High-altitude training masks simulate a reduced oxygen environment in order to produce “normobaric hypoxia”—oxygen deprivation similar to that at high altitude, but at normal atmospheric pressure. Although normobaric-hypoxic training may induce some of the physiological adaptations of altitude acclimatization, these are only evident in a normobaric environment—that is, at normal atmospheric pressure. At altitude, these devices produce only a small reduction in susceptibility to acute mountain sickness, and no improvements in sleep quality or performance have been reported in published peer-reviewed studies.
Decreasing the barometric pressure while reducing the oxygen content of air, as in hypobaric-hypoxic training, is an evidence-based practice used to acclimate prior to altitude exposure, but it requires special equipment and medical oversight.
In summary, normobaric-hypoxic training with easily available commercial devices is problematic in terms of oversight, expense, and potential risks in unfit individuals and those with cardiac or pulmonary conditions, hemoglobin abnormalities, hypertension, or other health issues. Use by those who are unfit or have medical concerns is not well studied, but possible complications have been voiced. Read HPRC’s “Intermittent normobaric-hypoxia exposure conditioning program” to learn more.