficiency by electrostatically attracting the particles to the fibers. These filters protect adequately against most industrial dusts, but one precaution should be observed in their use. Certain agents, such as oil mists, and storage in very humid air remove the electrostatic charge. Therefore this type of filter should be stored in its original package, kept out of oil mists and high (>80%) humidity, and used as soon as possible after purchase.

Figures 2-16 and 2-17.  Diffusion capture mechanism¹ and Electrostatic capture
Figure 2-18.  Typcial resin-impregnated felt dust filter

¹ Japuntich, Daniel A.  Respiratory Particulate Filtration.  J. Ind. Soc. Respir. Prot. 1984; (2)(1):137-169

The resin impregnated felt filter is readily identified by rubbing it between the fingers and then rubbing the fingers together. The fingers will feel slightly sticky.

Another type of dust filter is shown in Figure 2-19. The filtering medium is only loosely packed in the filter container so it is much thicker than the compressed type. Such filters are generally made of fibrous glass, although nonfelted, resin impregnated natural wool fibers have been used. They are not as common as the felted type. Typical dust respirators are shown in Figure 2-20.

Figure 2-21 shows a typical high efficiency dust, fume, and mist filter and Figure 2-22 shows high efficiency respirators. The filter is a flat sheet of material that is pleated and placed in the filter container. The pleating provides a large filtering area to improve the particle loading capacity and lower the breathing resistance. When viewed from the top, this type of filter shows a series of concentric rings or rows of pleats. This configuration is common, but other methods of construction are also used.

3. Particulate Respirator Classifications
   

For the 30 CFR 11 Subpart K certification tests particulate respirators are classified as designed for protection against a variety of dusts, fumes, mists. The following types are presently certified by MSHA/NIOSH:

1. Replaceable or Reusable Dust and Mist
   

Respirators, either with replaceable or reusable filters, designed as respiratory protection against (1) dusts and mists having an exposure limit not less than 0.05 milligram per cubic meter of air, or (2) dusts and mists having an exposure limit not less than 2 million particles per cubic foot of air.

2. Replaceable Fume
   

Respirators, with replaceable filters, designed as respiratory protection against fumes of various metals having an exposure limit not less than 0.05 milligram per cubic meter.

3. Replaceable Dust, Fume, and Mist
   

Respirators, with replaceable filters, designed as respiratory protection against dusts, fumes, and mists of materials having an exposure limit less than 0.05 milligram per cubic meter or 2 million particles per cubic foot of air.

Figure 2-19.    Typical dust filter with losse packed medium
Figure 2-20a.  Typical dust respirators.
Figure 2-20b.  Typical dust respirators.
Figure 2-21.    Typcial high efficiency filter
Figure 2-22a.  Typical half- and full-facepiece high efficiency respirators.
Figure 2-22b.  Typical half- and full-facepiece high efficiency respirators.

4. Single-use
   

Respirators designed as respiratory protection against pneumoconiosis- and fibrosis-producing dusts, or dusts and mist. In the single-use respirator, the filter is either an integral part of the facepiece or it is the entire facepiece itself (see Figure 2-23).

4. Filter Efficiency
   

Filter efficiency may be classified as follows:

1. High Efficiency
   

The highest efficiency filters (99.97 percent against 0.3 µ dioctyl phthlate particle) are used on high efficiency respirators certified for protection against dusts, fumes, and mists having an exposure limit less than 0.05 milligram per cubic meter or 2 million particles per cubic foot of air.

2. Lower Efficiency
   

Respirators for dusts, fumes, and mists having an exposure limit not less than 0.05 milligram per cubic meter, have lower efficiency filters as classified in 30 CFR 11 (approximately 99 percent against a lead fume aerosol).

Dust, mist, single-use dust and mist respirators also have lower efficiency filters as classified in 30 CFR 11 (approximately 99 percent against a silica dust particle with a geometric mean diameter of 0.4 to 0.6p and a standard geometric mean deviation not greater than 2).

2. Vapor and Gas Removing Respirators
   

The other major class of airborne contaminants consists of gases and vapors. Air-purifying respirators are available for protection against both specific gases and vapors, such as ammonia gas and mercury vapor, and classes of gases and vapors, such as acid gases and organic vapors. In contrast to filters, which are effective to some degree no matter what the particulate, the cartridges and canisters used for vapor and gas removal are designed for protection against specific contaminants.

Figure 2-23a.  Typical single use respirators
Figure 2-23b.  Typical single use respirators


 

1. Removal Mechanisms
   

Vapor and gas removing respirators normally remove the contaminant by interaction of its molecules with a granular, porous material, commonly called the sorbent. The general method by which the molecules are removed is called sorption. In addition to sorption, some respirators use catalysts which react with the contaminant to produce a less toxic gas or vapor

Three removal mechanisms are used in vapor and gas removing respirators.

 

1. Adsorption
   

Adsorption retains the contaminant molecule on the surface of the sorbent granule by physical attraction. The intensity of the attraction varies with the type of sorbent and contaminant. Adsorption by physical attraction holds the adsorbed molecules weakly. If chemical forces are involved, however, in the process called chemisorption, the bonds holding the molecules to the sorbent granules are much stronger and can be broken only with great difficulty.

A characteristic common to all adsorbents is a large specific surface area, up to 1500 m²/g of sorbent. Activated charcoal is the most common adsorbent. It is used primarily to remove organic vapors, although it does have some capacity for adsorbing acid gases. Activated charcoal also can be impregnated with other substances to make it more selective against specific gases and vapors. Examples are activated charcoal impregnated with iodine to remove mercury vapor, with metallic oxides to remove acid gases, and with salts of metals to remove ammonia gas. Other sorbents which could be used in vapor and gas removing respirators include molecular sieves, activated alumina, and silica gel.

2. Absorption
   

Absorbents may also be used to remove gases and vapors. Absorbents differ from adsorbents in that, although they are porous, they do not have as large a specific surface area. Absorption is also different because the gas or vapor molecules usually penetrate deeply into the molecular spaces throughout the sorbent and are held there chemically. Probably, absorption cannot occur without prior adsorption on the surface of the particles. Furthermore, adsorption occurs instantaneously, whereas absorption is slower. Most absorbents are used for protection against acid gases. They include mixtures of sodium or potassium hydroxide with lime and/or caustic silicates.

