Along with the bends, which in contrast are a risk during the ascent of a dive, narcosis is one of the most dangerous conditions to affect the SCUBA diver. The most dangerous aspects of narcosis are the loss of motor function and decision-making ability, and impairment in judgment. The diver may start to feel invulnerable, disregarding normal safe diving practices (frequently the diver throws away his mask and accelerates his descent to the bottom of the sea). The diver may not even be aware that he is operating at an impaired level. Other effects include vertigo, tingling and numbness of the lips, mouth and fingers, and extreme exhaustion. Affected divers are likely to panic, sometimes remaining on the bottom, too exhausted to ascend. Or the disease can take the form of exhilaration and giddiness, but it can also manifest itself as extreme anxiety and depression or a paranoid state.
Though some divers seem to be able to cope with the symptoms and even claim to be less susceptible than others, tests have shown that all divers are affected by nitrogen narcosis. Supposedly some divers can better cope with the effects than others by acclimation, training, special breathing techniques, etc. but the effects are still there. Also, the onset is hard to recognize, its severity is unpredictable, and it can kill, from its own toxic effect or from the result of resulting illogical behaviour of the diver.
Although narcosis is most apparent below 30 meters, there is no exact way to predict the severity of the effect on the diver. Similar to altitude sickness, its effects depend on many factors, both which vary from individual to individual and even from time to time in the same individual. Excellent cardiovascular health is no protection against its occurrence, nor is poor health necessarily a predictor. Certain factors increase the severity of nitrogen narcosis: cold, stress, heavy work and fatigue, and CO2 retention.
The mechanism of the narcosis is related to the solubility of nitrogen in the blood occurring at elevated atmospheric pressures. Once it leaves the tank and enters the diver's lungs it will have the same pressure as the surrounding water, i.e., the ambient pressure. So, experienced divers recommend a constant vertical speed, avoiding sudden changes of inclination which would cause an irregular solution of gas in blood due to a "delay" of pressures adapting.
One of the important factors leading to its occurrence seems to be the speed of the diver's descent. Pressure increases as the diver descends, but nitrogen compresses more slowly than other gases in blood. Very important is the vertical speed of descent: when the pressure increases due to increased depth, nitrogen compresses more slowly than other gases in blood. Irregular compression might be caused by small variations of vertical speed.
Similar to the mechanism of alcohol's effect, this change may cause altered permeability properties of neural cell lipid bilayers. The Meyer-Overton hypothesis states that narcosis happens when the gas penetrates the lipids of the brain's nerve cells. Here it apparently interferes with the transmission of signals from one nerve cell to another.
The relation of depth to narcosis is informally known as "Martini's law": It's like one martini per 10 meters below 20 meters. This can only serve as a rough not serious guide, which can never be a substitute for the real diving safety rules. Professional divers never suggest such calculation attempts, stressing instead that deep dives can be made only after a gradual training to increasing depths, and always with a linear vertical speed.
Good divers frequently check their mental state while immersed using the "thumbs test": from time to time, the two companions (careful divers never dive alone) show each other their fingers, one showing a number of fingers (e.g. 2), and the other must answer showing back one more or one less (i.e. 3 or 1, depending on previous agreement). If either of them botches the arithmetic, suspect narcosis.
Because of similar (and additive) effects in addition to those of excess nitrogen, alcohol should be avoided before any dive. A reasonable recommendation is total abstinence at least 24 hours before diving; by that time the effects of alcohol should be gone. As if there weren't enough other reasons for not drinking and diving, alcohol combined with nitrogen is likely to bring on a far more serious case of narcosis. Even a hangover, combined with the reduced physical capacity that goes with it, makes nitrogen narcosis more likely.