evidence: Mountain climbers are at risk of developing altitude sickness (also known as mountain sickness), which may be harmful or even fatal if its onset is ignored. It is caused by gaining altitude too rapidly, which doesn't allow the body enough time to adjust to reduced oxygen and changes in air pressure, and causes hypobaric hypoxia (a lack of oxygen reaching the tissues of the body). [...] This form of therapy floods the body with oxygen and relieves the symptoms of altitude sickness. A portable oxygen cylinder enables oxygen to be administered via a facemask or hood. [...] - Schommer K, Bärtsch P, 2011, ‘Basic medical advice for travellers to high altitudes’, Deutsches Ärzteblatt International vol. 108, no. 49, pp. 839–848. More information here.