Why is helium mixed with oxygen for deep-sea divers?

Helium is mixed with oxygen for deep-sea divers primarily to address the physiological hazards associated with breathing compressed air at high pressures.12

There are two main reasons for this practice:

• Prevention of Nitrogen Narcosis: At significant depths, the nitrogen in standard air acts as an anesthetic, causing a reversible impairment of consciousness known as nitrogen narcosis. Because helium is less soluble in body tissues—particularly the lipid-rich tissues of the nervous system—it does not produce the same narcotic effects as nitrogen.13
• Reduced Gas Density: As a diver descends, the density of the breathing gas increases, which significantly raises the work required to breathe. Using helium as a diluent provides a gas mixture of lower density, which improves pulmonary function and reduces the respiratory burden during deep dives.2

Additionally, because helium dissolves less readily in the bloodstream than nitrogen, it is a preferred component in specialized breathing mixtures for high-pressure operations.13