Nitrogen Narcosis: Symptoms and the Depth Where It Starts

What nitrogen narcosis is, in plain words

It is a temporary anaesthetic effect of nitrogen breathed under pressure. Air is 79% nitrogen, and the partial pressure of that nitrogen rises with depth: about 3.2 atm at 30 m, four times the surface value. Enough of it dissolves in nerve tissue to slow the brain down. The nitrogen is not changed chemically, and the effect wears off when the pressure drops.

Nobody knows the exact mechanism. The old rule is that a gas's narcotic strength follows how well it dissolves in fat, which is why argon is more narcotic than nitrogen and helium hardly at all. StatPearls calls that explanation incomplete and points to effects on specific receptors in the brain (GABA-A and NMDA), the same kind of targets anaesthetic drugs act on.

Narcosis rarely harms you directly. The danger is what you do while impaired. StatPearls says narcosis has been implicated in about 6-9% of recreational diving deaths, usually as a contributing factor to drowning. In the British Sub-Aqua Club's incident database, reviewed by James Clark in 2015, dives deeper than 30 m had 3.5 times as many incidents known to involve narcosis as shallower dives, although other training data suggest only about 30% of dives go that deep. Helium mixes avoid the problem, and our trimix calculator shows how narcotic a given mix feels at depth. This page is general information, not medical advice.

At what depth does narcosis start?

There is no single line. Clark's review found unpractised tasks, such as sorting cards, impaired as shallow as 10-20 m, with most symptoms shallower than 30 m mild and hard to recognise. Beyond 30 m on air (4 bar absolute), the same review says narcosis will invariably occur. StatPearls puts meaningful cognitive deficits from about 30 m and says most divers are significantly impaired at 60-70 m, while some are affected at around 30 m.

Thirty metres is not a safe zone either. In an open-water study cited by StatPearls, divers tested at 30 m on a decision-making task scored significantly worse than divers at the surface, and age, sex, body mass and experience made no difference. StatPearls also notes that the effect does not keep building while you stay at one depth. It grows as you go deeper and eases as you come up.

The table follows the one in Clark's review. Treat the depths as rough bands, because susceptibility varies between divers and between dives. The bottom rows are far beyond the oxygen limit of air: Clark notes that at those depths oxygen toxicity would likely cause convulsions first. DAN gives more than 122 m (400 ft) as the depth where divers can lose consciousness.

Nitrogen narcosis symptoms: what you and your buddy notice

It starts in the higher functions: judgement, reasoning, short-term memory and concentration. StatPearls describes an early euphoria like mild alcohol intoxication, then clumsier hands, fixed ideas and, deeper, hallucinations and stupor. DAN's list adds drowsiness and mood changes, with overconfidence and euphoria in some divers and fear or insecurity in others.

The catch is that you are a poor judge of your own state. StatPearls calls self-recognition of mild to moderate narcosis unreliable, and Clark notes that narcosis dulls pain and makes cold feel less uncomfortable, so a diver can get colder than they realise. A DAN case report shows how this plays out. A diver on a deep course, after an overnight work shift, felt tunnel vision, an intoxicated feeling and pounding in his ears on the way to 35 m (115 ft). At depth a burning pain started in his thigh, and he decided it was in his head. The narcosis cleared when he rose to 24 m (80 ft). The pain did not: it was most likely a box jellyfish sting, and it led to hours of symptoms after the dive.

What narcosis leads to is often easier to see than the narcosis itself. StatPearls and Clark list:

• Rapid or uncontrolled ascents.
• Missed decompression stops.
• Running low on gas without noticing.
• Misusing equipment.
• Slow responses to sights, sounds and signals, which Clark's table lists from about 30 m.

What makes narcosis worse: CO2, cold, fatigue and alcohol

Anything that already dulls the brain adds to it. StatPearls, DAN and Clark name the same group of factors, and carbon dioxide has the best measurements behind it.

• Carbon dioxide. In a 1978 chamber study, a rise of 10 mmHg in exhaled CO2 cut mental and manual performance by about 10%. Hard work at depth raises CO2, and dense gas makes breathing harder. StatPearls describes the effect as additive to nitrogen's, not multiplied by it.
• Descent speed. Clark cites studies in which a slower build-up of pressure produced less narcosis.
• Cold. A 2024 controlled study cited by StatPearls found that mild hypothermia and narcosis together caused additive, lasting impairment of memory, attention and fine motor skills. Good thermal protection helps on both counts.
• Fatigue and exertion. DAN lists fatigue first among its risk factors.
• Alcohol and drugs. StatPearls advises no alcohol for at least 12 hours before diving, and names benzodiazepines, antihistamines, opioids and cannabis as drugs that can add to narcosis.
• Anxiety, which StatPearls says training and experience help control.
• You. DAN counts a personal history of susceptibility as a risk factor, and StatPearls says the same diver can react differently on similar dives.

Can you adapt to narcosis?

Your sense of it adapts. Your performance does not. That is the finding of the two chamber studies that tested it directly.