3. Catalysis
   

A catalyst is a substance that influences the rate of chemical reaction between other substances. A catalyst used in respirator cartridges and canisters is hopcalite, a mixture of porous granules of manganese and copper oxides which speeds the reaction between toxic carbon monoxide and oxygen to form carbon dioxide.

As applied to respirators, the foregoing processes are essentially 100% efficient until the sorbent's capacity to adsorb gas and vapor or catalyze their reaction is exhausted. Then the contaminant will pass completely through the sorbent and into the facepiece. This is in contrast to mechanical particulate removing filters which become more efficient as matter collects on them and plugs the spaces between the fibers. This difference is important to remember. Water vapor reduces the effectiveness of some sorbents and increases that of others. For example, increasing moisture content of a sorbent designed to sorb acid gases may increase sorbent efficiency since most acid gases normally dissolve in water. Vapor and gas removing cartridges should be protected from the atmosphere while in storage.

2. Cartridges vs. Canisters
   
1. Sorbent Volume
   

The basic difference between cartridges and canisters is the volume of sorbent contained, not its function. Cartridges are vapor and gas removing elements that may be used singly or in pairs on quarter- and half-masks and on full-facepieces. The sorbent volume of a cartridge is small, about 50-200 cm³, so the useful lifetime is usually short, particularly in high gas or vapor concentrations. Therefore, use of respirators with cartridges generally is restricted to low concentrations of vapors and gases. The user should refer to NIOSH recommendations, certification labels, or specific standards set forth by regulatory agencies for specific maximum use concentrations.

Canisters have a larger sorbent volume and may be chin-, front- or back-mounted. Respirators with canisters can be used in higher vapor and gas concentrations (up to the immediately dangerous to life or health level) than those with cartridges. Chin-style canisters have a volume of about 250-500 cm³ and are used on full-facepiece respirators. Front- or back-mounted canisters are held in place by a harness and connected to the facepiece by a corrugated, flexible breathing tube. They have a sorbent volume of 1000-2000 cm³. Front- or back-mounted and chin-style canisters are used with full-facepieces as part of "gas masks." The "gas mask" is certified for single or specific classes of gases and vapors. It differs from the chemical cartridge respirator only in its larger sorbent volume and the higher concentrations of vapors and gases against which it provides protection.

2. Labeling
   

As vapor and gas removing cartridges and canisters are designed for protection against specific contaminants, or classes thereof, how does the user know he has the proper device? The printed certification label clearly lists these contaminants. An American National Standard, ANSI K13.1-1973, established a color code for the various types of sorbent cartridges and canisters which identifies the contaminants they are designed to protect against. Users should not rely on memorizing the color code, but should always READ THE LABEL! This is the only foolproof way of ensuring use of the correct cartridge or canister. The color code of the ANSI K13.1 standard has been included verbatim in the OSHA regulations, 29 CFR 1910.134(g).

3. Construction
   

The type of sorbent found in vapor and gas removing cartridges and canisters for use against a particular substance may vary from manufacturer to manufacturer. However, cartridge and canister construction varies little. The basic construction problems are the same: to provide enough sorbent bed depth and volume to ensure that 1) the contaminant is totally removed in the times specified in 30 CFR 11 bench tests, and 2) the sorbent remains mechanically stable in the container.

Figure 2-24 shows a typical chemical cartridge certified for use with a half-mask. The bed of sorbent granules is retained in the cylindrical "can" by a screen and coarse filter pad at the top and by a coarse particulate filter pad and a screen at the bottom (Figure 2-25). The pads only keep the fine granules in the sorbent from escaping from the cartridge; they are not designed for protection against particulate contaminants. Various precautions for use of these cartridges are discussed in Chapter 5, Respirator Use Under Special Conditions.

One problem in design and manufacture of sorbent canisters is to prevent passage of large quantities of air through small areas of the bed of packed sorbent granules. Such air channeling through the canister reduces its useful service life. Selection of the proper sorbent granule size and careful packing in the canister minimize air channeling. There is a tendency toward channeling where the irregular sorbent granules touch the smooth canister wall. Sometimes channeling is prevented by forming ridges in the canister shell like those in Figure 2-26. The retaining screens and pads hold the granular sorbent bed in place. The spring ensures that the sorbent remains tightly packed.

Figure 2-24.  Typical half-mask chemical cartridge
Figure 2-25.  Typicial chemical cartridge
Figure 2-26.  Typical chin style canister

Even with these precautions, sorbent canisters may be damaged by dropping. This can crush the granules, disturb the retaining screens or pads, or create channels between the sorbent granules and the canister wall. Cartridges and canisters should also be stored upright. In short, treat sorbent canisters with care.

3. Vapor and Gas Respirator Classifications
   
1. Chemical Cartridge Respirators
   

Figure 2-27 shows a typical chemical cartridge air-purifying respirator with an array of various cartridges that can be used with it. Chemical cartridge respirators can be either powered or non-powered, and either disposable or with replaceable cartridges or canisters. A listing of the vapors and gases and maximum concentrations for which chemical cartridge respirators are certified is included in 30 CFR 11.150. Note the accompanying restrictions on maximum use. These concentrations pertain to the cartridge and thus are the limiting concentration for the respirator regardless of whether a full or half facepiece is used.

In addition to the gases and vapors listed, 30 CFR 11.150 also allows MSHA/NIOSH to certify chemical cartridge respirators for gases and vapors other than those listed. For example, MSHA/NIOSH have certified respirators for use against:

Gas/Vapor	Maximum Use Concentration
Mercury*	0.5 mg/m3
Hydrogen sulfide*	100 parts per million
Chlorine dioxide	1 part per million
Formaldehyde	30 parts per million

*Respirators may be certified for gases and vapors with poor warning properties if there is a regulatory agency standard which permits their use and an effective end-of-service-life indicator is provided (Reference: FR 49 No. 140, pages 29270-29272, July 19, 1984).

2. Gas Masks
   

The following types of gas masks have been certified by MSHA/NIOSH:

Front- or back-mounted canisters
Chin-style canisters
Escape

Figure 2-27.    Full-facepiece chemical cartridge respirator with alternate cartridges

Front- or back-mounted. Front- or back-mounted canisters are usually certified for use with a full-facepiece. However, some half-mask or mouthpiece gas masks are certified. A "super size" or "industrial" size canister is fastened to the user's body, and a breathing tube connects the canister to the facepiece inlet. A typical front-or back-mounted canister is shown in Figure 2-28. Note that the construction does not differ markedly from that of the chemical cartridge shown in Figure 2-24. Other than the volume of sorbent contained (1000-2000 cm³), the greatest difference is that the canister, rather than the facepiece, usually contains the inhalation valve. Figures 2-29 and 2-30 show typical front- and back-mounted canister gas masks.

