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Airborne Contaminants & Noise

Atmosphere-Supplying Respirators

Quick Tips #375

Respirators protect users in two basic ways. The first is by the removal of contaminants from the air. These types of respirators include particulate respirators and air-purifying respirators (APRs). The Occupational Safety and Health Administration (OSHA) defines an APR as a respirator with an air-purifying filter, cartridge or canister that removes specific air contaminants by passing ambient air through the air-purifying element. The second way that respirators protect users is by supplying clean, respirable air from another source—atmosphere-supplying respirators. This Quick Tip provides an overview of the various types of atmosphere-supplying respirators. For a closer look at APRs, refer to Quick Tips #275: Types of Respirators, and #141: Particulate Respirators, 42 Code of Federal Regulations (CFR) Part 84.

OSHA subdivides atmosphere-supplying respirators into the following three classifications: supplied-air respirators (SARs), combination atmosphere-supplying respirators and self-contained breathing apparatuses (SCBAs). Atmosphere-supplying respirators, when used in accordance with the requirements of OSHA’s Respiratory Protection standard (29 CFR 1910.134), offer the wearer a level of protection equal to or exceeding APRs with an enhanced level of comfort. The drawbacks are that they are more expensive and harder to use and maintain than APRs.


When it comes to determining the level of protection provided by a respirator, it is important to first understand OSHA’s definitions for “Assigned Protection Factor” and “Maximum Use Concentration.” Assigned Protection Factor (APF) refers to the workplace level of respiratory protection that a respirator or class of respirators is expected to provide to employees when the employer implements a continuing, effective respiratory protection program. Maximum Use Concentration (MUC) means the maximum atmospheric concentration of a hazardous substance from which an employee can be expected to be protected when wearing a respirator. MUC is determined by the assigned protection factor of the respirator or class of respirators and the exposure limit of the hazardous substance.

Within atmosphere-supplying respirators, APFs can range from 10, for a SAR operated in the demand mode, all the way up to 10,000 for SCBAs. In addition, because atmosphere-supplying respirators provide the wearer with fresh breathing air from a trusted source, certain classifications of atmosphere-supplying respirators are approved for oxygen-deficient conditions (oxygen levels below 19.5%) as well as for airborne concentrations of contaminants that exceed Immediately Dangerous to Life or Health (IDLH) levels.

Atmosphere-Supplying Respirator Classifications

Supplied-Air Respirators (SARs) - The basic SAR is comprised of a respirator facepiece—which could be a tight fitting half mask or full face mask, or a loose-fitting hood or helmet assembly—connected via an air supply hose to a source of breathing air. The air supply could be either a low pressure or high pressure source. A low pressure source would be an ambient pump (an air compressor designed specifically for respiratory protection applications) located in, or drawing air from, a clean environment. A high pressure supply source would be either a cylinder or “cascaded” cylinders of breathing air, or the compressed air supply within a facility as long as it is filtered and monitored to ensure it meets OSHA’s purity requirements (see below) for breathing air.

Basic SARs are most commonly used in either one of two modes: continuous flow (the breathing air flows into the facepiece at a standard, steady rate) or pressure demand (the facepiece is continually pressurized with breathing air, and additional air is drawn in based upon the wearer’s breathing requirements). Pressure demand SARs require a high pressure supply source due to their performance requirements.

Combination Atmosphere-Supplying Respirators - This classification incorporates an integrated bottle of breathing air that can be accessed by the wearer should the primary supply of air become interrupted. This auxiliary supply of air is a limited volume—between five to 10 minutes worth—that enables the worker to safely escape from a hazardous environment. Because combination atmosphere-supplying respirators have this emergency escape or “egress” feature, OSHA allows combination systems that incorporate full facepieces to be used in environments where IDLH levels could be exceeded. Basic SARs are never appropriate for IDLH environments.

Combination atmosphere-supplying respirators operate in a pressure demand mode and therefore always require a high pressure supply source.

