CNS (Central Nervous System) Oxygen Toxicity

CNS oxygen toxicity is a dangerous condition caused by breathing high concentrations of oxygen under pressure. It affects the brain and central nervous system. While oxygen is essential for life, too much of it under pressure can become harmful.

This is different from pulmonary oxygen toxicity, which affects the lungs over long exposure periods. CNS toxicity, on the other hand, can appear suddenly and is more dangerous in the short term—especially underwater.

Why It Matters to Recreational Divers

CNS oxygen toxicity is usually associated with technical diving, but recreational divers can still be at risk in certain situations:

• Diving nitrox at or beyond its maximum operating depth (MOD)
• Using a rebreather incorrectly
• Ignoring proper gas planning
• Exceeding oxygen exposure limits over time

It is rare in recreational diving, but the consequences are severe, so understanding the risks is important.

Even though rebreathers are becoming more common among recreational divers, they greatly increase the risk of CNS oxygen toxicity. These systems maintain a constant partial pressure of oxygen, which can silently push a diver beyond safe exposure levels without warning signs. Rebreathers also add immediate consequences to problems that would be minor or recoverable on open circuit—like poor buoyancy, skipped checks, or task fixation. Most recreational divers are not trained or prepared to manage this level of risk. For that reason, I strongly discourage the use of rebreathers by recreational divers who are not formally trained in technical diving and committed to a disciplined, team-based approach.

What Causes It?

The higher the pressure, the more oxygen your body absorbs. At surface pressure, this is not a problem. But underwater, as depth increases, so does the partial pressure of oxygen (PO2). If PO2 gets too high, oxygen starts to damage nerve cells and brain tissue.

Your body's reaction to high oxygen depends on:

• The actual PO2 level
• The amount of time you're exposed to it
• Individual factors like fitness, CO2 buildup, cold, and stress

CNS toxicity is more likely if several of these stressors are present at once.

Signs and Symptoms

CNS toxicity affects the brain and nerves. Symptoms can appear suddenly and without much warning. A useful memory aid is the acronym VENTID-C:

• Vision changes (tunnel vision, blurred sight)
• Ear ringing (tinnitus)
• Nausea
• Twitching (especially facial muscles or lips)
• Irritability or anxiety
• Dizziness or confusion
• Convulsions (seizures)

The most serious symptom is a seizure. This can lead to drowning if it happens underwater.

Why It's So Dangerous

Unlike other dive problems, CNS oxygen toxicity doesn't always give clear warnings. In some cases, a diver may go from feeling fine to having a seizure in seconds.

This is especially dangerous because:

• A convulsion underwater may cause water inhalation
• The diver cannot protect their own airway or manage buoyancy
• Rescue requires immediate, decisive action from teammates

How to Avoid It

The best way to stay safe is to control your exposure to high oxygen levels. Here’s how:

• Stay within PO2 limits: Most agencies recommend a max PO2 of 1.4 ATA during the dive and 1.6 ATA only during decompression stops or emergencies
• Respect your MOD: Do not exceed the maximum depth for your nitrox mix
• Use a dive computer: Modern dive computers track your oxygen exposure and will warn you if you get close to your limits
• Don't treat MOD as a goal: Plan your dive to stay comfortably shallower than the limit

What To Do If It Happens

If a diver experiences a seizure underwater due to CNS oxygen toxicity, take the following steps:

• Approach from behind and gain control without compromising your own safety.
• Leave the regulator in the diver’s mouth if it is already in place. Do not attempt to replace it if it has fallen out.
• Hold the diver securely, typically around the chest or using gear straps if needed.
• Begin a controlled ascent immediately. Maintain neutral buoyancy, and apply slight upward pressure on the chest to allow for exhalation.
• Do not drop your own weight belt. If needed, drop the diver’s weight only if positive buoyancy cannot be maintained.

At the surface:

• Inflate the diver’s BCD
• Remove the regulator and secure any hoses if needed
• Open the airway once convulsions have stopped
• Check for breathing and begin rescue breaths if necessary
• Administer 100% oxygen and activate emergency response

This is a life-threatening emergency. Timely, skilled intervention is critical to survival.

Summary

CNS oxygen toxicity is rare in recreational diving but extremely serious. The risk increases when diving nitrox beyond its depth limits or staying too long at high oxygen pressures. It can cause seizures without warning and lead to drowning. With careful planning, adherence to PO2 limits, and proper respect for MOD, it is entirely preventable.