Oxygen toxicity (CNS, pulmonary)

Oxygen toxicity occurs when breathing oxygen at high partial pressures damages your body. This risk comes in two forms: Central Nervous System (CNS) and pulmonary.

How it works in practice

High pressure changes how oxygen interacts with your cells. CNS toxicity affects your brain while you dive. It often causes sudden seizures because the oxygen levels become too high for your nervous system. You cannot predict a seizure once it starts.

Pulmonary toxicity targets your lungs after long exposures. You might feel a burning chest or a cough during your deco stops. This happens because the oxygen concentration irritates your lung tissue over several hours. It is a slow process.

I remember diving in Anilao in 2016. We stayed at 18 m for 60 minutes using Nitrox 32. My lungs felt tight later that evening.

Why it matters for safety

You must track your oxygen exposure carefully. Always track your CNS clock on your dive computer. This digital counter shows how much of your daily limit you have used. It helps prevent a seizure.

Do not exceed the limits set by your training agency, such as PADI or SSI. Staying within safe oxygen limits prevents life-threatening emergencies. Most divers use a limit of 100% CNS clock for a single day.

Depth is the main driver of this risk. If you dive to 40 m on air, your partial pressure of oxygen (PO2) reaches 1.0 ata. While this is safe, switching to Nitrox 32 at that depth pushes PO2 to 1.6 ata. This level is the absolute maximum for active diving.

Common mistakes in Southeast Asia

Many divers ignore their computer settings. I saw a diver in Koh Tao hit 40 m on air in 2018 and nearly suffered a seizure. He forgot to update his gas settings on his Suunto computer. He was lucky.

Using the wrong gas mix is another error. Some recreational divers use Nitrox 36 for deep dives because they want more bottom time. This mistake increases your CNS percentage very quickly. You must calculate your PO2 before every descent.

Check your equipment often. A faulty regulator can sometimes deliver higher pressures than expected. This small error changes your oxygen math instantly.

Connection to deco and gas

Oxygen toxicity is closely linked to decompression sickness (DCS). Both issues arise from how gases behave under pressure. If you push your PO2 too high, you might also increase your nitrogen load.

Nitrox helps reduce nitrogen absorption. However, it increases your oxygen exposure if you stay deep. You are trading one risk for another. You must balance these two factors during your dive planning.

Watch your depth carefully. Use a dive computer that allows you to input specific gas percentages like 32% or 36%.