The effects of nitrogen narcosis are highly variable among divers with all divers being significantly impaired while breathing air at 60 to 70 meters, whereas some divers are affected at 30 meters. The effects are not progressive with time while depth is maintained, but symptoms progress and new symptoms develop as a diver descends deeper to greater pressures. The narcotic symptoms observed are quickly reversible upon ascent.

The symptoms seen in nitrogen narcosis begin first with effects of the higher function such as judgment, reasoning, short-term memory, and concentration. The diver may also experience a euphoric or stimulating feeling initially similar to mild alcohol intoxication. Further increases in the partial pressure of nitrogen in the blood from descending deeper lend to impairments in manual dexterity and further mental decline including idea fixation, hallucinations, and finally stupor and coma. Death can result from unconsciousness associated with severe narcosis or from severely impaired judgment leading to an accident of some form during the dive. Other factors have been linked to increased risk of nitrogen narcosis during dives while breathing compressed air and they include alcohol, fatigue, anxiety, and hypothermia. The concentration of carbon dioxide in the blood is thought to have an additive, rather than synergistic effect to nitrogen narcosis.

Measuring the effects of nitrogen narcosis have been studied and have been broken down into looking at cognition and behavior as well as neurophysiological changes that occur during narcosis. One study on simple arithmetic and reaction time showed the qualitative progressive decline in the participant’s ability to complete the test’s tasks with increasing pressures while breathing compressed air. Another study designed to see the difference between nitrogen narcosis in a laboratory setting versus in an open water setting. It reported that "divers were tested at 3 and 30 meters at a shore base and in the open sea. Intellectual functions, as assessed by memory test, sentence comprehension, and simple arithmetic, showed evidence of narcosis in both 30-meter dives but the decrement was greater in the ocean dives. This may be due to the greater psychological stress in the open sea," hinting at a greater impairment from narcosis when the diver is also experiencing anxiety from other factors outside of the narcosis.

Neurophysiological testing has been done using an electroencephalogram to measure brain wave activity. Studies have found decreased voltages in the basal rhythm of the brain and the appearance of low voltage theta waves. Cortical evoked potentials were also studied. These evoked potentials are differences on electroencephalogram readings that show a response in the brain to sensory stimuli, such as visual images. US Navy divers were studied using these visual evoked responses and showed that reliable and significant differences in visual evoked responses were apparent in the divers when they were breathing compressed air at depth, and these differences were not apparent when breathing compressed helium-oxygen mixtures.

Nitrogen narcosis can be prevented in different ways. Limiting the depth of a dive is one of the least invasive. It is agreed upon that the maximum depth limit for a diver to use compressed air is 30 to 50 meters. Beyond this, a gas mixture other than air is suggested for use to prevent nitrogen narcosis. For dives greater than 50 meters the diving community commonly substitutes helium or helium-nitrogen as the diluent gasses for oxygen. Helium has not been shown to have an anesthetic or narcotic effect on divers at depth breathing compressed gases.  Helium imposes a greater decompression burden upon divers than does nitrogen.

Treatment of narcosis involves early recognition of symptoms by the diver or the diver’s partner while at depth. Symptoms resolve completely within minutes upon ascent of the diver. If the symptoms persist, the dive should be aborted.[7][8][9][10]

• Arterial gas embolism

• Caustic cocktail from rebreathing circuit

• Contaminated gas mixture

• Decompression sickness

• Face squeeze

• Hypothermia

• Nitrogen narcosis

• Oxygen toxicity

• Pulmonary barotrauma (pulmonary overpressurization syndrome)

• Sinus barotrauma

Inert gas narcosis completely resolves upon ascent. It poses no problem in the long term and does not lead to chronic issues or predispose to increased (or decreased) susceptibility to recurrent exposures. However, inert gas narcosis will recur on exposure to pressure, it is unpredictable in severity and can lead to fatalities while diving, due to cognitive impairment in the marine environment.

Although helium does not cause inert gas narcosis, it can cause at deep depths. This is an unrelated phenomenon which is interestingly treated by adding nitrogen back into the breathing mixture.

The management of nitrogen narcosis is usually done with an interprofessional team that includes an emergency department physician, neurologist, intensivist and an HBO expert. The key is to educate the patient who undertake water sports. Nitrogen narcosis can be prevented in different ways. Limiting the depth of a dive is one of the least invasive. It is agreed upon that the maximum depth limit for a diver to use compressed air is 30 to 50 meters. Beyond this, a gas mixture other than air is suggested for use to prevent nitrogen narcosis. For dives greater than 50 meters the diving community commonly substitutes helium or helium-nitrogen as the diluent gasses for oxygen. Treatment of narcosis involves early recognition of symptoms by the diver or the diver’s partner while at depth. Symptoms resolve completely within minutes upon ascent of the diver. If the symptoms persist, the dive should be aborted.

The outcomes after nitrogen narcosis depend on the depth of the dive, rapidity of ascent and other comorbidities. Those who have neurological signs may have residual impairment after treatment, but for the most part, full recovery is expected in most divers.