Knowing when to change the cartridge of an air-purifying respirator (APR) is necessary, but it can be tricky. These cartridges, particularly those for gases and vapors, contain sorbent material that has a maximum capacity to trap contaminants. Once the element is “full,” any contaminant in the inhaled air breaks through the cartridge into the facepiece, exposing the user. Obviously, the cartridge must be changed before this occurs. But how can you tell when it’s time to change the filter?

To help you answer that question, OSHA addressed the issue of cartridge service life in the revised respiratory protection standard (29 CFR 1910.134). In addition to creating regulatory guidelines, the agency now provides some helpful tools — both of which will be discussed in this article.

OSHA’s requirements

First, it should be noted that the cartridge-change schedule requirement applies only to respirators used for protection against gases or vapors, not particulates. The reason: as more particles are trapped in a particulate filter, the filter actually becomes more efficient and breakthrough is less likely. Also, breathing resistance increases and the user recognizes the need to change the cartridge to make breathing easier. So a change schedule is not expressly required for particulate respirators.

However, OSHA’s revised standard requires that air-purifying respirators (for gases and vapors) can only be used if they are equipped with an end-of-service-life indicator (ESLI) or if the “employer implements a change schedule for canisters and cartridges that is based on objective information or data.”

ESLIs are devices in the cartridge or canister that give a visible sign that the adsorbent has been exhausted, warning the user to change the cartridge before breakthrough occurs.

Employers who use APRs without ESLIs must establish a schedule for changing cartridges. Users must be trained to recognize the warning properties of chemicals they work with in case of leakage, but they cannot wait until they experience the warnings to change their cartridges.

Cartridge-change schedules must be based on objective information, and the information and data used to establish the schedule must be included in the written respiratory protection program.

Cartridge service life factors

When calculating a cartridge-change schedule, you must look at many factors that could affect the useful service life, including:

  • Breathing rate of the user. This determines how much of the contaminant enters the cartridges and how quickly the adsorbent is used up. Heavy work will usually require more frequent changing. And different workers doing the same work can have different breathing rates. Because there is so much variation, you can classify work as low, moderate, or high and use standard breathing rate values for that workload in your calculations.

  • Temperature and humidity of the work area. High temperatures tend to reduce the adsorbent material’s ability to hold the contaminant, requiring more frequent changes. And water vapor in humid air will saturate the adsorbent material and reduce its ability to trap the contaminant. So it will be necessary to change cartridges more frequently in hot, humid work areas.

  • Amount and capacity of adsorbent in the cartridge. Since this factor can vary from manufacturer to manufacturer, and because different models made by the same manufacturer can use a variety of cartridges, calculations must be based on the specific equipment being used.

  • Chemical concentration in the air. Having more than one chemical in the air at the same time poses a real problem in calculating service life. If one chemical is more readily adsorbed than another, it may quickly saturate the adsorbent and allow the other chemical to break through sooner. If multiple chemicals might be present, a conservative estimate of the service life calculated for the least well-adsorbed chemical should be used.

Simplifying the process

It may seem that you’ll need to use a complicated formula to create your cartridge-change schedule, but there are resources that can help simplify the process.

First, OSHA provides a Respiratory Protection Advisor on its Web site ( This tool offers easy-to-use guidelines that help you understand and comply with the standard. One section explains how to calculate a cartridge-change schedule and includes a table of pre-calculated service times for a limited number of chemicals.

It also has an “Advisor Genius” that makes the calculation based on information you provide about the work environment, the respiratory equipment, and some properties of the chemical.

Most respirator manufacturers also offer service life calculation software on their Web sites. The advantage of using the manufacturer’s calculator is that the cartridge-specific information is already provided in their database, saving some research on your part.

To use these calculators, the chemical can be selected from a database and you simply enter information about the work (breathing) rate and environmental conditions. The Web site then calculates the service time and provides you with a report that you can print out for your records.

While some sites may allow you to download the program to your own computer, it’s a good idea to run it off the Web site to be sure you have the most current data.

Although these calculators simplify the process, there are still some challenges. For instance, you’ll need an accurate assessment of your work environment. For open-air environments, the local weather service can give you information about the temperature and humidity. If your facility is enclosed with heating or air conditioning or if there are unusual sources of heat or humidity, you may need to take measurements yourself. And measuring the chemical concentration in the air is a difficult but vital part of that assessment.

Employers have always needed to be sure respirator users changed cartridges before breakthrough occurred, but new tools like these calculators have been developed and made widely available to ensure better protection for users of air-purifying respirators.