Altitude sickness is the side effect caused by exposure to high altitudes. As a person reaches higher altitudes, the air contains less oxygen which begins to negatively affect the human body. Symptoms usually develop from around 2,500 meters of altitude. First signs of altitude sickness or acute mountain sickness (AMS) include headache, nausea or shortness of breath. More severe symptoms include dry cough, fever, vomiting or retinal haemorrhage. Extreme cases can include fluid build up in the brain characterized by loss of coordination, confusion, inability to walk and even coma. If left untreated, AMS can be lethal.