Intermediate 8 minutes
Nitrogen narcosis: the danger is what you decide, not what you feel
A reversible effect that fades in seconds and can still ruin an entire dive
It's the only diving injury that doesn't hurt, leaves no mark, and clears up on its own on the way back. It's also one of the biggest causes of incidents, precisely because while you have it, you feel perfectly fine.
In this article you'll see why nitrogen under pressure affects the nervous system, at what depth it starts to become noticeable, what factors bring it on earlier and make it worse, how to recognize it in yourself and your buddy, and why managing it is simple even though preventing it is not optional.
Narcosis doesn't cause harm by itself: it causes harm because it degrades judgment and reaction ability exactly in the part of the dive where you have the least margin for error. The problems it triggers are always indirect, which is exactly why it's so easily underestimated.
What it is and why it happens
Nitrogen is an inert gas in the metabolic sense: the body doesn't consume it or transform it. But at high pressures it stops being neutral for the nervous system and produces an anesthetic-like effect, comparable in how it shows up to that of alcohol or a mild sedative.
What determines the effect isn't the amount of gas you breathe but its partial pressure: that is, the share of the total pressure that corresponds to nitrogen, a concept you already saw in the article on the gas laws. At forty meters you're breathing the same air as always, but each of its components is at five times the pressure it had at the surface.
The precise mechanism by which an inert gas alters nerve transmission is still under research, but its behavior is predictable and consistent: it appears with depth, intensifies with depth, and disappears on ascent without leaving after-effects or requiring treatment.
Nitrogen isn't the only gas capable of producing it. Accumulated carbon dioxide itself has a noticeable narcotic effect, which is why inefficient breathing makes the picture markedly worse, as you'll see further on.
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