“Love is like oxygen. You get too much, you get too high; not enough and you’re gonna die.”  — Andy Scott, of Sweet

OSHA standards define safe lower and upper limits to the concentration of oxygen in the atmosphere to which workers are exposed. The limits are a minimum of 19.5% and a maximum of 23.5% in both the Respiratory Protection Standard ( 29 CFR 1910.134(i)(1)(ii)(A) ) and the Permit Required Confined Spaces Standard ( 29 CFR 1910.146(b) ). Why those limits?

Oxygen is the only material for which OSHA has established a lower limit and that lower limit is easy to understand. The effects of oxygen deficiency are tabulated in many sources. For instance, the CSB shares that below 19% oxygen, people begin to suffer from impaired cognitive abilities without being aware of the danger, as well as suffering from adverse physiological effects. So, OSHA set the lower limit at 19.5%, above the level at which people begin to suffer adverse effects, but still allowing for some departure from the 20.9% oxygen concentration of earth’s atmosphere.

We need oxygen. But how did OSHA decide that 23.5% oxygen was too much for worker exposure?

Simple Asphyxiant?

There are three reasons to limit the exposure to gases or vapors in the atmosphere:

  • Asphyxiation
  • Toxicity
  • Flammability
  • Oxidizing

Simple asphyxiation results when we breathe an atmosphere that is oxygen-deficient.

Since it is the absence of oxygen that causes simple asphyxiation, oxygen cannot be a simple asphyxiant. Simple asphyxiation is not the reason for OSHA’s upper limit on oxygen concentration.

Oxygen Toxicity?

Some gases don’t simply displace oxygen, but actively interfere with essential bodily functions, usually at concentrations well below those that would result in an oxygen deficient atmosphere. Gases that interfere with respiration—getting oxygen from the lungs into the blood, and then from the blood into cells—are called chemical asphyxiants or toxic asphyxiants. Gases that interfere with other functions of the body are just called toxic gases.

Oxygen is not a chemical asphyxiant. However, there is a phenomenon known as oxygen toxicity. It occurs at high partial pressures of oxygen and primarily affects patients on hyperbaric oxygen therapy and deep-sea divers. Exposure to high oxygen partial pressures can lead to impacts on the central nervous system, endocrinal damage, pulmonary damage, and retinal damage.

The damage from oxygen toxicity follows a dose/time function. The lower the oxygen partial pressure, the longer the exposure can be without ill effects. An oxygen partial pressure of 0.5 bar (7.25 psi) can be tolerated indefinitely. That means that air cannot result in oxygen toxicity unless it is being breathed at a pressure above 20 psig. On the other hand, an enriched-oxygen atmosphere can eventually lead to oxygen toxicity when the concentration is above 50%.

If oxygen toxicity were OSHA’s concern, the upper safe limit for oxygen would be much higher than 23.5%. Oxygen toxicity is not the reason for OSHA’s upper limit on oxygen concentration.


Category 1 flammable gases meet at least one of two flammability criteria. Either they have a Lower Explosive Limit (LEL) equal to or less than 13%, or the range from Upper Explosive Limit (UEL) is equal to or greater than 12%. Or both. Category 2 flammable gases can burn in air but meet neither of the flammability criteria. Ammonia, for instance, with an LEL of 16% and a UEL of 25%, is a Category 2 flammable gas.

Oxygen doesn’t burn, so it doesn’t have an LEL. Oxygen flammability is not the reason for OSHA’s upper limit on oxygen concentration.

Oxygen is an Oxidizer

Oxygen doesn’t burn, but it does contribute to the combustion of materials that do. Oxygen is an oxidizer. It’s not the only gas that acts as an oxidizer. Ozone, halogens, fluoride, and nitrogen oxides are among the other gases that act as oxidizers. Unlike oxygen, however, these other oxidizers are so poisonous that their PELs are based on their toxicity.

As with all oxidizers, the higher the concentration, the easier it is for flammable or combustible materials to burn.

