To help deep-sea divers breathe, they carry cylinders of oxygen mixed with

Deep-sea divers use cylinders containing a mixture of oxygen and helium, commonly known as heliox [1]. While atmospheric air consists primarily of nitrogen and oxygen [1][2], nitrogen becomes narcotic at high pressures encountered during deep dives, leading to nitrogen narcosis [1]. To mitigate this risk and reduce the likelihood of decompression sickness, helium is used as a diluent because it is an inert gas with negligible narcotic properties. Helium is also thinner and more compressible, which improves the efficiency of breathing regulators at extreme depths. Although nitrox (oxygen and nitrogen) is used for shallower dives, deep commercial and technical diving requires helium-based mixtures like heliox or trimix (oxygen, nitrogen, and helium) to ensure safety and mental clarity at depths exceeding 60 meters.

Sources

1. [1] https://en.wikipedia.org/wiki/Gas_blending_for_scuba_diving
2. [2] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.1.1 Is air a mixture? > p. 118