Canisters can be designed for one or more type(s) of gas(es) or vapor(s). Several specific gases and vapors for which MSHA/NIOSH can issue certifications are listed in 30 CFR 11.90. In addition, MSHA/NIOSH have certified gas masks for gases and vapors not listed but which have adequate warning properties (e.g., hydrogen fluoride, formaldehyde and phosphine). MSHA/NIOSH have also certified gas masks for ethylene oxide. However, since ethylene oxide has poor warning properties, these canisters are required to have an end-of-service-life indicator.

Canisters designed for protection against more than one vapor or gas have their sorbents either arranged in layers or intermixed. Figure 2-31 shows these two arrangements as either might appear in a chin-style canister. In certain instances, one type of construction has an advantage over the other, but mostly it is a matter of manufacturing convenience.

Chin-style. Chin-style gas masks typically have a medium-sized (250-500 cm³) canister rigidly attached to a full-facepiece (Figure 2-32). The useful lifetime is less than that of a front- or back-mounted canister (owing to the smaller sorbent volume), but greater than that of chemical cartridges. Gas masks can either be powered or non-powered. The maximum use concentration for both the front- or back-mounted and chin style gas masks is the immediately dangerous to life or health (IDLH) level of the substance.

Escape masks. Gas masks for use during escape from (not entry or reentry into) atmospheres immediately hazardous to life and health are certified under 30 CFR 11, Subpart I. They consist of a facepiece or mouthpiece, a canister, and associated connections. Where eye irritation is a consideration, a full-facepiece gas mask is necessary. An example of an escape gas mask is the "filter" self-rescuer for carbon monoxide used in escaping from mines (Figure 2-33).

Figure 2-28.   Typical front- or back-mounted canister
Figure 2-29.   Typical front- and back-mounted canister gas mask
Figure 2-30.   Typical back-mounted canister gas mask
Figure 2-31.   Typical chin-style canister for more than one vapor
Figure 2-32a. Chin-style canister gas masks
Figure 2-32b. Chin-style canister gas masks
Figure 2-33.   Filter self-rescuer

3. Particulate Vapor and Gas Removing Air-Purifying Respirators
   

Cartridges and canisters are available to protect against both particulates and vapors and gases. These devices look much like the sorbent cartridge or sorbent canister alone. Figure 2-34 shows the two methods of attaching a particulate filter to a typical cartridge. In A, the particulate filter is inside the cartridge container, in B it is outside the can and held in place by a snap-on cover. Other variations may be found, but the principle is the same. Where filters are used in combination with cartridges, the filter must always be located on the inlet side of the cartridge. Pesticide and paint spray respirators use combination respirator cartridges, although paint spray respirators are certified under Subpart L of 30 CFR 11 (Chemical Cartridge Respirators), and pesticide respirators under Subpart M. Typical combination particulate, vapor, and gas removing respirators are shown, in Figure 2-35, being used in paint spraying.

High efficiency particulate filters are included on some types of combination canisters like the front-mounted canisters shown in Figure 2-30.

A very specialized type of combination particulate and vapor and gas removing canister is the so called "Type N," or "Universal" canister (Figure 2-36). It looks much like a front- or back-mounted canister, being about the same size and held on the body in the same way. Internally, however, there is a great deal of difference. The Type N canister may contain several different sorbents for ammonia, acid gases, and organic vapors; a catalyst, hopcalite, to convert carbon monoxide to carbon dioxide; layers of drying agent to protect the catalyst from water vapor; and a high efficiency filter for particulates.

All of these layers are packed into a space equivalent in size to the conventional canister; therefore, the sorptive capacity of any single layer of sorbent in the Type N canister is less than that of the large sorbent bed in the industrial size canister for use against a single contaminant. Consequently, the useful service life of the Type N canister is relatively short.

All canisters approved for entry into carbon monoxide atmospheres must have an indicator, usually behind a small window, that shows when the canister will no longer remove the carbon monoxide. Actually, it indicates the condition of the drying agent upstream of the catalyst. The CO catalyst, hopcalite, is rendered useless by moisture, and this indicator tells only the condition of the hopcalite, not that of the acid gas, ammonia gas, or organic vapor sorbent. Therefore, it cannot be used as an indication of the overall canister condition.

Figure 2-34.   Typical combination particulate- and gas- and vapor-removing cartridges
Figure 2-35a. Combination particulate-, gas- and vapor-removing respirator
Figure 2-35b. Combination particulate-, gas- and vapor-removing respirator
Figure 2-36.   Typical Type N canister 

Figure 2-37 shows a typical front-mounted Type N canister attached to a full-facepiece.

3. Powered Air-Purifying Respirators
   

The powered air-purifying respirator (PAPR) uses a blower to pass contaminated air through an element that removes the contaminants and supplies the purified air to a respiratory inlet covering. The purifying element may be a filter to remove particulates, a cartridge to remove vapors and gases or a combination filter and cartridge, canister or canister and filter. The covering may be a facepiece, helmet, or hood. These respirators are certified under 30 CFR 11, Subparts I, K, L, and M.

Powered air-purifying respirators come in several different configurations. One configuration consists of the air-purifying element(s) attached to a small blower which is worn on the belt and is connected to the respiratory inlet covering by a flexible tube as shown in Figure 2-38. This type of device is usually powered by a small battery (either mounted on the belt separately or as part of the blower), although some units are powered by an external DC or AC source.

Another type consists of the air-purifying element attached to a stationary blower, usually mounted on a vehicle, powered by a battery or an external power source and connected by a long flexible tube to the respiratory inlet covering.

The third type of powered air-purifying respirator consists of a helmet or facepiece to which the air-purifying element and blower are attached. Only the battery is carried on the belt.

The respiratory inlet covering for a powered air-purifying respirator may be a tight fitting half-mask (Figure 2-39) or full-facepiece, or a loose fitting hood or helmet (Figure 2-40). A powered air-purifying respirator with a tight fitting facepiece must deliver at least four cubic feet of air per minute (115 liters per minute). A powered air-purifying respirator with a loose fitting hood or helmet must deliver at least six cubic feet of air (170 liters per minute) at all times.

