Mountaineers carry an oxygen cylinder along with them. It is because when they climb mountains, as altitude increases, the amount of oxygen level in the atmospheric air decreases as trees are not found at higher altitudes. As a result, mountaineers suffer from dizziness. It can also lead to hypoxia. Hypoxia is a condition wherein a region of the body is deprived of sufficient supply of oxygen at the tissue level. Hence, to avoid such issues arising due to insufficient oxygen, mountaineers carry oxygen cylinders.