Nitrogen narcosis is an altered state of consciousness experienced by scuba divers whilst in the water. It is a consequence of the behavior exhibited by the inert gas nitrogen when breathed underwater at a greater than atmospheric pressure. It results in impaired mental functioning, which when you're underwater can be fatal, the same way that driving a car drunk can be fatal. Decreased mental coherence leads to mistakes being made and unnecessary risks being taken. The diving term for this behaviour is 'getting narced', and while some people may not notice the symptoms when they do occur, all divers below a certain depth will be affected to some extent.
Mechanism
Water can not be compressed so there is a linear pressure increase gradient associated with water depth: surface pressure is 1 ATM; at 10m depth pressure is equal to 2 ATM; at 20m depth is equal to 3 ATM; and so on and so forth. When someone is diving, their body is subject to the compression forces that high pressure exerts. However, as most body compartments are composed of water this has few noticeable effects at the pressures that recreational divers usually keep to – an advised maximum of 5 ATM (40m). However, gases are compressible, and so gas-filled spaces (such as your lungs, sinuses, middle ears and scuba tank) will become subject to changes in volume depending on water depth.
A scuba regulator is designed to deliver air to the diver at a pressure equal to the surrounding atmosphere. When breathing underwater, you inhale the same volume of gas that you would at the surface. However, more molecules of the gases are inhaled, as the increase in pressure means that a larger number of molecules occupy the same volume in comparison to surface pressure (similarly, oxygen canisters are required by mountain climbers because the decrease in atmospheric pressure at 8000m means there are less oxygen molecules per breath than at sea level). The increase in pressure also means that the partial pressures exerted by the oxygen, nitrogen and carbon dioxide (and others) in the mix of gases in the scuba tank are altered (Dalton's law). In combination with Henry's law, the result is the increased absorption of inert nitrogen gas by the body.
In actual fact, it's not just nitrogen that can cause a narcosis; all inert gases are readily lipid soluble under the right pressure conditions, and so nitrogen narcosis should be more accurately termed 'inert gas narcosis'. Argon, hydrogen and xenon are similarly able to dissolve in the body given the right level of pressure; xenon is in fact absorbed readily at atmospheric pressure. The inert gases are lipid-soluble, and since the nervous system of the body contains a large amount of lipid-rich tissue, the gases will dissolve into nerves, the spinal column, and brain. Here they will bind to cellular receptors, modifying neuronal activity and nerve conduction velocity as well as ionic transport across cell membranes. Narcosis is a dangerous and potentially fatal condition. It can cause the diver to make stupid, elementary mistakes, but it is also fatally toxic in itself if a diver is breathing atmospheric air mixtures (20.9% O2, 79% N2) at depths greater than 90m.
Effects
Inert gas narcosis goes by many names, the most poetic (in my opinion) being 'the rapture of the deep'. Its effects are very similar to alcohol intoxication; symptoms and signs can include euphoria, incoordination, loss of judgment with regard to actions, anxiety, and depression amongst others. In higher concentrations, the effect is anaesthetic; the diver may lose consciousness and drown. The results of narcosis can be slight or extreme. Some people barely notice any symptoms and end the dive as usual. Some may become so intoxicated that they pull their mask off, regulator out, and start actively swimming to the bottom of the ocean. Different people are affected differently at different depths, and the same person can be affected differently on different dives. This makes it hard to predict one person's susceptibility to nitrogen narcosis. The "Martini's law" is often used as a guide: at 10m, the effects of narcosis are similar to the effects of drinking one martini; at 20m, 2 martinis; at 30m, 3 martinis (and so on.) However, this isn't a hard and fast rule, and so it is vital that buddy pairs do frequent checks on each other's mental status using the thumb test (where one buddy holds up a certain number of fingers, and their partner show them back the same number of fingers +/-1 as agreed in advance) or similar during a deep dive.
Personally, I've never recognised any narcosis symptoms in myself while diving even on my deep dives (and I've been down to 50m). Of course, the fact that I can still walk in a straight line even after ten martinis probably has something to do with this. This doesn't mean that I wasn't affected by narcosis, just that the symptoms weren't severe enough for me to consciously register them. However, I have noticed obvious narcosis symptoms in someone at 20m; it really is that random in regards to its severity. An acquaintance was once diving at 45m with his underwater camera when he came across (and I quote) "a fantastically beautiful, mesmerising forest of kelp or seaweed" that he just had to stare at and take loads of pictures of. When he later developed the roll of film, he found he'd taken 13 photos of his left hand.
Treatment
The treatment for narcosis is wonderfully simple: ascend to a shallower depth. Feeling woozy at 35m? Go up to 20m and the symptoms will have lessened, if not disappeared. If only dealing with alcohol intoxication was so easy. The only danger is in ascending too quickly and getting yourself bent, which is more than a slight possibility considering your judgment is impaired. Diving is a dangerous enough sport when you do have your wits about you; there are all manner of horrible ways to die if you don't. That said, it's a fantastic hobby and I advise all to give it a go at least once. Go on, you know you want to.
Prevention
Preventing yourself from experiencing narcosis is a little more difficult. Basic safety sense dictates that a diver should avoid alcohol and other recreational drugs for 24hrs before diving; maintain adequate hydration; and maintain adequate health and fitness. The most obvious way of avoiding narcosis is to avoid going on deeper dives (>20m), but this isn't always an option depending on what kind of diving you enjoy. Some divers swear by a constant descent rate at the beginning of the dive, but there's no proof to back this method up.
The best method of prevention is to get yourself trained in using alternate gas ratios in your scuba tank. Nitrox divers (also called enhanced air, enriched air, EANx and a variety of other names) still use only oxygen and nitrogen, but in different mix ratios; the usual choice is for 32%O2 (Nitrox I) or 36%O2 (Nitrox II) in recreational diving. Technical divers can use up to 80%O2, but those guys are hard-core and need to do proper decompression stops and often carry four scuba tanks with different air mixes when diving. The decreased amount of N2 in these gas mixes means that less nitrogen is absorbed, and so the theory goes that a diver can complete a longer dive time before reaching their saturation limits, and have a decreased risk of experiencing narcosis. In practice, most divers regard it as a way of doing a deeper than normal dive (>40m) with the same saturation limit dive times. Human folly, as technically the increased amount of O2 means that there is an increased risk of a diver giving themselves oxygen toxicity if they descend below 30m, but it's not like I haven't done it myself from time to time. My bad.
For those that do practice technical diving, two other gas blending options exist: trimix and heliox. Trimix is a mixture of oxygen, nitrogen and helium, used for the deep portion of technical diving (up to 130m in depth) with the exact ratio custom calculated for each dive to avoid both narcosis and oxygen toxicity. Heliox is a blend where the nitrogen is completely removed and replaced with helium, the usual ratio being 21% O2 to 79% He. Technical diving limits are recognised as being around 130m, because of the danger of high pressure nervous syndrome caused by breathing heliox.
References
- PADI Open Water, Rescue Diver and Divemaster training material
- Nitrox diving training material