Acute mountain sickness is an illness caused by reduced air pressure and lower oxygen levels at high altitudes (usually above 8,000 feet or 2,400 meters). People at higher risk for acute mountain sickness are those who live at or near sea level and travel to a high altitude, or people who have had the illness before. The faster one climbs to a higher altitude, the more likely acute mountain sickness will occur. Signs and symptoms depend on the speed of ascension and the level of exertion, and can range from mild to life-threatening. Mild to moderate symptoms include difficulty sleeping, dizziness or light-headedness, fatigue, headache, loss of appetite, nausea or vomiting, rapid pulse, and shortness of breath with exertion. Symptoms of more severe acute mountain sickness include blue color to the skin, chest tightness or congestion, confusion, cough, coughing up blood, decreased consciousness, gray or pale complexion, cannot walk in a straight line or walk at all, shortness of breath at rest, and death due to lung problems or brain swelling.