Nitrogen Narcosis

What it feels like

Nitrogen narcosis affects the brain in a way that many divers compare to drinking alcohol. Mild narcosis can bring a light-headed, cheerful feeling, overconfidence or slowed thinking. Deeper, it can cause poor decisions, difficulty reading gauges, fixation on one task, anxiety, confusion and, at extreme depths, loss of consciousness. The French diving pioneer Jacques-Yves Cousteau called it "l'ivresse des grandes profondeurs", the rapture of the deep.

The danger is not the feeling itself but what it does to judgment. A narced diver may not notice their gas running low, may misread their computer or may follow a fish deeper without thinking.

Why it happens

At higher pressure, more nitrogen dissolves in the body, including the nervous system, and its partial pressure rises. Above a certain level it interferes with how nerve signals pass between cells. Scientists still debate the exact mechanism, but the effect is closely tied to the partial pressure of the gas: the deeper you go, the stronger it becomes.

Oxygen and carbon dioxide also appear to add to the narcotic effect. Breathing enriched air nitrox is generally not considered a way to reduce narcosis, because the extra oxygen is thought to be narcotic too.

Depth and variation

Most divers breathing air start to notice narcosis somewhere around 30 m (100 ft), and nearly everyone is affected by 40 m (130 ft), the usual recreational depth limit. An old teaching rule, sometimes called the "martini law", compares each 15 m (50 ft) of depth to one alcoholic drink. It is a rough way to explain the idea, not a measurement.

How strongly narcosis hits varies between divers and between dives. Cold, fatigue, anxiety, hard work, alcohol or medication, poor visibility and a fast descent can all make it worse. Experience helps divers recognise it, but nobody becomes immune.

Managing narcosis

• Plan the dive, write down the key limits and stick to them.
• Descend at a controlled rate and stop to check yourself and your buddy.
• If you or your buddy notice symptoms, ascend a few metres. Narcosis usually eases within a minute or two.
• Keep tasks simple at depth and use hand signals your buddy knows well.
• For deeper dives, technical divers breathe trimix, which replaces some nitrogen with helium, a gas that isn't narcotic at these depths.

No lasting harm

Narcosis itself leaves no after-effects. Once you ascend, your head clears and you'll often have little memory of how impaired you were. The lasting harm comes only from accidents that happen while narced, which is why depth limits and good planning matter.

Frequently asked questions