Helium mixed with oxygen under pressure is given to sea divers for respiration. Because pure oxygen can be toxic at a great concentration at the depth. Therefore oxygen can be mixed with helium to reduce oxygen concentration while eliminating nitrogen. The main reason for adding helium to the breathing mix is to reduce the proportion of nitrogen and oxygen below those of air, to allow the gas mix to be breathed safely on deep dives. A low proportion of nitrogen is required to reduce nitrogen narcosis and other physiological effects of gas at depth.