Nitrogen
Eighty percent of the gas molecules in air are nitrogen (N2). Our bodies do not need or use nitrogen for metabolism, so it serves little function in a breathing gas mixture, other than to dilute the concentration of oxygen. When high concentrations of nitrogen become dissolved in our bodies, however, nitrogen can affect our central nervous system. Familiar to all divers is the effect known as "nitrogen narcosis". Jacques Cousteau called it "rapture of the deep", and its effects have been likened to alcohol inebriation. When breathed at high concentrations, nitrogen can impair our neurological abilities. The exact biochemistry is not known, but the symptoms include impaired judgment, loss of short-term memory, slowed response time, and sometimes euphoria. Obviously, just as one should not drive while intoxicated, diving with impaired mental abilities is at the very least unwise. As with oxygen toxicity, there is a wide range of variation in susceptibility to nitrogen narcosis both between, and within individuals. There is some evidence that repeated exposure can lead to an "adaptation" effect, but this is a topic subject to continued debate. Some divers begin to notice the symptoms while breathing air as shallow as 90 feet (27 meters) or so, while other claim to suffer no incapacitation at depths in excess of 200 feet (61 meters). Impairment likely occurs at lower PN2 levels than those at which divers begin to detect overt symptoms. In any case, the greater the inspired PN2, the more severe the narcosis. There is some evidence that oxygen also contributes to narcosis, but probably only at concentrations above which CNS oxygen toxicity would be of primary concern.
Nitrogen plays another important role in limiting conventional scuba diving: it's involvement with decompression sickness. This will be discussed in greater detail in the following section on decompression.
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