Zhou et al. [23] measured the pH of blood in individuals before they ascended to high altitude, in addition to PaCO 2 , and PaO 2 . In that study, the blood pH was significantly associated with susceptibility to HAPE, and there was a significant difference in blood pH value between subjects who developed HAPE and without HAPE. They found that during intense physical labor following a rapid ascent to 4000 m, the likelihood of developing acute mountain sickness was significantly greater among individuals with blood pH > 7.45 than among those with blood pH < 7.45. However, they found no significant differences in PaCO 2 or PaO 2 between these two groups of subjects. Thus, they concluded that blood pH, but not PaCO 2 or PaO 2 , could be used as a predictor of HAPE.