For any given breathing mixture the level of narcosis is related to the depth of the dive. However there is more to it than that. Depth is only one of the factors involved. The environmental and personal factors also play a significant part in narcosis. Environmental factors can increase your susceptibility to narcosis and can increase the symptoms at any given depth. Visibility is one of the biggest factors in susceptibility to narcosis. Consider a dive where you have 20m visibility and plenty of ambient light but then a week later you do the same dive but this time the visibility is less than a meter and there is no ambient light. The second dive is much more likely to produce symptoms of narcosis than the first. Current can also be a major factor in bringing on narcosis. If you are fighting against a current and breathing faster than usual due to working hard to swim down a shotline then you are at a higher risk of experiencing narcosis. It is believed that Carbon Dioxide can increase the risk of narcosis, this would explain why working hard against a current may be such a common cause of narcosis. Minor equipment problems can also induce narcosis. A slight equipment problem which, in itself will not cause any issues, may be enough to induce narcosis. This is related to other psychological causes of narcosis. Concern over the dive, diving with unfamiliar equipment or unfamiliar buddies, cold, drugs, fatigue, stress, motion sickness and motion sickness medications have all been linked to an increase in the likelihood of narcosis. All of this means that the depth in itself is not the only factor that determines your level of narcosis. As such it’s impossible to draw an arbitrary line where you can say air/nitrox is safe at this depth but no deeper. It is clear that psychological factors affect narcosis. There have been a number of studies which have attempted to show the psychological aspect of narcosis. Tom Mount and Dr Gilbert Milner carried out a study in 1965 that demonstrated that divers tend to experience a level of narcosis that is consistent with the level they expect to experience. They carried out an experiment in which three different groups were given different information about narcosis. One was told that they would definitely experience severe narcosis at 40m, the second group was taught about narcosis, the symptoms and the fact that it may be experienced from 30m but that it wasn’t likely to occur and the third group were given extensive information on the symptoms, risks, dangers and known research. They were also told that divers with strong will power could mentally prepare themselves and greatly reduce the effects. The three groups were then subjected to a series of tests at varying depths. The results of the study were that the group that had been told to expect severe narcosis performed worst on the test and were unable to complete the tests at the deeper depths. The second group performed better on all of the tests than the first group although they still showed a significant level of impairment. Finally the third group, the one that had been told that it was possible to reduce the effects of narcosis through mental preparation, performed better then either of the other two groups. There was no suggestion that any of the participants had managed to avoid narcosis, but that the level of narcosis was affected by their expectation of the level they were likely to experience. A more recent study carried out for the HSE by the Diving Diseases Research Centre and Plymouth University supported the importance of psychological aspects in addition to the bio-physical impact of narcosis. One of the conclusions from this study was that narcosis is not simply an objective measurable phenomenon; it also has a subjective facet.