Oxygen enrichment is the general term for any gas or liquid that has more than 21% oxygen by volume in the air. In enclosed spaces, high levels of oxygen can be dangerous to humans.
Why 21% Oxygen?
The average oxygen level on earth is approximately 21% oxygen by volume. The oxygen level on the planet has stabilized at 21% because the amount of oxygen consumed is approximately equal to the amount of oxygen generated over time.
There is nothing “magic” about 21%. It is simply the volume of oxygen in our present day atmosphere. The same balances hold true for nitrogen (78% of air by volume) as well as the trace gases like argon, carbon dioxide and water vapor. If you go back far enough in geological time, these numbers will be different.
The earth formed over 4.5 billion years ago. At that time, there was virtually no free oxygen in the atmosphere. Fossil remains of oxygen-breathing mammals have been found that date back around 150 million years and human remains have been recorded over the last 200,000 years, so logic tells us that oxygen levels have risen to 21% over time. While there have been minor variations in the earth’s total oxygen level, over time humans and animals have become physically adapted to breathing air that contains approximately 21% oxygen.
Therefore, it makes sense that because humans and animals are adapted to breathing 21% oxygen in air, anything much different from 21% would be hazardous to our health. This is why OSHA considers any oxygen level below 19.5% as oxygen deficient or anything above 23.5% as oxygen enriched air. Both are potentially dangerous.
How much oxygen is too much?
While OSHA defines anything above 23.5% oxygen as dangerous, medical scientists have experimented with subjecting patients to hyperoxia, or breathing air at 100% oxygen levels for years. Hyperbaric oxygen therapy (HBOT) uses 100% oxygen at an ambient pressure higher than atmospheric pressure. Patients are placed in a sealed container with increased pressure and oxygen levels. For example, this is used to treat decompression sickness, to give supplemental oxygen to premature babies, and is used to treat a host of diseases under the general term of hyperbaric oxygen therapy.
The problem with breathing air at increased oxygen levels is that over time it can damage cell membranes, a collapse of the alveoli in the lungs, retinal detachment, and seizures. However, studies show that long term recovery is possible once oxygen levels are reduced.
Oxygen enrichment hazards
Leaking pipe, flange or manifold connections. This can be particularly hazardous in areas where there is not sufficient ventilation causing the oxygen concentration to increase.
Breaks in systems under oxygen pressure. A sudden release of oxygen under pressure can result in a relatively large jet of escaping oxygen. For example, a fork lift operator might rupture an oxygen pipe. Even a small volume of liquid oxygen can produce a large volume of oxygen gas.
Oxygen use in cutting and welding. In industrial processes where oxygen is used, oxygen may be allowed to freely escape during the process. Over time, the oxygen remains in the atmosphere and, if ventilation is inadequate, the air can become enriched with oxygen.
Liquid oxygen spill. A spill of liquid oxygen creates a dense cloud of oxygen enriched air when evaporating. In an open space, a hazardous oxygen concentration would only exist for a moment. In a closed space, this can result in a dangerous increase in the oxygen level.
Venting from cryogenic gas cylinders. As the temperature rises, liquid oxygen tank pressure can cause evaporation of the liquid oxygen. This is a normal occurrence. However, if the oxygen is not used for some time, the pressure relief valve can release oxygen into the surrounding atmosphere.
Desorption. Oxygen is released when cold materials which have absorbed oxygen, such as absorbents (molecular sieve, silica gel) or insulation materials are warmed to room temperature.
Oxygen enrichment and fire
While breathing moderately oxygen enriched air is not a problem, it does increase the risk of fire. To create fire you need heat, fuel and oxygen. By increasing any of these three elements you increase the risk of fire starting.
Firefighters have known for years that when the windows in a burning building break, the quick introduction of oxygen creates a backdraft, the rapid explosion of superheated gases in a fire with too little oxygen.
Another example of the dangers of oxygen enrichment are welding fires. According to the American Welding Society (AWS) Fact Sheet, Fire and Explosion Prevention:
- Sparks can travel up to 35 ft. (10 m) horizontally and even farther when falling. They can pass through or become lodged in cracks, clothing, pipe holes, and other small openings. Even at 35 ft., sparks can be hotter than 2,500 degrees F.
- Torch flames can ignite substances within several feet of the flame.
- Material in contact with the hot work piece, even away from the flame source and actual weld, can ignite.
- When this high heat and high levels of oxygen combine, wood, sawdust, rags, clothing or flammable vapors can become a potential fuel source for fire.
Oxygen enrichment safety
To protect workers and staff against high levels of oxygen, GasLab offers the RAD-0012 Oxygen Enrichment Safety Alarm. This wall-mounted oxygen sensor continuously detects oxygen levels in an enclosed area and sounds an alarm if oxygen levels begin to rise. It is used in industries that utilize bulk oxygen tanks like steel manufacturing, welding and cutting, cryogenics, for medical breathing gas, diving tanks, or in areas where bulk oxygen is produced.