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Feeling sick from lower oxygen levels after arriving at a high altitude.

Most cases of altitude sickness are mild, but some may be life-threatening.

Symptoms tend to occur within hours after arrival at high altitude and include headache and nausea. More-serious sickness may cause shortness of breath, a cough that won't go away and trouble exercising.

Mild altitude sickness may resolve in 1 to 3 days. More-serious sickness may require oxygen, medicines and moving to a lower altitude.