High altitude causes hypoxia (decreased oxygen) ,symptoms improve in a process known as acclimatization(over time). Three major problems with high altitude which can present occasionally acute mountain sickness, high-altitude lung fluid accumulation and high-altitude brain swelling. Other related problems, such as impaired sleep at high altitude, often coexist with the major syndromes. Symptoms of acute mountain sickness are nausea, dizziness, breathlessness, panting, headache, gastrointestinal (anorexia, nausea or vomiting), fatigue or weakness, dizziness or lightheadedness, difficulty sleeping. Symptoms of high altitude lung fluid accumulation are cough, shortness of breath, chest tightness, and fatigue. Symptoms of high-altitude brain swelling are loss of consiousness during sleep. Unacclimatized persons with coronary artery disease may develop increased anginal symptoms following ascent to altitude because of an increase in cardiac work. Many more symptoms do occur.