High-Altitude Training Masks
Commercial devices are available that purportedly help a person acclimatize to high altitude while they are still training at normal atmospheric pressure, but their effectiveness is questionable.
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—i.e., 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. More detail is available in the summary by Stephen Muza, Thermal and Mountain Division, US Army Research Institute of Environmental Medicine.