Geng et al. [21] investigated the diffusion of carbon monoxide (CO) in the lung of 27 subjects who rapidly ascended from 2260 m to 4700 m. They found that CO diffusion was correlated with the development of acute mountain sickness. In 10 patients with HAPE, the diffusing capacity of CO (DLCO) increased with increasing altitude, but the amplitude of this increase was greater in subjects without HAPE. Thus, they speculated that insufficient DLCO was a cause of acute mountain sickness and that DLCO is an objective parameter to predict adaptability to high altitude and the occurrence of HAPE.