One potential disadvantage of powered air-purifying respirators is that since there is a constant flow through the air-purifying element instead of flow only during inhalation; the useful service lifetimes of the air-purifying elements on powered air-purifying respirators could be shorter than the service lifetimes of comparable elements attached to a negative pressure respirator. In order to overcome this problem, some powered air-purifying respirators have a spring loaded exhalation valve assembly. This causes the blower assembly to slow down when the wearer exhales. This helps to extend the service lifetime of the air-purifying elements.

Figure 2-37.   Typical front-mount Type N canister gas mask
Figure 2-38.   Powered air-purifying respirator with chemical cartridges and breathing tube
Figure 2-39.   Tight fitting half-mask powered air-purifying respirator
Figure 2-40a. Helmeted powered air-purifying respirator
Figure2-40b.  Helmeted powered air-purifying respirator 

Powered air-purifying respirators using chemical cartridges and canisters have the same limitations, insofar as the air-purifying elements are concerned, as the negative pressure respirators approved for the same gases or vapors.

In the past, powered air-purifying respirators were considered positive pressure respirators, since they normally supplied air at positive pressure. It was assumed that any leakage was outward from the facepiece. They were given correspondingly high protection factors. However, recent field studies by NIOSH and others have indicated that the level of protection provided by these respirators may not be as high as previously reported. Because of the potential for overbreathing at the minimum airflow rates, NIOSH now recommends much lower protection factors.

4. Advantages and Disadvantages of Air-Purifying Respirators
   

Air-purifying respirators are generally small and are easily maintained. (The exceptions to this are the combination Type C supplied-air and air-purifying respirator and powered air-purifying respirator.) They restrict the wearer's movements the least. The many combinations of facepieces, mouthpieces, filters, cartridges and canisters allow the user to match the respirator to the particular situation.

Air-purifying respirators should not be used in atmospheres containing less than 19.5 percent oxygen nor in atmospheres immediately dangerous to life or health (except escape gas masks). They should not be used for protection against gases or vapors with poor warning properties except for escape only or where permitted by a regulatory agency and the respirator is equipped with an end-of-service-life indicator for that particular substance. The cost of replacement elements for air-purifying respirators can be high. Chemical cartridge respirators have fairly low maximum use concentrations, even when used with a full-facepiece.

1. Particulate Respirators
   

The advantages of particulate filter respirators include their light weight, small size and ease of maintenance. In general, these respirators will not affect the mobility of the worker and may present little physiological strain to the wearer. The air flow resistance of a particulate-removing respirator filter element increases as the quantity of particles it retains increases. This resistance increases the breathing resistance offered by a nonpowered respirator and may reduce the rate of air flow in a powered respirator. Filter element plugging by retained particles may also limit the continuous use time of a particulate filter type respirator. Rapid plugging means that the element has to be replaced frequently. Elements should be replaced at least daily or more often if breathing resistance becomes excessive or if the filter suffers physical damage (tears, holes, etc.). Filter elements designed to be cleaned and reused also should be cleaned at least daily in accordance with the manufacturer's instructions. Between uses, reusable respirators should be packaged to reduce exposure to conditions which cause filter degradation, such as high humidity.

Performance of some fibrous filter materials (electrostatic felts) is hurt by storage in very humid atmospheres, so care should be taken in storing filter elements. Performance also may deteriorate during use because of water vapor or oil mists in the workplace atmosphere. Airborne liquid particles (aqueous and nonaqueous) and extremely small solid particles may deteriorate the functioning of these materials. Solid particles plug fibrous filter materials (including electrostatic felts), and, although this plugging increases the resistance to air flow and hence may exacerbate respirator faceseal leakage, significant plugging increases the materials' efficiency in removing particles from air.

2. Vapor and Gas Removing Cartridges and Canisters
   

Gas and vapor removing cartridges and canisters have the same advantages as particulate filter respirators. Certain cartridges and canisters have higher breathing resistance than particulate filter respirators and thus will present a slightly higher physiological burden to the wearer. If a vapor or gas lacks adequate warning properties (odor, taste, irritation) in a concentration above the established breathing time-weighted average concentration (TWA), a vapor and gas removing air-purifying respirator should not be used unless the respirator incorporates an adequate end-of-service-life indicator .

Another disadvantage is the limited capacity of the cartridges and canisters in these respirators to remove vapors and gases from air, or to catalyze a reaction converting toxic vapors or gases to nontoxic products or products that can be removed from air. Theoretically, cartridges and canisters containing sorbents are totally efficient against vapors and gases until their capacity for adsorption or catalysis is exhausted. Then, the vapor or gas passes through the sorbent bed of the cartridge or canister and into the facepiece. If the wearer detects an odor or taste of gas in the inspired air, or feels eye or throat irritation, he/she should leave the hazardous area immediately and go to a safe area that contains respirable air. Then the wearer should replace the cartridge or canister. Because of the limited useful service time of canisters and cartridges, they should be replaced daily or after each use, or even more often if the wearer detects odor, taste, or irritation. Discarding the cartridge/canister is recommended at the end of the day, even if the wearer does not detect odor, taste or irritation. This is due to the possibility of desorption of the gas or vapor occurring during overnight storage.

If a respirator wearer detects an odor, taste, or irritation for a very short time and then the sensation disappears, penetration of an air contaminant into the respiratory inlet covering has not necessarily ceased. The nerve endings that cause a sensation of odor, taste, or irritation often are fatigued or their response is dulled by low concentrations of substances. Thus, one may fail to detect low concentrations of some substances in air. This often happens when the concentration increases very slowly.

In addition to odor thresholds, users can institute change-out schedules based on reliable service-life data. Users should be warned to replace cartridges whenever they detect the odor of the substance and at the end of the service time indicated by the change-out schedule.

Some sorbents used in cartridges and canisters are harmed by high humidity, whereas others are harmed by very dry atmospheres. Therefore, when replacing these elements, unsealed cartridges and canisters should not be used. Also, remember that if the hazardous atmosphere is very moist or dry, the useful service time may be markedly reduced.

3. Nonpowered Air-Purifying Respirators
   

In addition tothose limitations imposed by respiratory inlet coverings (see Chapter 2), particulate filter elements, and sorbent cartridges and canisters, further limitations of nonpowered air-purifying respirators should be considered.

