What is oxygen toxicity in scuba diving and how to avoid it...

Oxygen toxicity in scuba diving happens when a diver breathes oxygen at a high partial pressure (PPO₂) for too long.

Although oxygen is essential for life, under pressure underwater it can become toxic to the body, especially the brain (CNS toxicity) and lungs (pulmonary toxicity).

Why it happens

As depth increases, gas pressure increases.
Even normal air (21% oxygen) becomes “stronger” under pressure.

Example:

• At the surface: PPO₂ of air = 0.21 ATA

• At 30 m / 100 ft: PPO₂ of air ≈ 0.84 ATA

• With enriched air nitrox (EAN32 or EAN36), PPO₂ rises faster.

Most recreational agencies limit:

• Working PPO₂: 1.4 ATA

• Maximum contingency PPO₂: 1.6 ATA

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Types of Oxygen Toxicity

1. CNS Oxygen Toxicity (Central Nervous System)

This is the dangerous one for divers because it can cause sudden convulsions underwater.

Symptoms

Remember the acronym VENTID-C:

• Vision changes (tunnel vision)

• Ears ringing

• Nausea

• Twitching (especially lips)

• Irritability

• Dizziness

• Convulsions

A seizure underwater can lead to drowning if the regulator is lost.

Main causes

• Exceeding maximum depth for the gas

• High PPO₂

• Long exposure

• Stress, fatigue, cold, CO₂ buildup

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2. Pulmonary Oxygen Toxicity

Usually affects technical divers or hyperbaric treatments after very long oxygen exposure.

Symptoms

• Chest burning

• Coughing

• Difficulty breathing

Rare in recreational diving.

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Nitrox and Oxygen Toxicity

With EAN32 (32% oxygen):

• MOD at PPO₂ 1.4 = about 33 m / 110 ft

• MOD at PPO₂ 1.6 = about 40 m / 132 ft

With EAN36:

• MOD at PPO₂ 1.4 = about 28 m / 95 ft

This is why nitrox divers must always analyze tanks and respect MODs.

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How to Avoid Oxygen Toxicity

1. Stay within MOD (Maximum Operating Depth)

Never exceed the depth limit for your gas mix.

Formula:
[
MOD = \left(\frac{PPO₂}{FO₂} -1\right)\times10
]
(metric)

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2. Monitor PPO₂

Modern dive computers display PPO₂ during nitrox dives.

Keep:

• Preferred ≤ 1.4 ATA

• Emergency max ≤ 1.6 ATA

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3. Avoid CO₂ buildup

High carbon dioxide increases risk dramatically.

Avoid:

• Skip breathing

• Overexertion

• Poor buoyancy causing hard swimming

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4. Limit exposure time

Even acceptable PPO₂ becomes risky over long periods.

Technical divers track:

• CNS %

• OTUs (oxygen tolerance units)

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5. Analyze your gas

Always personally verify:

• Oxygen percentage

• MOD

• Computer settings

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6. Stay calm and rested

Fatigue, stress, dehydration, and cold may increase susceptibility.

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What to Do if Symptoms Appear

If twitching, dizziness, or visual disturbances occur:

1. Ascend slightly to reduce PPO₂

2. Stop exertion

3. Signal your buddy

4. End the dive safely

If a diver convulses underwater:

• Keep regulator in mouth if possible

• Do not ascend immediately during active seizure

• Wait until convulsions stop

• Then perform controlled ascent

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Recreational Diving Reality

For normal recreational diving:

• Air diving rarely reaches dangerous PPO₂ levels

• Oxygen toxicity risk mainly appears with:

  • Nitrox

  • Deep diving

  • Technical diving

  • Rebreathers

That’s why proper nitrox training is important before using enriched air.