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|Respirator cartridges don't last forever!|
| A change schedule is the part of the written respirator program which says how often cartridges should be replaced and what information was relied upon to make this judgement. A cartridge's useful service life is how long it provides adequate protection from harmful chemicals in the air. The service life of a cartridge depends upon many factors, including environmental conditions, breathing rate, cartridge filtering capacity, and the amount of contaminants in the air. It is suggested that employers apply a safety factor to the service life estimate to assure that the change schedule is a conservative estimate. |
If you know what the chemical is and how much of it you are exposed to, then you are ready to estimate out how long your respirator cartridges will work and apply the safety factor.
|Keep In Mind|
|You may not rely on odor thresholds and other warning properties as the primary basis for determining the service life of gas and vapor cartridges and canisters.|
|Worker Exertion Level: a worker breathing twice as fast as another will draw twice the amount of contaminant through the respirator cartridge |
The service life of a cartridge or canister respirator depends upon the total amount of contaminant captured by the adsorbant. The total amount of captured contaminant is directly related to the work rate or breathing rate; i.e., a worker breathing twice as fast as another will draw twice the amount of contaminant through the respirator cartridge. Most cartridge studies have used a breathing rate, 50-60 liters per minute, that approximates a high end of moderate workrate. For workrates that exceed this level (e.g., heavy shoveling, running)you may need to apply or take into account a correction factor when determining a service life.
The service life of a respirator cartridge is directly related to the amount of active material in the cartridge. For instance, most dual cartridge organic vapor respirators contain between 35-50 grams of activated charcoal in each cartridge. If the specific cartridge being evaluated can be reproducibly determined to have a certain amount of active material, then modifications to the service life may be justified. You can obtain information on cartridge specifications from manufacturers.
High tempertaures can adversely affect the adsorptive capacity of respirator cartridges and canisters. The high temperature may act by thermally loosening the attractive forces that make adsorption happen or may act in concert with humidity by increasing the moisture carrying capacity of air. This latter mechanism may represent the greatest likely effect on service lives of cartridges. Temperature effects alone have been reported to reduce the service life 1-10% for every 10 degrees Celsius rise depending on the specific solvent (Nelson, et. al., 1976). Corrections to cartridge estimated service life for this effect alone are probably not necessary under normal working temperatures.
Relative Humidity is a measure of the amount of water vapor the air will hold at a specified temperature and is expressed in percentage values. Since warmer air will hold more water than colder air, the same relative humidity at a higher temperature represents a significantly greater amount of moisture. High relative humidity is a significant negative factor in the capacity of organic vapor cartridges since the large quantity of water vapor will compete with the organic vapors for active sites on the adsorbent. Most of the laboratory work determining adsorbent capacity has been performed at a low relative humidity of 50% at approximately 70 degrees F.
If the actual use of the organic vapor respirators will take place in a significantly more humid environment, then you may need to apply or take into account a safety factor when determining a service life. The exact magnitude of the humidity effect is complex, dependent in part upon chemical characteristics and concentrations of both the contaminant and the water vapor. Based upon relatively few studies, a reduction by a factor of 2 in the cartridge service life originally estimated based upon 50 % relative humidity, may be made when the relative humidity reaches 65% (Nelson, et. al., 1976; Werner, 1985). If the relative humidity exceeds 85%, you should consider experimental testing or another method to more specifically determine the service life. Mathematical modeling may be an appropriate, albeit complex, approach to predict the effect of humidity at various chemical concentrations (Wood, 1987; Underhill, 1987).
Multiple contaminants introduce a great deal of variability into the prediction of service life for respirator cartridges. Much of the laboratory testing and the mathematical models have utilized a single contaminant to determine service lives. Only a limited number of multiple contaminant situations have been studied and reported in the literature (e.g.Yoon, 1996; Jonas et. al., 1986). Cartridge service life for mixtures of compounds with significantly different chemical characteristics is probably best determined by experimental methods. Predictions based upon models without experimental data should probably be very conservative and ascribe the service life derived from the least well adsorbed compound to the total mixture concentration in terms of parts per million. The displacement of a less well adsorbed compound by a more highly adsorbed one may alter the actual service life from the estimated one in some cases.
|These generalizations should only be used in concert with one of the other methods of predicting service life for specific contaminants.|
If you need help developing a respiratory protection program, call EHSO at 770-645-0788!