An important disadvantage is the negative air pressure created inside the respiratory inlet covering during inhalation which can cause air contaminants to penetrate the covering if it fits poorly. Care should be taken to provide each wearer with a respirator that fits properly. This can best be accomplished by individual fittings and fit tests.

Other disadvantages of nonpowered air-purifying respirators include resistance to breathing and need for frequent replacement of air-purifying elements (except for disposable respirators).

4. Powered Air-Purifying Respirators
   

One advantage of powered air-purifying respirators is that they provide an airstream to the wearer. This airstream has the advantage of providing a cooling effect in warm temperatures, but can present a problem in cold temperatures. The decreased inhalation resistance makes the respirator possibly more comfortable to wear. Powered air-purifying respirators with loose fitting hoods or helmets have the advantage that since there are no large sealing surfaces on the face, some people who cannot wear a tight-fitting facepiece for such a reason as facial scars or facial hair can wear them.

Powered air-purifying respirators normally do not restrict mobility. In addition, these respirators offer minimal breathing resistance since the blower supplies the filtered air to the breathing zone of the wearer. Powered air-purifying respirators have limitations in addition to those imposed by respiratory inlet coverings, particulate filter elements and cartridges containing sorbents. A powered respirator's battery should be recharged periodically to ensure that the blower will deliver enough respirable air to the respiratory inlet covering. A battery has a limited useful life and cannot be recharged indefinitely. Battery replacement can be expensive.

The blower in most powered respirators has a high speed motor which will eventually wear out. Therefore, the blower will have to be replaced periodically. If the blower fails, the wearer of a powered respirator should go to the nearest safe area.

Other disadvantages include weight, bulk, complex design, the need for continual maintenance, at least daily replacement of air-purifying elements, and periodic replacement of batteries and blowers. Out-of-doors use presents special problems if hot or very cold air is supplied to the respiratory inlet covering.

Until recently, powered air-purifying respirators were considered positive pressure devices. Field studies by NIOSH as well as others, have indicated that these devices are not positive pressure, and that their assigned protection factors are inappropriately high.

1. Self-Contained Breathing Apparatus
   

The distinguishing feature of all self-contained breathing apparatus (SCBA) is that the wearer need not be connected to a stationary breathing gas source, such as an air compressor. Instead, enough air or oxygen for up to 4 hours, depending on the design, is carried by the wearer. As Fig. 2-4 shows, SCBAs are classified as "closed circuit" or "open circuit."

1. Closed Circuit
   

Another name for closed-circuit SCBAs is "rebreather" device, indicative of the mode of operation. The breathing gas is rebreathed after the exhaled carbon dioxide has been removed and the oxygen content restored by a compressed or liquid oxygen source or an oxygen generating solid. Descriptions and certification tests for the closed-circuit apparatus are given in Subpart H of 30 CFR 11.

These devices are designed primarily for 1- to 4-hour use in oxygen deficient and/or IDLH atmospheres such as might be encountered during mine rescues or in confined spaces. They have been used since the early 1900s when the Gibbs and McCaa devices were developed. Few major design changes have been made since then, a significant commentary on their acceptance and good performance. [NOTE: 30 CFR 11 prescribes certification for mine rescue only devices that give 1-hour or more performance. Devices that give 30-minute or longer performance may be certified for auxiliary mine rescue service.]

Because negative pressure is created in the facepiece of non-positive pressure apparatus during inhalation, there is increased leakage potential. Therefore, negative pressure closed-circuit SCBA should be used in atmospheres immediately dangerous to life or health (IDLH) only where their long term use capability is necessary, as in mine rescue. For use in oxygen deficient atmospheres over long periods, closed-circuit SCBA are satisfactory. Positive pressure closed-circuit SCBA are a significant new respirator development and are described in Chapter 6, New Developments at NIOSH.

Two basic types of closed-circuit SCBA are presently available. One uses a cylinder of compressed oxygen and the other a solid oxygen generating substance. Figure 2-41 shows a typical closed-circuit SCBA with a small cylinder of compressed oxygen. Breathable air is supplied from an inflatable bag. The exhaled air passes through a granular solid adsorbent that removes the carbon dioxide, thereby reducing the flow back into the breathing bag. The bag collapses so that a pressure plate bears against the admission valve, which opens and admits more pure oxygen that reinflates the bag. Thus, the consumed oxygen is replaced. The advantage of the rebreathing process is that only the oxygen supply need be provided, as all the other air constituents except the waste carbon dioxide are recirculated. The advantage of this type of device is its long term (1- to 4-hour) protection. Disadvantages include the bulk of the SCBA and the negative pressure created in the facepiece during inhalation from some closed-circuit SCBA. As previously discussed, it is now possible for certification of positive pressure devices which offer a higher level of protection. Figure 2-42 shows a closed-circuit SCBA in use.

The second type of closed-circuit SCBA (Fig. 2-43) uses an oxygen-generating solid, usually potassium superoxide (KO₂). The H₂O and CO₂ in the exhaled breath react with the KO₂ to release O₂.

2 KO₂ + CO₂ + H₂O --> K₂CO₃ + 1.5 O₂ + H₂O,

2 KO₂ + 2CO₂ + H₂O --> 2KH CO₃ + 1.5 O₂.

The O₂ is not released until the wearer's exhaled breath reaches the canister. Thus, there is a short time lag between when the canister is initiated and O₂ flow begins. This has been overcome in some devices by providing a "quick start" feature known as a chlorate candle, a canister section filled with mixed sodium chlorate and iron. Oxygen flow is started by striking the device, somewhat like lighting a match. This is designed to provide enough oxygen until the potassium superoxide in the canister begins to function.

Oxygen is continually released at a high flow rate into the breathing bag(s) which acts as a reservoir to accommodate breathing fluctuations. A pressure relief valve and saliva trap release the excess pressure created in the facepiece by oxygen flow and nitrogen buildup.

This closed-circuit apparatus is lighter and simpler than the cylinder type. However, it is useful for only about one hour and, once initiated, cannot be turned off. The precautions are the same as for the compressed oxygen unit.

Recently, as a result of regulations promulgated by MSHA under the Coal Mine Health and Safety Act, a new device of closed-circuit SCBA, known as a self-contained self-rescuer (SCSR) was certified for use in underground mines in emergency situations. These devices are similar in design and operation as those already described. They include both compressed-oxygen and oxygen-generating types and offer a duration of one hour.

