Comparisons of altitude tolerance and hypoxia symptoms between nonsmokers and habitual smokers.

BACKGROUND: Increased levels of carboxyhemoglobin (COHb) in smokers are blamed for inducing pre-hypoxic tendency classified as anemic hypoxia. If COHb can be simply converted to altitude, there should be significant differences between smokers and nonsmokers with respect to hypoxia tolerance. However, the studies of the effects of carbon monoxide and/or smoking habit on the physiological functions at altitude do not have consistent conclusions, and many pilots still have smoking habits. This study was designed to assess whether there is a definite significant difference for time of useful consciousness (TUC), subjective symptoms, or performance degradation between nonsmokers and smokers.
METHODS: During the hypoxia experience of routine physiological training, TUC and 12 typical subjective symptoms were examined at the chamber altitude of 25,000 ft (7620 m) in 589 nonsmokers and 582 smokers in Study 1. The time until the deterioration of handwriting was assessed by 6 physiological training observers in 51 nonsmokers and 70 smokers in Study 2. The results were compared between the groups.
RESULTS: Smokers revealed significantly fewer subjective symptoms in 5 out of 12 symptoms. There were no significant differences in TUC and the rate of handwriting deterioration between the groups.
CONCLUSIONS: Paradoxically, smokers are slightly resistant to hypoxia with respect to emerging subjective symptoms. However, bluntness to hypoxia could postpone the detection of the possible hypoxic occurrence in pilots.