Question: When does narcosis start to affect some divers?
About the article "Scuba diving", section "Nitrogen narcosis".

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Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high pressure gas at depth.
The mechanism is similar to that of nitrous oxide, or "laughing gas," administered as anesthesia.
Being "narced" can impair judgment and make diving very dangerous.
Narcosis starts to affect some divers at 66 feet (20 m).
At this depth, narcosis manifests itself as a slight giddiness.
The effects increase drastically with the increase in depth.
Almost all divers are able to notice the effects by 132 feet (40 meters).
At these depths divers may feel euphoria, anxiety, loss of coordination and lack of concentration.
At extreme depths, hallucinogenic reaction and tunnel vision can occur.
Jacques Cousteau famously described it as the "rapture of the deep".
Nitrogen narcosis occurs quickly and the symptoms typically disappear during the ascent, so that divers often fail to realize they were ever affected.
It affects individual divers at varying depths and conditions, and can even vary from dive to dive under identical conditions.
However, diving with trimix or heliox dramatically reduces the effects of inert gas narcosis.
Narcosis is potentially one of the most dangerous conditions to affect the scuba diver below about .
Except for occasional amnesia of events at depth, the effects of narcosis are entirely removed on ascent and therefore pose no problem in themselves, even for repeated, chronic or acute exposure.
Nevertheless, the severity of narcosis is unpredictable and it can be fatal while diving, as the result of illogical behavior in a dangerous environment.
Tests have shown that all divers are affected by nitrogen narcosis, though some experience lesser effects than others.
Even though it is possible that some divers can manage better than others because of learning to cope with the subjective impairment, the underlying behavioral effects remain.
These effects are particularly dangerous because a diver may feel they are not experiencing narcosis, yet still be affected by it. 
 and nitrogen narcosis.
Some scuba divers use the term "deep water blackout" loosely to refer to the final stage of nitrogen narcosis.
Nitrogen narcosis is a neurological impairment caused by dissolved blood nitrogen under pressure.
The progressive signs and symptoms of nitrogen narcosis are:  Impairment of reasoning, judgement, memory and concentration;  Sense of well-being and levity;  Anxiety;  Loss of coordination and physical dexterity;  Hallucination, terror and vertigo;  Unconsciousness and death.
The onset of symptoms may start around 30 metres and unconsciousness may occur at around 90 metres.
Because free-divers start and finish the dive with only a single lungful of nitrogen to absorb, nitrogen narcosis has long been assumed to be an issue only for divers on compressed air.
However, this issue may be more complex than that and needs to be resolved.
There is a lot more to nitrogen narcosis than blackout and it is imprecise and probably not helpful to use the term "deep water blackout" to refer to a single aspect of it.
Therefore, for the purposes of this article "deep water blackout" refers to latent hypoxia at depth and is unrelated to nitrogen narcosis. 
Narcosis while diving (also known as nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect), is a reversible alteration in consciousness that occurs while diving at depth.
It is caused by the anesthetic effect of certain gases at high pressure.
The Greek word "ναρκωσις" (narcosis) is derived from "narke", "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates.
Narcosis produces a state similar to drunkenness (alcohol intoxication), or nitrous oxide inhalation.
It can occur during shallow dives, but does not usually become noticeable at depths less than .
Except for helium and probably neon, all gases that can be breathed have a narcotic effect, although widely varying in degree.
The effect is consistently greater for gases with a higher lipid solubility, and there is good evidence that the two properties are mechanistically related.
As depth increases, the mental impairment may become hazardous.
Divers can learn to cope with some of the effects of narcosis, but it is impossible to develop a tolerance.
Narcosis affects all divers, although susceptibility varies widely from dive to dive, and between individuals.
Narcosis may be completely reversed in a few minutes by ascending to a shallower depth, with no long-term effects.
Thus narcosis while diving in open water rarely develops into a serious problem as long as the divers are aware of its symptoms, and are able to ascend to manage it.
Diving beyond is generally considered outside the scope of recreational diving.
Below these depths, as narcosis and oxygen toxicity become critical risk factors, specialist training is required in the use of various helium-containing gas mixtures such as trimix or heliox.
These mixtures prevent narcosis by replacing some of the breathing gas with non-narcotic helium. 
Narcosis is a phenomenon that occurs when a combination of organic toxicants, each at low concentrations, have a toxic effect on an organism.
Narcosis has a nonspecific mode of action, where it does not target one specific site of action, but instead affects the membranes and protoplasm.
These individual organic toxicants affect the fish at sublethal levels, however, the combination of contaminants can lead to mortality of the organism.
However, if the stressor is removed narcotic effects can be reversed.
Fish influenced by narcosis may display lethargic symptoms.
Narcosis can also result in respiratory-cardiovascular responses to fish.