KNOW YOUR GAS MASK
Drawing by S. J. Pearce from “Respiratory Protective Devices Manual,” published 1963, American Industrial Hygiene Association and the American Conference of Governmental Industrial Hygienists. Reproduced by permission.
TWENTY YEARS AGO, before selfcontained breathing apparatus became available, gas masks furnished most of the respiratory protection to firemen. Today, although self-contained breathing apparatus is generally preferred, about one-half the respiratory protection available to firemen is still furnished by gas masks.
Gas masks have passed the test of time by providing reliable protection to fire fighters for the past 40 years. Accidents can usually be traced to misuse rather than to any inherent defects in the mask itself. Furthermore, it is the effects of this misuse which causes some people to lose confidence in the ability of the gas mask to protect them.
In the recent past, personnel of the Bureau of Mines have become more closely associated with those in the fire service. As a result there has been a mutually beneficial exchange of information and ideas. This exchange has indicated the existence of some serious misconceptions regarding the gas mask.
Canister more than filter
The main function of the canister, as used in the fire service, is to remove carbon monoxide and smoke particles from the otherwise breathable air. The secondary function is to remove other gases and vapors formed or released by fires. Since carbon monoxide is the most common gas encountered in fire fighting, it is important to understand how the canister operates against this gas.
Unlike most other gases and vapors which are removed by absorption or adsorption and held fast by the granular chemicals inside the canister, carbon monoxide is removed by burning or oxidizing to form harmless carbon dioxide. The material in the canister which performs this operation is a catalyst called Hopcalite. It promotes the oxidation of all the carbon monoxide for as long as the Hopcalite is protected from moisture. Moisture or water vapor poisons the Hopcalite and decreases its catalytic effectiveness. Therefore, a means of determining when Hopcalite is no longer protected against moisture is necessary.
Photo courtesy Mine Safety Appliances Co.
Window-indicator vs. mechanical timer
The window-indicator does this directly by a paper impregnated with a moisture-sensitive salt placed behind a window in the canister adjacent to the Hopcalite layer. The paper changes color when moisture reaches the Hopcalite, and the color change can be viewed through the window. The canister must not be used for protection against carbon monoxide after the color has changed. Directions are printed on each canister explaining the operation of the window-indicator.
The timer is an indirect way of indicating the residual life of universal canisters against carbon monoxide. It is indirect because it does not actually indicate the condition of the Hopcalite. Each inhalation of the wearer actuates a gear train that shows on a dial which on some timers is graduated in hours, on others from FULL to EMPTY. The amount of moisture in the air that is drawn through the canister does not affect the timer reading. Although both the window-indicator and the timer have been tested and approved by the Bureau of Mines, the windowindicator canister is definitely safer for fire fighting because moisture content of the air varies widely. All timerequipped canister masks used for fire fighting should, therefore, be converted to the window-indicator type. Conversion kits for this purpose are available from mask manufacturers.
Some fire departments remove universal gas mask canisters from service because of overheating, which is defined as being too hot to touch. It was explained to the Bureau, that absorption of moisture in the air by the canister liberates heat and that a hot canister has absorbed too much moisture and should be discarded.
This reasoning is only partially true because the comparatively small amount of heat generated by moisture absorption can be detected only on the bottom few inches of the canister. Further, the heat generated by moisture absorption will not make the canister too hot to touch.
On the contrary, an overheated canister is probably performing properly and need not be discarded. The overheating is probably due to efficient carbon monoxide oxidation; the amount of heat liberated by the oxidation varies directly with the amount of carbon monoxide oxidized.
To illustrate, suppose a mixture of 2 percent carbon monoxide and air was passed through the canister at 2 cubic feet a minute. Let us further assume the relative humidity is 50 percent and the temperature is 25°C (77°F). The heat generated by the adsorption of the moisture from the air will raise the temperature of the lower half of the canister to about 70°C (158°F), and it will feel warm when touched. But the heat generated by the oxidation of the 2 percent carbon monoxide will raise the temperature of the top half of the canister to about 170°C (340°F), which will be very hot to the touch.
To summarize, when a canister used in fire fighting becomes overheated, it is not a sign of canister exhaustion but a sign that the canister is performing properly. A fireman who is experienced in using the universal gas mask knows that as the air he breathes becomes hotter, the carbon monoxide concentration is greater, and that he must be more careful. This discussion assumes that the canister is not overheating from the radiant heat of a fire, in which case everything would be hot—not just the canister.
Two percent limitation
The second item that needs clarification pertains to the Bureau of Mines limiting the approval of universal gas masks for atmosplwres containing not more than 2 percent carbon monoxide. This statement has been interpreted to mean that universal canisters are not effective against higher concentrations.
Actually, the main reason for the 2 percent limitation is the heat generated by 2 percent carbon monoxide is about as much as the wearer can tolerate, although the canister can oxidize concentrations greater than 2 percent carbon monoxide. Also, since carbon monoxide is converted into an equivalent amount of carbon dioxide, the effects of carbon dioxide concentrations greater than 2 percent would become significant.
Furthermore, canister masks should not be worn when the carbon monoxide concentration is greater than 2 percent because the oxygen content of the air may be approaching a dangerously low concentration. Canisters should not be worn in carbon monoxide concentrations greater than 2 percent for these reasons and not because the canister is unable to handle them.
Canister Resistance to inhalation
A certain amount of effort is required to draw air into the facepiece through the canister and breathing tube. The effort required is proportional to the rate of air flow and to the resistance to air flow.
The Bureau of Mines has set the maximum permissible resistance for approved universal gas masks at 3.5 inches of water on inhalation and 1.0 inch of water on exhalation. All resistances are measured with the air flowing at 85 liters per minute (3 cfm). When the resistance to inhalation increases, the cause is invariably plugging in the canister. The canister should then be replaced.
However, it cannot be assumed that a canister remains effective so long as the resistance to inhalation remains low. A canister can be exhausted and still have a low resistance to inhalation.
Canister filter for smoke
All gas mask manufacturers have two models of universal canisters which are sold to fire departments. One has a high-efficiency filter for smoke removal while the other type does not. Each can be identified by the wording on the canister approval label.
If the word limited appears on the label describing the canister’s filter, the filter is not the high-efficiency type. Such a statement would read, “This approval also applies to limited protection against dusts, fumes, mists, and smokes.”
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KNOW YOUR GAS MASK
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The actual requirement for limitedtype canister filters is 50 percent efficiency against 0.3-micron-diameter smoke when tested at a flow of 85 liters per minute. While the limitedtype filter may just pass this requirement, the high-efficiency type will have an efficiency of more than 95 percent. Most of them have an efficiency of more than 99.9 percent.
All active Bureau of Mines-approved universal canisters with window-indicators, are listed here alphabetically and grouped according to efficiency of the canisters against smoke. For use in fire fighting, a selection should be made from those canisters with highefficiency filters.
All of these canisters are also available without the window-indicator. The letter “A” following the Bureau of Mines approval number means it is a window-indicator canister.
Use in low oxygen
A fireman cannot accurately estimate the amount of oxygen in the atmosphere surrounding a fire in an enclosed area, unless an instrumented analysis is made. For this reason, a gas mask should not be worn in an area where a low-oxygen content is even suspected. Low oxygen should always be suspected when ventilation is lacking, when no flames are visible, or when working below ground.
The device of choice for all-around respiratory protection in fire fighting is the self-contained breathing apparatus. Nevertheless, within their limitations, gas masks will continue to be used effectively in the fire service for many years to come.
Proper selection of a respirator is important, but it is only part of the job. Careful training in the use and maintenance of respiratory-protective equipment is most important.