a term that can mean acute altitude sickness; also used for chronic disease characterized by low oxygen saturation of hemoglobin, due to low partial pressure of oxygen in inspired air plus alveolar hypoventilation that develops in some individuals, especially older people. Polycythemia leads to florid skin color but cyanosis appears on mild exertion, along with dyspnea, fatigue, headache, and mental torpor. A person so afflicted returns to normal shortly after return to lower altitude.