Air purifying respirators use cartridges to remove chemical contaminants from the air before the air enters the breathing zone. These cartridges work by an absorption process—contaminants in the air are attracted to the media in the filter. Since there is a limited amount of media in the cartridges, they have a limited service life and must be changed when the media becomes nearly spent. 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 also suggested that employers apply a safety factor to the service life estimate to assure that the change schedule is a conservative estimate.
A change schedule is the part of the written respirator program which indicates how often cartridges should be replaced and what information was referenced to make this determination. It is no longer acceptable to rely on odor thresholds and other warning properties as the primary way of determining cartridge life. Fortunately for employers, the Occupational Safety and Health Administration (OSHA) provides three valid ways to estimate a cartridge’s service life.
The revisions to the OSHA Respiratory Protection standard, 29 Code of Federal Regulations (CFR) 1910.134, became effective April 8, 1998. 29 CFR 1910.134(d)(3)(iii)(B)(2) states: “If there is no end of service life indicator (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.”
Currently, there are very few cartridges equipped with National Institute for Occupational Safety and Health (NIOSH) approved ESLIs. Therefore employers must develop their own change schedules, but they do not have to search for and analyze test data themselves. Employers can simply acquire information from other sources that have the expertise to develop change schedules and note the source for this information in their written respiratory program. If no information can be obtained to develop an accurate respirator cartridge change schedule, the employees must use a supplied-air system.
Developing a Respirator Cartridge Change Schedule
The following factors must be considered when developing a respirator cartridge change schedule:
- Frequency of use (e.g., continuously, intermittently) throughout the shift
- Wearer’s work rate
- The presence of potentially interfering chemicals
Once this information is gathered, a respirator cartridge change schedule is developed by using one of the following three methods:
1. Conduct Experimental Tests
An end user, outside consultant or outside laboratory can perform experimental testing to determine cartridge service life once all pertinent information has been gathered about the workplace and the contaminants.
2. Use Manufacturer's Recommendations
A manufacturer's service-life recommendation could come from the chemical supplier or, more likely, the respirator manufacturer. This method is not as reliable as conducting your own tests, but is still a good alternative. Unfortunately, respirator manufacturers might not have information for your specific chemicals or compounds.
3. Use Mathematical Models
Mathematical models use computer programs or complex mathematical formulas to determine cartridge service life. If computer programs are not available, then predictive or descriptive mathematical models are to be used:
- Predictive Model: The predictive model developed by G.O. Wood looks at chemical and physical properties of compounds to determine cartridge life. This model is the least accurate method because it does not look at actual experimental data.
- Descriptive Model: The descriptive model looks at existing experimental data to set up a basic model. Once this model has been set up, it can be used to calculate values for points where experimental data is not available.
Both models have drawbacks. Neither model relies heavily on experimental data, thus reducing accuracy, and both equations are very complex, opening them to human error. These models also only work well when you are dealing with single-contaminant situations.
One tool that can be used to help estimate organic vapor cartridge life is the Rule of Thumb method. This method is from the American Industrial Hygiene Association (AIHA) publication "The Occupational Environment — Its Evaluation, Control and Management." The Rule of Thumb is:
- If the concentration of the chemical is less than 200 parts per million (ppm) and the chemical's boiling point is greater than 70°C, you can expect a service life of eight hours at a normal work rate
- Service life is inversely proportional to work rate
- Reducing concentrations by a factor of 10 will increase the service life by a factor of five
- Humidity above 85 percent will reduce service life by 50 percent
Note: The Rule of Thumb method should not be the sole method of determining service life. It can only be used as a guide.
Establishing cartridge service life for mixtures of contaminants is a complex task and one that requires considerable professional judgment to create a reasonable change schedule. Cartridge service life for mixtures is best determined using experimental methods. OSHA states the change schedule for a mixture should be based on reasonable assumptions that include a margin of safety for the worker wearing the respirator. Where the individual compounds in the mixture have similar breakthrough times (i.e., within one order of magnitude), service life of the cartridge should be established assuming the mixture stream behaves as a pure system of the most rapidly migrating component or compound with the shortest breakthrough time. Where the individual compounds in the mixture vary by two orders of magnitude or greater, the service life may be based on the contaminant with the shortest breakthrough time. OSHA suggests that an approach such as this reflects good health and safety practice where neither objective nor experimental data is available for the mixture.
In the event that a user experiences breakthrough, the cartridge or filter becomes soiled or breathing becomes difficult prior to the scheduled change out interval, they should change the cartridge/filter immediately and notify their employer so that use conditions can be re-evaluated and adjustments made to the change schedule.
Mine Safety Appliances (MSA)
North Safety by Honeywell
OSHA 29 CFR 1910.134
OSHA Logic Flowchart
OSHA Respirator eTool
The Occupational Environment — Its Evaluation, Control and Management, American Industrial Hygiene Association