Figure 2-41.    Closed-circuit SCBA
Figure 2-42a. Closed-circuit SCBA
Figure 2-42b. Closed-circuit SCBA
Figure 2-43.  Oxygen-generating closed-circuit SCBA

SCSR are much smaller and weigh considerably less than closed-circuit SCBA for entry. Their weights range between 7 and 16 pounds. The SCSR are escape only apparatus and need not meet all the entry unit requirements of 30 CFR 11. Factors contributing to size and weight reduction include: a mouthpiece in place of a facepiece; the elimination of structural breathing bag protection; filament wound pressure gas vessels; smaller candles; lighter breathing bag material; single pendulum flow breathing tube; the elimination of bypass valve and warning whistle requirements; a more efficient utilization of carbon dioxide sorbent and/or oxygen-generating chemicals; lighter weight packaging material; and others. Figure 2-44 shows an oxygen-generating SCSR. These devices are not usually worn by the miner during mining operations as were the former filter self-rescuers (CO scrubbing only or air-purifying respirators), because they are larger and heavier than the filter self-rescuer. MSHA has strict enforceable storage and location requirements for SCSR. Since they are sealed and may not be opened except for emergency use, there are specific daily and 90 day required SCSR inspection periods and inspection procedures. SCSR with pressure vessels use active pressure gauge indicators. The chemical SCSR use passive storage life color indicators and inspection criteria.

2. Open Circuit
   

An open-circuit SCBA exhausts the exhaled air to the atmosphere instead of recirculating it. 30 CFR 11 does not specify which breathing gas must be used for these devices, but it is almost always compressed air. Compressed oxygen cannot be used in a device designed for compressed air because minute amounts of oil or other foreign matter in the device components can cause an explosion. In fact, 30 CFR 11 prohibits certification of any device designed to permit interchangeable use of oxygen and air. It is an accepted safety rule that:

OXYGEN NEVER BE USED IN A DEVICE UNLESS IT IS SPECIFICALLY DESIGNED FOR THAT PURPOSE.

Figure 2-45 shows typical open-circuit SCBA. A cylinder of high pressure (2000-4500 psi) compressed air supplies air to a regulator that reduces the pressure for delivery to the facepiece. This regulator also serves as a flow regulator by passing air to the facepiece on demand. The regulator is either mounted directly to the facepiece or a flexible corrugated hose connects the regulator to the respiratory inlet covering, usually a full-facepiece.

Figure 2-44.     Oxygen-generating self-contained self-rescuer
Figure 2-45a.  Open-circuit SCBA
Figure 2-45b.  Open-circuit SCBA

Because it has to provide the total breathing volume requirements, since there is no recirculation, the service life of the open-circuit SCBA is usually shorter than the closed-circuit SCBA. Most open-circuit SCBA have a service life of 30 minutes to 60 minutes based on NIOSH breathing machine tests as prescribed in 30 CFR 11 (11.85-10). NIOSH certifies units with less than 1 hour, but not less than 30 minutes service for auxiliary mine rescue. Open-circuit SCBA are widely used in fire fighting and for industrial emergencies. SCBA with less than 30 minutes service time are certified, generally for escape use only. Escape SCBAs are also certified in combination with supplied-air, airline respirators.

Two types of open-circuit SCBA are available, "demand" or "pressure demand." The difference is very important and best explained by describing the operation of a typical open-circuit SCBA regulator. In a "demand" or negative pressure type regulator, air at approximately 2000 psi is supplied to the regulator through the main valve (Fig. 2-46). A bypass valve passes air to the facepiece in case of regulator failure. Downstream from the main valve, a two-stage regulator reduces the pressure to approximately 50-100 psi at the admission valve, which is actuated by movement of a diaphragm and its associated levers. The admission valve stays closed as long as positive pressure in the facepiece (during exhalation) forces the diaphragm away from the valve assembly. Inhalation creates negative pressure in the facepiece, and the diaphragm contracts, opening the admission valve and allowing air into the facepiece. In other words, air flows into the facepiece only on "demand" by the wearer, hence the name.

Recent studies indicate that a demand-type SCBA is no more protective than an air-purifying respirator with the same facepiece. Therefore, a demand-type open-circuit SCBA should not be used in IDLH atmospheres. Like closed-circuit SCBA, however, they may be adequate against oxygen-deficient atmospheres.

A pressure-demand or positive pressure regulator is very similar to a demand type except that there is usually a spring between the diaphragm and the outside case of the regulator. This spring tends to hold the admission valve slightly open, theoretically allowing continual air flow into the facepiece. This would be true except that all pressure-demand devices have a special exhalation valve that maintains about 1.5-3 inches H₂O positive back pressure in the facepiece, and opens only when the pressure exceeds that value. This combination of modified regulator and special exhalation valve is designed to maintain positive pressure in the facepiece at all times. Under certain conditions of work a momentary negative pressure may occur in the wearer's breathing zone, although the regulator still supplies additional air on "demand." Because of the positive pressure, any leakage should be outward; therefore, a pressure-demand SCBA provides very good protection. Contrary to common belief, the pressure-demand SCBA has the same service time as a demand version of the same device, if it seals well on the wearer's face. Any leakage increases air consumption and decreases service time.

Figure 2-46.  Open-circuit demand SCBA regulator

A FACEPIECE WHOSE EXHALATION VALVE IS DESIGNED FOR DEMAND OPERATION CANNOT BE USED WITH A PRESSURE-DEMAND REGULATOR, AS AIR WILL FLOW CONTINUALLY AND QUICKLY EXHAUST THE AIR SUPPLY.

Some open-circuit SCBA can be switched from demand to pressure-demand operation. The demand mode should be used only for donning and adjusting the apparatus in order to conserve air and should be switched to "pressure demand" for actual use.

Several required safety features on all certified entry (both closed and open circuit) SCBA provide additional protection. Among these are:

• pressure gauges or liquid level gauges visible to the wearer which indicate the quantity of gas or liquid (air or oxygen) remaining in the cylinder
• remaining service life indicators or warning devices that signal alarm when only 20-25% of service time or service volume remains
• bypass valves, in case the first and second stage reducer or regulator fails and it is necessary to conserve or provide respirable air
• fittings on devices that use compressed or liquid oxygen which are incompatible with compressed or liquid air fittings.

The choice of demand or pressure-demand open-circuit SCBA should be based on thorough evaluation of the respiratory hazards. MSHA and NIOSH continue to issue certifications for both types since the demand type is still used in many industrial applications. In a potentially IDLH atmosphere, a pressure-demand SCBA should most certainly be used.

