Chronic Mountain Sickness (CMS) is a condition in which your blood has a higher proportion of red blood cells than normal, but the amount of oxygen in the blood is very low (hypoxemia). This condition usually takes years to develop, and most cases occur at elevations above 10,000 feet. It’s the result of the body’s attempt to adapt to life in a low-oxygen environment. Common symptoms include headaches, ringing of the ears, dizziness, shortness of breath, and heart palpitations. It can also cause symptoms of fatigue, trouble sleeping, a diminished appetite, and mental confusion.