helium (He) /hē″lē·əm/ [Gk, helios, sun] , a colorless, odorless gaseous element; the second lightest element. Its atomic number is 2; its atomic mass is 4.00. Helium is one of the rare or inert gases and does not combine with other elements. It occurs in the atmosphere at concentrations of five parts per million. Because of its lightness and lack of flammability, it is also used to lift airships and balloons. In the liquid state it is used for low-temperature activities. The main physiological and medical uses of helium are in respiratory therapy and testing and the prevention of nitrogen narcosis and decompression sickness in hyperbaric environments. Helium is one third as soluble in lipids as is nitrogen. That characteristic accounts for its preferred use in hyperbaric atmospheres, such as those associated with deep-sea diving. A mixture of 80% helium and 20% oxygen is commonly breathed by deep-sea divers to prevent gas emboli and by patients undergoing treatment to clear obstruction of the respiratory tract. Problems associated with such uses involve the high velocity of acoustic transmission in helium and the high thermal conductivity of the gas. These characteristics produce voice distortions and hypothermia in persons who inhale it. The low density of helium reduces the effort of breathing any gas mixture of which it is a component. Helium is used in pulmonary function testing to calculate the diffusion and residual capacities of the lungs.
