Why is helium used in diving apparatus?

Explanation

Because of low solubility (as compared to `N_2`) in blood, a mixture of `O_2` and He is used in diving apparatus used by scuba divers.
Scuba divers or deep sea divers must cope with high concentration of dissolved gases while breathing air at high pressure underwater. Increased pressure increases the solubility of atmospheric gases in blood (Henry.s Law). When the divers come towards surface, the pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in blood. This blocks capillaries and creates a medical condition called .bends. which are painful and dangerous to life. To avoid bends as well as, the toxic effects of high concentrations of `N_2` in the blood, the tanks used by scuba divers are filled with air diluted with He `(11.7% He + 56.2% N_2 +32.1% O_2)`.