<unk>  is a breathing gas mixture of helium (He) and oxygen (O2).<unk>  is a medical treatment for patients with difficulty breathing. The mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs.<unk>  has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of the low density of the gas.<unk>  is also used in saturation diving and sometimes during the deep phase of technical dives.
Heliox
Khat
Tabun 
Riken
Phencyclidine
Medicen
Heroin
Keldur