In addition to entry, SCBA are also certified for escape from IDLH. These escape-only SCBA are generally of short duration that is, 3, 5 or 10 minutes, and are small in both size and weight. The compressed-air container is usually hip- or back-mounted with the air valve in a readily accessible position for immediate activation. The facepiece may be donned quickly by simply tightening the headband straps or a hood may be furnished for quick donning of the escape SCBA. Figure 2-47 shows two hood-type, escape-only SCBA.

2. Supplied-Air Respirators
   
1. Airline respirators (Types C and CE)
   

Airline respirators as described in 30 CFR 11, Subpart J use compressed air from a stationary source delivered through a hose under pressure. 30 CFR 11 specifies that the pressure shall not exceed 125 psi at the point where the hose attaches to the air supply. A manufacturer submitting an airline respirator for certification must specify the operating pressure and the hose length, from 25 to 300 feet. At the lowest pressure and longest hose length, the device must deliver at least 170 Lpm to a helmet or hood. At the highest pressure and shortest hose length the flowrate must not exceed 425 Lpm to a helmet or hood. The equivalent airflows to a tight-fitting facepiece are 115 Lpm and 425 Lpm, respectively.

Airline respirators are available in demand, pressure-demand, and continuous-flow configurations (see Figure 2-5). The respiratory inlet covering may be a facepiece, helmet, hood, or complete suit, although there are presently no approval tests for suits.

A demand or pressure-demand airline respirator is very similar in basic operation to a demand or pressure-demand open circuit SCBA, except that the air is supplied through a small diameter hose from a stationary source of compressed air rather than from a portable air source. Because the air pressure is limited to 125 psi, regulators for demand and pressure-demand airline respirators need only single stage reduction. Otherwise, the demand and pressure-demand airline regulators are similar in operation to the demand and pressure-demand SCBA regulators respectively. Figure 2-48 shows a typical demand type regulator. Figure 2-49 shows a typical pressure-demand airline respirator with a tight-fitting facepiece. Note that the regulator sometimes is mounted on the facepiece or worn on the wearer's chest.

Continuous-flow airline respirators maintain air flow at all times, rather than only on demand. In place of a demand or pressure-demand regulator, an air flow control valve or orifice partially controls the air flow. According to 30 CFR 11, a flow of at least 115 Lpm to a tight fitting facepiece and 170 Lpm to a loose-fitting hood or helmet must be maintained at lowest air pressure and longest hose length specified. This means that by design, either the control valve cannot be closed completely, or a continually open bypass is provided to allow air to flow around the valve and maintain the required minimum rates.

Figure 2-47.  Typical escape-only ESCBA
Figure 2-48.  Typical demand-type air flow regulator
Figure 2-49.  Pressure-demand airline respirator

Some special valves known as vortex tubes are available with some certified airline respirators. These valves fractionate the airstream into two high speed airflow components. One component becomes cool from adiabatic expansion while the other component becomes warm from adiabatic compression. Either component can be utilized in valve design to cool or heat the respirable air provided to the user for comfort and physiological support.

Figure 2-50 depicts a typical continuous-flow airline respirator with a tight-fitting facepiece. Notice the air-purifying element on the air-supply line. Figure 2-51 shows typical airline respirators, which may be obtained with half masks and full-facepieces. Figure 2-52 shows continuous-flow airline respirators with hoods.

Although addition of an air-purifying element in the supply line upstream of the air-supply hose attachment can help clean the air, other precautions also should be taken to ensure breathing air quality. The air supply to airline respirators is required to meet the requirements for Type I gaseous air (Grade D or higher quality) set forth by the Compressed Gas Association Commodity Specification for Air, G-7.1. Furthermore, OSHA requires that a breathing air compressor have certain safety devices to protect the air quality (see Chapter 3).

Airline respirators with special items to protect the wearer's head and neck from rebounding abrasive material may have facepieces, helmets, or hoods. Plastic, glass, and metal wire screen are used to protect the lenses of facepieces and the windows of helmets and hoods against the rebounding material. These respirators are known as abrasive-blasting airline respirators or Type "CE" supplied-air respirators.

Figure 2-53 shows Type "CE" respirators in use. Note the protective screen over the lens and the heavy apron on the abrasive- blasting hood.

Full suit airline respirators are available. They provide air not only for breathing but also to isolate the whole body from the surrounding atmosphere. They are used against substances that irritate or corrode the skin or which may penetrate the skin to produce toxic effects. Presently, 30 CFR 11 does not provide for certification of airline suits.

Figure 2-50.  Continuous-flow airline respirator
Figure 2-51.  Half mask and full-facepiece continous flow airline respirators
Figure 2-52.  Continuous flow airline respirators with hoods
Figure 2-53.  Typical Type CE abrasive-blast airline respirator

2. Hose Masks
   

Hose masks supply air from an uncontaminated source through a strong, large diameter hose to a respiratory inlet covering. Two types are available. One has a hand or motor operated air blower that pushes low pressure air through the hose to the respiratory inlet covering. The blower is designed so that air flows freely through it when it is not in operation. Therefore, if the blower fails, the wearer can still inhale respirable air by normal breathing. The other type of hose mask has no blower and requires the wearer to inhale through the hose.

The hose mask with a blower is categorized by 30 CFR 11 Subpart J as a Type "A" supplied-air respirator and is certified for use in atmospheres not immediately dangerous to life or health. The hose mask without a blower is categorized as Type "B" and is certified for use only in atmospheres not immediately dangerous to life or health. The hose mask with blower may have a facepiece, helmet, or hood, but the hose mask without blower must have a tight fitting facepiece. Hose masks may have special equipment to protect the wearer's head and neck from rebounding material during abrasive blasting. Such a hose mask with blower is classified as a Type "AE" supplied-air respirator, and the one without blower is classified as Type "BE."

A certified hose mask with blower may have up to 300 feet of air supply hose in multiples of 25 feet, but one without blower may have only up to 75 feet in multiples of 25 feet. The hand or motor operated blower must deliver air through the maximum length of hose at not less than 50 Lpm. The motor operated blower of a device with 50 feet of hose must deliver no more than 150 Lpm. However, no maximum air flow rate is specified for the hand operated blower.

Currently there are only three hose masks certified. They are not widely used in industrial applications. They are heavy, cumbersome and offer only a very low protection factor.