Basic SARs and combination atmosphere-supplying respirators are similar in that the wearer is tethered by an air supply hose to the supply of breathing air. Where these two styles differ is in the additional level of protection offered in the combination style respirators.

According to Table 8 from 42 Code of Federal Regulations (CFR), the maximum length of hose allowed from the point of attachment at the air source to the worker is 300 feet for both basic SAR and combination systems. With certain exceptions, as called out in 42 CFR Part 84 Subpart J.149, the air supply hose must be provided in multiples of 25 feet. It is important to note that manufacturers of ambient air pumps may have maximum airline length ratings of considerably less than 300 feet, so always check the manufacturer’s recommendations if an ambient air pump is used as the breathing air source.

Another important point to keep in mind with these two classifications is that OSHA requires both the facepiece and the airline to be from the same manufacturer. You cannot use company X’s facepiece with company Y’s airline. If your source is an ambient air pump, the pump can be from a different manufacturer than the other components.

Self-Contained Breathing Apparatuses (SCBAs) - This third classification differs from the other two classifications in that the wearer transports his/her breathing air in a cylinder on his/her back. While SCBAs offer the advantage of giving the wearer additional freedom of movement, carrying around a cylinder can be fatiguing. Plus the cylinders used on SCBAs offer a finite amount of breathing air and need to be changed out every 30 to 60 minutes. So there are certainly advantages and disadvantages to SCBAs.

Just like the combination units equipped with full facepieces, SCBAs can be used in IDLH environments as long as the SCBAs are rated by the National Institute for Occupational Safety and Health (NIOSH) at a 30-minute or greater supply of air. And because SCBAs have an APF of up to 10,000, they offer the highest MUC of any form of respiratory protection. Because of these two factors, SCBAs are the go-to choice for emergency response scenarios.

SCBAs are available in 30-, 45- and 60-minute capacities. In addition, the option exists between high pressure (pressurized up to 4500 pounds per square inch (PSI)) and low pressure (pressurized up to 2216 PSI) systems. SCBAs in the 45- and 60-minute capacities use high pressure exclusively, while 30-minute SCBA are available in both high and low pressure models.

The advantage of high pressure SCBAs is that they pack more air into a smaller cylinder, making them more comfortable to wear. One disadvantage of high pressure SCBAs is that they are more expensive to purchase than their low pressure sibling. Refilling is another drawback to high pressure systems. It is more difficult to locate a facility capable of refilling the high pressure tanks. Local fire departments are the best bet when it comes to refilling the high pressure tanks; low pressure tanks can typically be refilled at scuba dive shops or your local fire department.

For a more in depth look at SCBAs, please see Quick Tips #193, SCBA Information.


Breathing Air Requirements

No matter what classification of atmosphere-supplying respirator used, OSHA requires that the employer provide workers using these systems with breathing gases of the highest purity, per 29 CFR 1910.134(i). OSHA specifies the use of Grade D breathing air as described in the American National Standards Institute/Compressed Gas Association Commodity Specification for Air, G-7.1. This standard was last updated in 2011. To meet the minimum requirements of this specification the air must include:

  • Oxygen content of between 19.5%–23.5%;
  • Hydrocarbon content of 5 milligrams per cubic meter (mg/m3) of air or less;
  • Carbon monoxide content of 10 parts per million (ppm) or less;
  • Carbon dioxide content of 1000 ppm or less; and a
  • Lack of noticeable odor.

Under 29 CFR 1910.134(i)(4) and 1910.134(i)(5), OSHA also specifics the moisture content requirements for breathing air in cylinders and air supplied through compressors.


29 CFR 1910.134

42 CFR Part 84 Subpart J

42 CFR Part 84 Subpart J Table 8

OSHA’s Assigned Protection Factors for the Revised Respiratory Protection Standard

OSHA’s Respiratory Protection eTool

Compressed Gas Association Commodity Specification for Air, CGA G-7.1-2011

(Rev. 8/2015)

The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.

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