In 1995, Hamilton and colleagues took 11 highly experienced divers to the equivalent of 54.6 m once a day for 5 days. Reaction times showed learning but no adaptation to narcosis. The divers' own rating of how narced they felt did adapt, clearly, by day 3. The authors concluded that divers' stories of getting used to depth describe the subjective side of narcosis, not the behavioural one. An earlier study held two groups of men at 5.5 bar (about 45 m) on 12 successive days and found their balance worse at depth every day, with no improvement across the series.

Clark's review reaches the same view. Practised tasks get easier, which he calls habituation rather than true adaptation, and the feeling of being "dived up" late in a season is anecdotal. So feeling clear-headed at 35 m on day five of a trip is not evidence that you are less impaired. What does help, according to Clark, is working up to depth gradually and practising often the tasks you would need at depth, such as rescue skills and emergency equipment.

Is oxygen narcotic? END and the two conventions

The evidence is split, so planners use two conventions. END (equivalent narcotic depth) is the depth on air that would feel as narcotic as your mix. Whether oxygen counts toward it changes the answer.

The case for counting it: in the 1978 study by Hesser and colleagues, raising oxygen to 1.65 atm cut mental performance by about 10%, as did raising nitrogen to 6.3 atm, and the authors concluded that nitrogen, oxygen and CO2 all have narcotic properties at raised pressure. On fat solubility alone, oxygen should be about 1.7 times as narcotic as nitrogen, according to the table in Clark's review. The case against: in a 2022 study, 12 people breathed 100% oxygen at 142 and 284 kPa (about 1.4 and 2.8 bar) with no change in brain connectivity measures or psychometric test scores, and the authors concluded that hyperbaric oxygen is not narcotic.

Our trimix calculator offers both. With P as absolute pressure in bar (depth ÷ 10 + 1), the oxygen-narcotic END is ((1 - FHe) × P - 1) × 10, and the oxygen-not-narcotic END is (FN2 ÷ 0.79 × P - 1) × 10, rounded up to 0.1 m. For trimix 21/35 at 45 m, P is 5.5. Counting oxygen: 0.65 × 5.5 = 3.575, so the END is 25.75 m, shown as 25.8 m. Not counting it: 0.44 ÷ 0.79 × 5.5 = 3.063, so 20.7 m.

The same choice decides whether nitrox is less narcotic than air. At 30 m, EAN32 has an END of 30 m if oxygen counts and 24.5 m if it does not. Nitrox's firm benefit is longer no-stop time, covered in is nitrox worth it. Counting oxygen as narcotic is the conservative choice.

What to do when narcosis hits

Go up a few metres until your head clears, then decide whether the dive continues. StatPearls calls ascent the only definitive treatment and says symptoms typically resolve within minutes. DAN says narcosis usually clears once you rise above the depth where it started. In DAN's case report, the diver who felt narced at 35 m was clear at 24 m, and DAN's comment was that he should have ended the dive as soon as the symptoms began.

• Agree before the dive on a maximum depth and a signal for "I feel narced". StatPearls names buddy awareness, preset depth limits and turn signals as the core of prevention.
• At the first sign, in yourself or your buddy, signal and ascend slowly. Watch the ascent rate, because a fast ascent is one of the things narcosis causes.
• Once clear, check gas and no-stop time again before doing anything else.
• If the symptoms do not clear after the ascent, end the dive. StatPearls advises exactly that.
• Symptoms that start or persist after surfacing are not narcosis. Numbness, weakness, confusion or joint pain after a dive can mean decompression sickness or gas embolism, which need oxygen and urgent care: see our DCS symptoms and first aid guide.
• After a deep air dive, give yourself time before tasks that need a clear head. StatPearls notes that reaction time can stay impaired after the symptoms fade, and Clark cites a change in a visual test still present 30 minutes after a dive to 30 m.

Why helium is used for deep dives

Helium barely dissolves in fat and causes no clinically meaningful narcosis, so replacing part of the nitrogen with it makes a deep dive feel shallower. Clark's table rates helium's narcotic potency at 0.2 against nitrogen's 1. A 2022 EEG study showed the difference directly: breathing air at 608 kPa (about 51 m) raised a brain connectivity measure by 35% and was experienced as mild narcosis, while heliox at the same pressure caused no significant change.

StatPearls gives 30-50 m as the commonly accepted maximum depth for air, notes that most recreational agencies set 40 m, and says helium mixes are commonly used for dives deeper than 50 m. Helium has costs of its own. It changes decompression and makes it more complex, and below about 150 m it brings high pressure nervous syndrome, with tremor and jerks. Adding 5-10% nitrogen to heliox, which turns it into trimix, uses nitrogen's mild narcotic effect to damp that down.

In practice, trimix divers pick the deepest END they will accept and blend to it. Our trimix calculator's best mix mode does that from a target depth, a ppO2 limit and an END; 30 m is a common choice. Trimix diving needs technical training and certification, and every tank has to be analysed before use.