The use of these gases forms part of technical diving and requires further training and certification.While the individual diver cannot predict exactly at what depth the onset of narcosis will occur on a given day, the first symptoms of narcosis for any given diver are often more predictable and personal. For example, one diver may have trouble with eye focus (close accommodation for middle-aged divers), another may experience feelings of euphoria, and another feelings of claustrophobia. Some divers report that they have hearing changes, and that the sound their exhaled bubbles make becomes different. Specialist training may help divers to identify these personal onset signs, which may then be used as a signal to ascend to avoid the narcosis, although severe narcosis may interfere with the judgement necessary to take preventive action.Deep dives should be made only after a gradual training to test the individual divers sensitivity to increasing depths, with careful supervision and logging of reactions. Scientific evidence does not show that a diver can train to overcome any measure of narcosis at a given depth or become tolerant of it.Equivalent narcotic depth (END) is a commonly used way of expressing the narcotic effect of different breathing gases. The National Oceanic and Atmospheric Administration (NOAA) Diving Manual now states that oxygen and nitrogen should be considered equally narcotic. Standard tables, based on relative lipid solubilities, list conversion factors for narcotic effect of other gases.
Professional divers use such a calculation only as a rough guide to give new divers a metaphor, comparing a situation they may be more familiar with.Reported signs and symptoms are summarized against typical depths in meters and feet of sea water in the following table, closely adapted from Deeper into Diving by Lippman and Mitchell:

Causes
The cause of narcosis is related to the increased solubility of gases in body tissues, as a result of the elevated pressures at depth (Henrys law). Modern theories have suggested that inert gases dissolving in the lipid bilayer of cell membranes cause narcosis. More recently, researchers have been looking at neurotransmitter receptor protein mechanisms as a possible cause of narcosis. The breathing gas mix entering the divers lungs will have the same pressure as the surrounding water, known as the ambient pressure. After any change of depth, the pressure of gases in the blood passing through the brain catches up with ambient pressure within a minute or two, which results in a delayed narcotic effect after descending to a new depth. Rapid compression potentiates narcosis owing to carbon dioxide retention.A divers cognition may be affected on dives as shallow as 10 m (33 ft), but the changes are not usually noticeable.