Merck Manual

Please confirm that you are not located inside the Russian Federation


Gas Toxicity During Diving


Richard E. Moon

, MD, Duke University Medical Center

Last full review/revision Aug 2019| Content last modified Aug 2019
Click here for the Professional Version
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Topic Resources

Problems during diving can result from toxic effects of gases such as nitrogen, oxygen, carbon dioxide, and carbon monoxide.

Air is a mixture of gases, mainly nitrogen and oxygen with very small amounts of other gases. Each gas has a partial pressure, based on its concentration in the air and on the atmospheric pressure. Both oxygen and nitrogen can have harmful effects at high partial pressures.

Oxygen toxicity

Oxygen toxicity occurs in most people when the partial pressure of oxygen reaches 1.4 atmospheres or greater, equivalent to slightly over 187 feet (57 meters) depth when breathing air (shallower depths when breathing oxygen concentrations greater than 20%). Although oxygen toxicity can rarely occur in a hyperbaric oxygen chamber, divers who use inappropriate concentrations of oxygen during deep dives are at higher risk.

Symptoms include tingling, focal seizures (such as facial, lip, or one-sided limb twitching), vertigo, nausea and vomiting, and constricted (tunnel) vision. About 10% of people have seizures or fainting, which typically results in drowning.

To prevent oxygen toxicity during deep dives, special gas mixtures and special training are required.

Nitrogen narcosis

Nitrogen narcosis (rapture of the deep) is caused by high partial pressures of nitrogen.

Symptoms resemble those of alcohol intoxication. People show very poor judgment and become disoriented and often euphoric. They may fail to surface on time or even swim deeper, thinking they are going to the surface. This effect becomes noticeable at 100 feet (about 30 meters) in some divers breathing compressed air and is usually incapacitating at 300 feet (about 90 meters).

To minimize these effects, divers who must dive to great depths typically breathe a special mixture of gases rather than regular air. Low concentrations of oxygen are used, diluted with helium or hydrogen rather than nitrogen, because helium and hydrogen do not cause narcosis. Diving with helium at depths greater than 500 to 600 feet can precipitate the high-pressure nervous syndrome. This resolves during ascent, which must occur at an appropriately slow rate to avoid decompression sickness.

Carbon dioxide toxicity

Some scuba divers have carbon dioxide toxicity because they do not increase their breathing adequately during exertion. Others retain carbon dioxide because the compressed air at depth is denser and requires greater effort to move it through the airways and breathing apparatus. Voluntary reduction in breathing rate to conserve air ("skip breathing") can also cause carbon dioxide buildup in the blood. Malfunction of a closed or semiclosed rebreathing apparatus is another potential cause of carbon dioxide toxicity.

A buildup of carbon dioxide in the bloodstream is the body’s signal to breathe. Divers, such as snorkelers, who hold their breath rather than use a breathing apparatus, often breathe vigorously (intentional hyperventilation) before a dive, breathing out a large amount of carbon dioxide but adding little oxygen to the blood.

This maneuver allows them to hold their breath and swim under water longer because their carbon dioxide levels are low. However, this maneuver (called dangerous underwater breath-holding) is also hazardous, because divers can run out of oxygen and lose consciousness (called breath-hold blackout or hypoxic blackout) before the carbon dioxide reaches a level high enough to signal the need to return to the surface and breathe. This sequence of events is probably responsible for many unexplained drownings among spearfishing competitors and others who hold their breath while diving or swimming underwater.

Did You Know...

  • Hyperventilating before swimming underwater in an attempt to increase breath-holding time can increase the risk of drowning.

Symptoms of carbon dioxide toxicity may include

  • Headaches

  • Difficulty breathing

  • Nausea

  • Vomiting

  • Flushing

High carbon dioxide levels can also lead to blackouts, increase the likelihood of seizures due to oxygen toxicity, and worsen the severity of nitrogen narcosis. Divers who frequently have headaches after diving or who pride themselves on using air at a low rate may be retaining carbon dioxide.

Carbon dioxide usually gradually decreases as a diver ascends. People who develop symptoms during a dive should return gradually to the surface. People who routinely have headaches after diving may need to modify their diving technique.

Carbon monoxide poisoning

Carbon monoxide is a product of combustion. Carbon monoxide can enter a diver’s air if the air compressor intake valve is placed too close to engine exhaust or if the lubricating oil in a malfunctioning compressor becomes hot enough to partially combust, producing carbon monoxide.

Symptoms include nausea, headache, weakness, clumsiness, and confusion. Severe cases of carbon monoxide poisoning can cause seizures, loss of consciousness, or coma. Diagnosis is with a blood test. As time passes, the blood level decreases, so in order to make the diagnosis the test should be done as soon as possible. The diver’s air supply can also be tested for carbon monoxide.

People are given oxygen. High blood levels of oxygen help eliminate carbon monoxide from the blood but do not always cause organ damage to resolve. For people with severe poisoning, people may be given high-pressure oxygen therapy in a hyperbaric chamber, available at certain medical centers.

High-pressure nervous syndrome

A poorly understood set of neurologic symptoms can develop when people dive deeper than 500 to 600 feet (150 to 180 meters), particularly when the dive is rapid and the diver breathes a mixture of helium and oxygen. Symptoms include nausea, vomiting, tremors, clumsiness, dizziness, fatigue, sleepiness, muscle jerks, stomach cramps, and confusion. The syndrome resolves on its own when people ascend or when the rate of descent is slowed.

More Information

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version

Also of Interest


View All
How to Fit and Use Crutches
How to Fit and Use Crutches
3D Models
View All
3D Model