Question: At the point when does narcosis begin to influence a few jumpers?
About the article "Scuba diving", section "Nitrogen narcosis".

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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. 
Nitrogen narcosis is caused by the pressure of dissolved gas in the body and produces impairment to the nervous system.
This results in alteration to thought processes and a decrease in the diver's ability to make judgements or calculations.
It can also decrease motor skills, and worsen performance in tasks requiring manual dexterity.
As depth increases, so does the pressure and hence the severity of the narcosis.
The effects may vary widely from individual to individual, and from day to day for the same diver.
Because of the perception-altering effects of narcosis, a diver may not be aware of the symptoms, but studies have shown that impairment occurs nevertheless.
Since the choice of breathing gas also affects the depth at which narcosis occurs, the table below represents typical manifestations when breathing air. 
Narcosis may refer to:  Narcosis, the unconsciousness induced by a narcotic drug  Nitrogen narcosis, an effect of diving deep with nitrogen  Hydrogen narcosis, an effect of diving deep with hydrogen  CO₂ narcosis, carbon dioxide retention to a reduction in the hypoxic drive  
The most straightforward way to avoid nitrogen narcosis is for a diver to limit the depth of dives.
Since narcosis becomes more severe as depth increases, a diver keeping to shallower depths can avoid serious narcosis.
Most recreational dive schools will only certify basic divers to depths of , and at these depths narcosis does not present a significant risk.
Further training is normally required for certification up to on air, and this training should include a discussion of narcosis, its effects, and cure.
Some diver training agencies offer specialized training to prepare recreational divers to go to depths of , often consisting of further theory and some practice in deep dives under close supervision.
Scuba organizations that train for diving beyond recreational depths, may forbid diving with gases that cause too much narcosis at depth in the average diver, and strongly encourage the use of other breathing gas mixes containing helium in place of some or all of the nitrogen in air – such as trimix and heliox – because helium has no narcotic potential.
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 diver's sensitivity to increasing depths, with careful supervision and logging of reactions.
Diving organizations such as Global Underwater Explorers (GUE) emphasize that such sessions are for the purpose of gaining experience in recognizing the onset symptoms of narcosis for an individual, which are somewhat more repeatable than for the average group of divers.
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.
For example, hydrogen at a given pressure has a narcotic effect equivalent to nitrogen at 0.55 times that pressure, so in principle it should be usable at more than twice the depth.
Argon, however, has 2.33 times the narcotic effect of nitrogen, and is a poor choice as a breathing gas for diving (it is used as a drysuit inflation gas, owing to its low thermal conductivity).
Some gases have other dangerous effects when breathed at pressure; for example, high-pressure oxygen can lead to oxygen toxicity.
Although helium is the least intoxicating of the breathing gases, at greater depths it can cause high pressure nervous syndrome, a still mysterious but apparently unrelated phenomenon.
Inert gas narcosis is only one factor influencing the choice of gas mixture; the risks of decompression sickness and oxygen toxicity, cost, and other factors are also important.
Because of similar and additive effects, divers should avoid sedating medications and drugs, such as marijuana and alcohol before any dive.
A hangover, combined with the reduced physical capacity that goes with it, makes nitrogen narcosis more likely.
Experts recommend total abstinence from alcohol for at least 12 hours before diving, and longer for other drugs.
Abstinence time needed for marijuana is unknown, but owing to the much longer half-life of the active agent of this drug in the body, it is likely to be longer than for alcohol.