Which gas is primarily replaced with oxygen in EANx mixtures?

In Enriched Air Nitrox (EANx) mixtures, nitrogen is the primary gas that is replaced with oxygen. EANx typically consists of a higher percentage of oxygen than regular air, which contains about 21% oxygen and 79% nitrogen. By increasing the oxygen content to levels such as 32% or 36%, divers reduce the overall percentage of nitrogen in the mix.

This reduction in nitrogen is significant because it decreases the amount of nitrogen that can be absorbed by the body while diving, which directly affects the risk of nitrogen narcosis and decompression sickness. The purpose of using EANx is to extend no-decompression limits and provide longer bottom times at certain depths by minimizing nitrogen exposure.

While helium is sometimes used in deep diving mixtures, it is not a gas typically replaced in EANx. Carbon dioxide and argon also do not play a direct role in the context of EANx mixtures, as EANx focuses primarily on adjusting the balance of oxygen and nitrogen.