What gas narcosis is associated with depth?

Nitrogen narcosis is the correct answer because this condition specifically occurs under increased pressure at deeper depths while scuba diving. As a diver descends, the partial pressure of gases in the breathing mixture increases, particularly nitrogen, which makes up a significant portion of normal air.

At depths greater than approximately 30 meters (100 feet), the increased partial pressure of nitrogen can lead to altered mental states and impaired cognitive function, akin to the effects of alcohol intoxication. This phenomenon is commonly referred to as "the rapture of the deep" and can affect divers' judgment and motor skills, making it crucial for them to be aware of their depth and how it affects their mental state.

The other gases mentioned do not typically cause narcosis in the same manner at recreational diving depths. While oxygen can become toxic at high pressures, this toxicity manifests differently than narcosis. Similarly, carbon dioxide narcosis is more related to insufficient ventilation or buildup of CO2 rather than depth alone. Hydrogen, while it can be used in specific deep diving mixtures, also does not induce narcosis in the conventional sense. Thus, nitrogen narcosis stands out as the recognized phenomenon associated with depth during diving.