On Friday, January 27, 1967, during a rehearsal for the launch of Apollo 1 on February 21, an electrical fire ignited the combustible nylon materials in the cabin. The cabin was pressurized at 2 psig with pure oxygen. Within a couple of minutes, all three members of the crew were dead from cardiac arrest induced by high carbon monoxide level, although by the time the ground crew got to them, they were burned beyond recognition.

That disaster changed forever how we view the hazards of oxygen-enriched atmospheres.

How Oxygen-Enrichment Affects Flammability

The ideal mixture for burning a fuel gas is fuel gas and pure oxygen at a stoichiometric ratio, when there is exactly as much oxygen as the fuel needs. This is the hottest burning mixture because it doesn’t need to heat any inert gases that are along for the ride. A mixture with fuel gas in excess of the stoichiometric ratio is called “rich” and at a certain excess, the mixture becomes too rich to burn. A mixture with fuel gas deficient of the stoichiometric ratio is called “lean” and at a certain deficiency, the mixture becomes too lean to burn. Oxygen enrichment opens up the range from too lean to too rich.

Oxygen enrichment also lowers the autoignition temperature (AIT), the temperature at which flammable or combustible materials ignite spontaneously, without an ignition source. Ignition sources work by heating a material above its AIT. The lower the AIT, the easier it is for ignition sources to ignite the material, or more importantly, the weaker the ignition source can be and still ignite the material. So, OSHA determined that there should be an upper limit for oxygen concentrations, at least in some cases.

Note: High pressures and high temperatures, temperatures and pressures well beyond what workers would be exposed to, greatly enhance the impact of oxygen-enrichment on flammability. This is why removing all flammable or combustible substances is essential to commissioning equipment for oxygen service.

So Why 23.5% As the Upper Limit for Worker Exposure to Oxygen?

The first time OSHA used 23.5% as the upper limit for oxygen concentration in air was in the Respiratory Protection Standard. In the first version of the regulation, adopted in 1971,  OSHA used 23.5% oxygen as the upper limit for breathing air because that was already the breathing air specification described in the CGA G-7.1-1966 Standard. OSHA noted that Grade D air was readily available. In addition to breathing air, with limits of 19.5% to 23.5% oxygen, the Respiratory Protection Standard also allow for the use of breathing oxygen, as long as it meets United States Pharmacopoeia requirements and is in equipment designed for oxygen service.

The next time OSHA used 23.5% as the upper limit for oxygen concentration was in the Permit Required Confined Spaces Standard, which went into effect on April 15, 1993. One definition of a hazardous atmosphere is where the oxygen concentration is outside the range of 19.5% to 23.5%. OSHA originally proposed 22% as the upper limit, but the regulated community complained that was too narrow a range and would lead to false positives in oxygen testing. At the time that OSHA was finalizing the standard, NIOSH testified to OSHA that it considered 25% oxygen to be oxygen-enriched. Ultimately, however, OSHA decided to go with 23.5% oxygen, because that was already the concentration that they had specified in the Respiratory Protection Standard.

And The Hazard Is…

Oxygen enrichment is a hazard because it increases the flammability of other materials, as demonstrated to the world in 1967 when there was a cabin fire in the Apollo 1. It is also a hazard at very high partial pressures because of oxygen toxicity. However, the reason that OSHA has set the maximum concentration for oxygen in breathing air and confined spaces at 23.5% is because that was the specification for breathing air that was already available. While OSHA has established limits in these two cases, OSHA has not established a PEL for oxygen to be used in other circumstances.

Accounting for Oxygen-Enrichment

OSHA has an enforceable upper limit for oxygen concentration when considering breathing air and when considering permit-required confined space entry. OSHA considers 23.5% oxygen to be safe, and the record demonstrates that level is very conservative. If your hazard analysis discovers a potential for oxygen-enrichment, be mindful that there are hazards associated with oxygen-enrichment. Some use 23.5%, not because it is a level that represents danger, but because it is a concentration that anyone can be comfortable is as safe as air.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.