Which gas increases the risk of narcosis at depth?

The correct answer is nitrogen. At depth, the partial pressure of gases inhaled during a dive increases, and nitrogen, which makes up a significant portion of breathing gas, becomes more pronounced. As a diver descends, the effects of nitrogen narcosis can occur due to the enhanced absorption of nitrogen into the body’s tissues and bloodstream, leading to a state similar to alcohol intoxication. This phenomenon tends to occur at depths greater than approximately 30 meters (about 100 feet) and can impair cognitive functions, motor skills, and decision-making abilities.

When discussing why others are less relevant, while oxygen is a critical gas for breathing and can become toxic at high pressures (as in oxygen toxicity), it does not typically cause narcosis in the same way nitrogen does. Helium is often used in technical diving to mitigate the narcosis risk associated with nitrogen, as it does not contribute to narcotic effects. Carbon dioxide, although it can lead to other issues such as hypercapnia (an excess of carbon dioxide in the bloodstream), does not directly cause narcosis in the same manner as nitrogen does. This makes nitrogen the primary gas associated with narcosis at depth.