3. Combination Respirators
   

MSHA/NIOSH may certify respirators assembled from two or more types of respirators in combination as prescribed in 30 CFR 11.63(b).

To date MSHA/NIOSH have certified several types of air-purifying units or SCBA in combination with the Type C supplied-air respirator.

1. Combination Supplied-Air/Air-Purifying Respirator
   

One type of combination respirator that MSHA/NIOSH has certified is the Type C supplied-air and air-purifying respirator as shown in Figure 2-54. These devices are certified under the class of the air-purifying element since it is the component in the combination which provides the least protection to the user. This type of respirator consists of facepiece; regulator or control valve, if necessary; breathing tube, if necessary; belt or harness; supplied-air hose; and air-purifying element. The air-purifying element may be a canister, chemical cartridge, or particulate filter. It is mounted either directly on the facepiece or on an adapter which is worn on the belt.

The supplied-air portion of the respirator can be either Type C continuous-flow or pressure-demand.

An advantage of this type of respirator is that the wearer has respiratory protection while entering (in some cases) and leaving without being connected to an airline. The air-purifying element weighs less than a self-contained breathing apparatus cylinder. The disadvantage is that they have the limitations of the air-purifying element, and therefore, can be used only for specific conditions. Depending upon the specific respirator, the air-purifying element will have one of the following restrictions (consult the certification label of the respirator to determine which applies):

a. no restrictions

b. air-purifying element can be used only to: (1) enter prior to connecting to air supply, (2) egress after disconnecting or loss of air, or (3) to move from one air supply to another

c. escape only after loss of air.

2. Combination Supplied-Air/SCBA Respirator
   

To be usable in an IDLH atmosphere, an airline respirator must have an auxiliary air supply to protect against potential failure of the primary supply. This is provided by adding a self-contained cylinder of high pressure compressed air to a Type "C" or "CE" airline respirator. The auxiliary air supply may be certified for 3-, 5-, or 10-minute service time, or for 15 minutes or longer (see Figure 2-55). The certification tests for these combination devices are found in 30 CFR 11, Subpart H, "Self-Contained Breathing Apparatus." The devices shown in Figure 2-55 are only representative of this general class; designs vary widely.

Figure 2-54.  Combination supplied-air respirator with escape only high efficiency filter
Figure 2-55.  Combination supplied-air/SCBA

Because of the short service time of the self-contained breathing air supply, combination units generally are used for emergency entry into and escape from IDLH atmospheres. The self-contained portion of the device is used only when the airline portion fails and the wearer must escape, or when it may be necessary to disconnect the air line temporarily while changing locations. A combination airline and SCBA may be used for emergency entry into hazardous atmosphere (to connect the airline), if the SCBA part is classified for 15 minutes or longer service and not more than 20% of the air supply's rated capacity is used during entry. It is seldom used as a routine means of protection, as the open-circuit SCBA might be.

4. Advantages and Disadvantages of Atmosphere-Supplying Respirators
   
1. Airline Respirators
   

A great advantage of the airline respirator is that it may be used for long continuous periods. Other advantages are minimal breathing resistance and discomfort, light weight, low bulk, moderate initial cost, and relatively low operating cost.

The biggest disadvantage of supplied-air respirators is that loss of the source of respirable air supplied to the respiratory inlet covering eliminates any protection to the wearer. Such loss may be caused by cutting, burning, kinking, or crushing the supply air hose, by air compressor failure, or by depletion of the respirable air in a storage tank. Possible loss of respirable air supports the NIOSH recommendation against airline respirator use in IDLH atmospheres. However, an airline respirator with an auxiliary self-contained air supply could be used in such atmospheres because the auxiliary self-contained air supply always can be used in escape.

The trailing air supply hose of the airline respirator severely restricts the wearer's mobility. This may make the airline respirator unsuitable for those who move frequently between widely separated work stations. A combination airline and self-contained breathing apparatus may be suitable if the supply of self-contained breathing air is adequate for the time required to move from place to place. A coiled airline hose provided with some MSHA/NIOSH certified devices will further promote wearer mobility at the worksite.

Airline respirators that operate in the demand mode have negative air pressure inside the respiratory inlet covering during inhalation which permits the contaminated atmosphere to leak into the respiratory inlet covering if it fits poorly. However, airline respirators that operate in the pressure-demand mode are designed to have positive air pressure inside the respiratory inlet covering which helps to ensure that contaminated air will not leak in. Thus, an airline respirator operating in the pressure-demand mode provides much better protection than one that operates in the demand mode.

2. Hose Masks
   

Advantages of the hose mask without blower are its theoretically long use periods and its simple construction, low bulk, easy maintenance, and minimal operating cost. An advantage of the hose mask with blower is its minimal resistance to breathing.

Obviously, air pressure inside the respiratory inlet covering of the hose mask with no blower is negative during inhalation, so contaminated air may leak in if the covering fits poorly. Therefore, hose masks, with and without blower, are certified only for use in non-IDLH atmospheres.

The trailing air supply hose of the hose mask also severely limits mobility, so it may be unsuitable for those who move frequently among widely separated work stations.

A severe restriction of the hose mask without blower is that it is limited to a maximum hose length of only 75 ft. Also, it requires the wearer to inhale against the resistance to air flow offered by the air hose which may become significant during heavy work. Inhaling against this resistance strains the wearer and may cause fatigue.

3. Self-Contained Breathing Apparatus
   

Because the SCBA wearer carries his own supply of respirable air, he is independent of the surrounding atmosphere. A great advantage of such apparatus is that it allows comparatively free movement over an unlimited area.

The bulk and weight of most SCBAs make them unsuitable for strenuous work or use in a constricted space. The limited service life makes them unsuitable for routine use for long continuous periods. The short service life of open-circuit type devices may limit them to use where the wearer can go conveniently and quickly from a hazardous atmosphere to a safe atmosphere to change the tank of supply air.

Open-circuit SCBA are normally less expensive to purchase and use than closed-circuit SCBA. Additionally, the open-circuit SCBA requires less maintenance and fewer inspections.

The demand-type open-circuit SCBA and most closed-circuit SCBA have negative air pressure inside the respiratory inlet covering during inhalation so contaminated air can leak in if they fit poorly. The pressure-demand type open-circuit SCBA and those closed-circuit SCBA that are positive pressure devices provide very good protection because the air inside the respiratory inlet covering is normally at positive pressure which helps to keep the contaminated atmosphere from leaking in.

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