Trimix (breathing gas)

Trimix is a breathing gas, consisting of oxygen, helium and nitrogen, and is often used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

The helium is included as a substitute for some of the nitrogen, to reduce the narcotic effect of the breathing gas at depth. With a mixture of three gases it is possible to create mixes suitable for different depths or purposes by adjusting the proportions of each gas. Oxygen content can be optimised for the depth to limit the risk of toxicity, and the inert component balanced between nitrogen, which is cheap but narcotic, and helium, which is not narcotic and reduces work of breathing, but expensive and conducive to heat loss.