What is narcosis and why does it matter?





We were all told about nitrogen narcosis when we first learned to dive.


But recent research by scientists in Europe suggests that much of what we were told was wrong.


Everyone is mentally and physically affected by nitrogen narcosis to some extent when they dive deep, whether they are breathing air or Nitrox. Responsiveness differs from person to person. It is commonly accepted that the effects begin at around 30m (100ft), although I have seen divers affected much shallower.





The signs and symptoms vary from mild performance impairment to hallucinations and general anaesthesia. The deeper you go, and the longer you stay, the more severe the symptoms become.


Nitrogen narcosis is a more serious concern than many divers believe. To dive safely, a diver has to master a number of different skills: these include manual dexterity, motor coordination and both short-term and long-term memory. And these are exactly the skills that are adversely affected by nitrogen narcosis.


What Causes Nitrogen Narcosis?


For years, divers have been taught that higher levels of carbon dioxide (CO2) in the blood stream, generated by working hard or swimming fast underwater, contribute to nitrogen narcosis, along with other factors, such as alcohol use, a hangover, fatigue, anxiety, the effects of motion sickness medication, rate of descent, task loading and time pressure stress.


However, from a scientific point of view, there is little evidence for most of this.


Historically, the effects of nitrogen narcosis have been measured by assessing the performance of tasks such as mental arithmetic, memory, reaction time and manual dexterity, while a diver is “at depth” in a hyperbaric chamber. Unfortunately this method is unreliable, as it has been shown that a diver’s motivation and experience can have an influence on the results.


A better way to go about testing the effects is to monitor changes in the brain using electro-encephalography (EEG), which involves placing electrodes on the scalp. Recently this was used for the first time to measure the effects of nitrogen narcosis during an entire dive AND for a period of time after the diver surfaced.


Diver training manuals have always advised divers that, in order to make the effects of nitrogen narcosis disappear, all they have to do is ascend to a shallower depth. One of the most remarkable findings of these EEG studies was that, in fact, not only does nitrogen narcosis NOT go away when you ascend, it actually persists for at least 30 minutes after you surface!


The studies also showed clearly that the ONLY factors that affect nitrogen narcosis are the surrounding pressure and the gas being breathed. Other factors seem to have little or no impact.


Can divers adapt?


The diving community generally believes that divers can adapt to nitrogen narcosis. However, scientific tests have generally been unable to confirm any habituation and researchers have concluded that sport divers do not actually develop tolerance to nitrogen narcosis. It is more likely the case that divers who perform the same tasks over and over again just gradually find them easier to do. This makes them think that they are adapting to narcosis but, in fact, they are just learning to cope with it. It is like habitual drunks, who learn to deal with the effects of alcohol by, for instance, talking slowly or moving carefully. Of course, this may help them sound sober ot stop spilling their drink, but they are still just as drunk!


Key points


Nitrogen narcosis impairs your ability to function effectively underwater at depth.


You may feel less “narked” when you ascend to a shallower depth but you are still just as “narked”.


The effects of nitrogen narcosis persist for at least 30 minutes after surfacing.


Divers cannot adapt to nitrogen narcosis although they can learn to cope with it.


What to do?


Be aware that, throughout a deep dive and even for a short time on the surface afterwards, narcosis is having an adverse effect on your concentration, co-ordination and other key mental and physical processes. Therefore, you need to act accordingly. Focus more than usual on things like your depth, time, gas supply, position in the water and the location of other team members.


Don’t let your thoughts drift or allow anything to take your your mind off your dive.


Read more about this in Scuba Physiological and Scuba Exceptional