Cartridge Change Schedule
	Factors That Can Reduce Cartridge Service Life
	Estimating a Cartridge's Service Life

When respirators are provided for protection against gases and vapors, the OSHA Respiratory Protection Program requires employers to provide:

1. An atmosphere-supplying respirator, or
   
   
2. If there is no ESLI appropriate for conditions in the employer's workplace, the employer implements a change schedule for canisters and cartridges that is based on objective information or data that will ensure that canisters and cartridges are changed before the end of their service life. The employer shall describe in the respirator program the information and data relied upon and the basis for the canister and cartridge change schedule and the basis for reliance on the data.

A change schedule is that part of the written program which explains how often respirator cartridges are to be replaced, and the information that was relied upon to make this judgment. The useful service life of a respirator cartridge 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
  
  
• the amount of contaminants in the air

OSHA suggests that employers apply a safety factor to the service life estimate to assure that the change schedule is a conservative estimate.

When establishing a cartridge change schedule, the following should be kept 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.
  
  
• You should account for environmental and user factors and use a conservative approach when evaluating service life testing data.
  
  
• You should apply a safety factor to any estimate to account for uncertainty.
  
  
• Mixtures, intermittent use and concentrations, storage practices and other variables may require the use of an administrative time limit, e.g., one day, even though the estimated life would be longer.
  
  
• There is a published "rule of thumb" that may provide a rough estimate of cartridge service life. However, you should NOT use this as the sole method of determining service life.
  
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Factors that can reduce cartridge service life^†

• Exertion level
• Cartridge variability
• Temperature
• Humidity
• Multiple contaminants
  
  
• 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 adsorbent. The total amount of capture 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 of 50 - 60 liters per minute. This approximates a high end of moderate work rate. For work rates that exceed this level (e.g., heavy shoveling, running, etc.) you may need to apply or take into account a correction factor when determining a service life.
  
  
• Respirator cartridge variability: Some cartridges contain more activated charcoal than others.
  The service life of a respirator cartridge is directly related to the amount of active material in the cartridge. For example, 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. Consult the manufacturer(s) for information on cartridge specifications.
  
  
• Temperature: The hotter it is, the shorter the service life.
  High temperatures 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 percent 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: Water vapor will compete with organic vapors for active sites on the adsorbent.
  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 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 percent 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 percent relative humidity, may be made when the relative humidity reaches 65 percent (Nelson, et. al., 1976; Werner, 1985). If the relative humidity exceeds 85 percent, 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: Predictions should be derived from the least well adsorbed compound.
  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.
  
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Estimating a cartridge's service life
Once you have determined the identity and airborne concentration of the chemical(s) to which your employees are exposed, you are ready to estimate how long your respirators will work and apply the safety factor. OSHA suggest three valid ways for estimating a cartridge's service life. These include:

1. Conducting Experimental Tests
2. Use the Manufacturer's Recommendation
3. Use a Math Model

Conducting Experimental Tests
Experimental testing is probably the best way to determine service life for multiple chemicals or specific conditions. The steps to follow when using this method include:

1. Obtain the following information:
   
   • Names of all airborne contaminants
   • Breathing rate of workers or;
   • Maximum flow rate of powered air purifying respirator;
   • Estimate of worst case exposure levels.


2. Determine who will conduct the experimental tests:
   
   • Your company's Industrial Hygienist
   • An outside consultant or laboratory


3. Provide the tester with the following:
   
   • Information from step 1.
   • Actual cartridges for the respirators.
   • The opportunity to test at the work site under typical conditions; or,
   • The range of variable factors or conditions to be given to the lab.


4. Obtain the results and create a written change schedule for the cartridges:
   
   • If you are relying on experimental testing data, it is important to keep in mind that:
     • There is no widely accepted, standard protocol for performing service life testing.
     • OSHA has devised a field testing approach which will demonstrate the validity of an established change schedule.
     • The EPA has published draft Interim Recommendations for cartridge testing.
     • A field testing approach is provided by Cohen, H. J., Development of a Field Method for Calculating the Service Lives of Organic Vapor Cartridges - Part IV. Results of Field Validation Trials, American Industrial Hygiene Association Journal, (1991), Pages 263 - 270.
     • Ideally, respirator cartridges should be tested under worst case conditions either in the workplace or in the laboratory.
     • The determination of breakthrough from workplace testing does not require the determination of the full breakthrough curve for the respirator cartridge.

Use the Manufacturer's Recommendation
Chemical and respirator manufacturer's may be able to estimate cartridge change schedules based upon their own expertise and testing data. The basic steps to follow when relying on the Manufacturer's Recommendation include:

1. Obtain the following information:
   
   • Names of all airborne contaminants
   • Concentrations of those contaminants (in parts per million)
   • Humidity in the work area.
   • Work rate


2. Contact the manufacturer of the respirators you plan to use.
   Many respirator manufacturers (e.g., MSA, 3M, etc.) have developed computer software that will estimate cartridge breakthrough based upon the above information and the type of respirator/cartridge being used. This software may be be found at the company's websites.



3. Provide the manufacturer with the following information.
   
   • Information from step 1.
   • Name of the respirator model.


4. Request the cartridge service life as well as the exact objective information they relied upon to project that service life.



5. Create a written change schedule for the cartridges.

Respirator manufacturers are likely to possess the most accurate data for their own respirators. However, the manufacturer may not have tested the respirator with the chemical(s) you work with, and therefore, may not be able to offer a reliable recommendation.

Use a Math Model
Mathematical equations have been used to predict the service lives of vapor respirator cartridges when used for protection against single contaminants. The OSHA website for Respirator Change Schedule uses an equation developed by Gerry O. Wood. This website includes a table of precalculated breakthrough times for 120 chemicals.

The basic steps in using a Math Model Table, such as the Wood Math Model Table, to estimate respirator service life include:

1. Determine the concentration level of airborne contaminants in the work area.
2. Obtain access to a predictive table that is based on research.
3. Use the table to come up with a cartridge service life estimate.
4. Account for differences in the real work environment and those assumptions used by the math model.
• Humidity and temperature
• Breathing Rate
5. Create a written change schedule for the cartridges.

The basic steps for using a Math Model Equation include:

1. Determine the following:
• Number of cartridges used by the respirator.
• Weight of sorbent in each cartridge in grams.
• Carbon micropore volume in cubic centimeters per gram.
• Density of the packed bed in units of grams per cubic centimeter.
• The maximum temperature expected in the workplace.
• The maximum concentration of contaminants in the workplace (parts per million)).
• The work-rate (volumetric flow rate) in units of liters per minute (LPM).
2. Put the information from Step 1 into a mathematical equation and calculate for the unknown service life.
3. Apply a safety reduction to the service life estimate, create a written change schedule for the cartridges, and include in the written respiratory protection program.

The math models are usually only directly applicable for single contaminant exposures. If you have a multiple contaminant situation, you may need to use other methods to determine the cartridge change schedule, or increase the safety factors.

†The information in this section was taken from the OSHA Respiratory Advisor Web site.