A mixture of 2% $O_2$ and 98% He is used in diving apparatus which gives the same partial pressure of $O_2$ as in normal air at 1 atm. Due to high partial pressure and greater solubility, $N_2$ gets dissolved in blood and form bubbles (bends or decompression sickness). Hence, $N_2$ is replaced by helium which is much less soluble in biological fluids.