Which gas is primarily responsible for narcosis at depth during dives?

Narcosis during dives is primarily caused by the effects of nitrogen under increased pressure. As divers descend deeper, the partial pressure of nitrogen in the breathing gas increases, leading to its increased solubility in the body's tissues, particularly in the lipid membranes of nerve cells. This phenomenon can impair cognitive function and coordination, resembling the effects of alcohol intoxication, hence the term "nitrogen narcosis."

While oxygen, carbon dioxide, and hydrogen can have their own physiological effects, they do not primarily induce narcosis at the pressures experienced during recreational diving. Oxygen can become toxic at greater depths when partial pressures rise substantially, but it does not contribute to the state of narcosis similar to nitrogen. Carbon dioxide is more associated with respiratory issues and can lead to hypercapnia, while hydrogen, although relevant in specific technical diving scenarios, is not a main contributor to narcosis at typical diving depths. Thus, nitrogen is identified as the gas primarily responsible for narcosis during dives, particularly among recreational divers.