Scuba divers are: at risk due to high concentration of dissolved gases while breathing air at high pressure under water. The tanks used by Scuba divers are filled with

Scuba divers face risks from high concentrations of dissolved gases at depth, specifically nitrogen narcosis and decompression sickness [t1][t3]. At high underwater pressures, nitrogen dissolves more readily into the blood and tissues. To mitigate these risks, the air in diving tanks is often diluted with helium [t4][t7]. Helium is used because it is an inert gas with lower solubility in blood compared to nitrogen and is less dense, which reduces the work of breathing [t3]. Common mixtures include Trimix (oxygen, nitrogen, and helium) and Heliox (oxygen and helium) [t2][t4]. By adding helium to dilute the nitrogen and oxygen, divers can descend to greater depths while avoiding the toxic effects of oxygen and the narcotic effects of nitrogen [t4][t5]. Therefore, tanks are filled with air diluted with helium to ensure safety during deep dives.

Sources

1. [1] https://en.wikipedia.org/wiki/Gas_blending_for_scuba_diving