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What is CNS in Scuba Diving?

Breathing oxygen at a partial pressure greater than 1.6 ATA exposes you to a toxic reaction that affects the lungs and the central nervous system (CNS).

The likelihood of CNS diving toxicity increases with oxygen partial pressure, exposure time, exercise, cold, anxiety, increasing carbon dioxide levels in the blood, and steroid therapy.

Precursor Symptom of CNS Oxygen Toxicity

In general, divers who breathe normal air from a scuba cylinder (21% oxygen content) will be avoiding the major risks of CNS in diving.

But, there is an increased risk of getting oxygen toxicity when (either):

Scuba diving on air at depths approaching (or exceeding) 60 metres.
Using a gas mixture that has an increased percentage of oxygen (e.g. EANx).

Oxygen toxicity signs and symptoms vary in severity. But, grand mal convulsions are the first precursor of CNS oxygen toxicity.

Pro Tip: Recompression treatment with oxygen for decompression illness symptoms or carbon monoxide toxicity occasionally results in a toxic reaction.

Oxygen Toxicity Symptoms

CNS toxicity symptoms may first occur underwater or shortly after surfacing. The diver may be suffering any or all of the following manifestations:

Convulsions
Difficulty in concentration (agitation)
Disorientation
Dizziness
Hearing sounds (e.g. ringing bells, roaring, machine sounds)
Lethargy
Light-headedness
Muscular twitching (especially in the lips)
Nausea
Tunnel vision
Twitching of muscles, particularly of the face
Vertigo
Visual abnormalities
Unconsciousness (can be the first indication of CNS in diving)

Oxygen Toxicity Treatment

If a diver begins convulsing underwater, get behind the diver and bring them to the surface as quickly and as safely as possible.
Do not remove their regulator mouthpiece during the ascent and make sure it stays in the victim's mouth when you reach the surface.
Establish positive buoyancy for yourself and the victim. It's best not to remove your weight system in case you need to descend again.
Monitor their consciousness, airway, and breathing. Contact the nearest emergency medical services and begin cardiopulmonary resuscitation (CPR) if necessary.

Special Considerations for Pulmonary Oxygen Toxicity

In most cases, pulmonary oxygen toxicity results from breathing 100% oxygen for an extended period at elevated partial pressures. Typical symptoms include:

Coughing
Pain behind the breastbone
Shortness of breath
Wheezing

Hence, it does not generally threaten air-breathing recreational divers, unless the diver requires recompression. If so, the oxygen level is reduced and monitored. However, divers breathing enriched air and technical divers need to monitor their use of high oxygen mixtures.

Related Information and Help Guides

Important: The tutorial video [16:21 seconds] presented by DAN explains how gas toxicities and pressure affect the human body during a dive.