Which gas is primarily responsible for increased risk during deep dives performed with EANx?

During deep dives using Enriched Air Nitrox (EANx), the primary concern is the increased partial pressure of oxygen. As divers descend, the surrounding pressure increases, resulting in a higher concentration of gases in the breathing mixture. For EANx, which typically contains a higher proportion of oxygen than regular air, this means that the partial pressure of oxygen can exceed safe limits at greater depths.

When the partial pressure of oxygen rises too high, it can lead to a condition known as oxygen toxicity, which includes symptoms such as visual disturbances, dizziness, nausea, and seizures. This makes it particularly critical for divers to monitor their depth and the percentage of oxygen in their gas mixture to prevent these adverse effects.

While nitrogen is present in the mixture, the risk associated with it is more related to nitrogen narcosis and decompression sickness rather than the acute toxicity associated with elevated oxygen levels. Helium is used in some deep dives to mitigate nitrogen narcosis rather than being a primary risk factor in EANx diving. Carbon dioxide, while important for monitoring breathing efficiency and managing dive dynamics, is not the primary concern when utilizing enriched air mixtures in deep dives.