One of the chief dangers is Acute Mountain Sickness, AMS (also called altitude sickness.) AMS is the body's effort to acclimate to the reduced oxygen levels at higher altitudes. As previously mentioned, during the acclimatization process one's breathing rate increases in an attempt to obtain additional oxygen. In some individuals, this process can lead to hyperventilation symptoms such as shortness of breath, dizziness, nausea, and headache. These symptoms can be extremely distressing and even debilitating.

Although most cases of AMS are minor, with a small percentage of those afflicted, a potentially life-threatening condition called high-altitude pulmonary edema (HAPE) may develop, which is caused by the leakage of fluids from the blood vessels into the lungs. The first symptom of HAPE is usually an extreme shortness of breath that occurs simultaneously with ascension to a higher altitude. Shortly thereafter, the heart rate and the breathing increase and a cough develops that sometimes contains bloody sputum. By the time that these symptoms appear, quick medical attention is necessary to prevent the sufferer from lapsing into a coma and dying.

Still another form of AMS, and one that is even more dangerous, is called high-altitude cerebral edema (HACE). Although the exact mechanism whereby this disorder develops is not completely understood, it usually begins with the classic symptoms of AMS and then progresses into neurological symptoms such as speech impairment, loss of coordination, alternated states of consciousness, and hallucinations which eventually lead to death if medical attention is not immediately administered. Although HACE is very rare, it is a high-altitude risk that cannot be ignored.