If not properly acclimatized to high altitude, a person may be negatively affected by the lower oxygen concentration available. These hypoxia-related illnesses include acute mountain sickness (AMS), high-altitude pulmonary edema, and high-altitude cerebral edema (HACE). High altitude cerebral edema is a severe and sometimes fatal form of altitude sickness that results from capillary fluid leakage due to the effects of hypoxia on the mitochondria-rich endothelial cells of the blood–brain barrier. The edema can be characterized by vasogenic cerebral edema with symptoms of impaired consciousness and truncal ataxia.