High altitude mountaineering physiology training pertinent to the exertion and duration of stay at high altitudes without supplementing oxygen is much more involved than general aviation physiology training.

Predisposition or vulnerability to acute mountain sickness (AMS), high altitude pulmonary edema (HAPE), and High Altitude Cerebral Edema (HACE) depend on the altitude, the rate of ascent and the degree of individual susceptibility. Generally the concerns are connected to elevations (altitudes) exceeding 2500 meters/8202feet above sea level.

The significant difference between high altitude mountaineering and high altitude flying is the rate of ascent and descent is much slower to the extent pre-breathing 100% oxygen to dissipate (flush) nitrogen out of body tissues and fluids to avoid decompression illness isn’t a necessity. Unfortunately the exposure to high altitude is generally longer (days) and the human performance military mountaineering activities are significantly more physically demanding. Consequently the engaging in high altitude mountaineering activities not only causes physical and physiological screening of potential high altitude mountaineering team members being desirable, it also imposes the necessity for high altitude mountaineering physiology training.

Although high altitude to a physiologist starts around 5000ft/1524m above sea level where an absolute atmospheric pressure of 12.2 psi is encountered, it is at altitudes at and greater than 10,000 feet where reduced atmospheric pressure can potentially place considerable stress on the individual’s cardiovascular system resulting in inadequate oxygenation of the blood and impairment to function at full capacity.

Although the atmosphere generally remains consistent, with a 78% nitrogen/21% oxygen mixture to about 70,000 feet, humans are unable to adapt physiologically to all of the physical changes that occur in the different regions of the atmosphere.

The efficient zone of human adaptability in terms of near-ideal physiological environment extends from sea level to 10,000 feet although some acclimation for some individuals may be required at elevations between 8,000 and 10,000 feet. Also, without the use of supplemental oxygen, a decrease in night vision capabilities will occur with ascent from sea level to above 4,000 feet.

The Physiologically Deficient Zone extends from 10,000 to 50,000 feet. This is where reduced atmospheric pressure causes major physiological problems...dehydration, hypoxia and decompression sickness.

The Space Equivalent Zone extending from 50,000 feet to the outer fringes of the atmosphere, the space equivalent zone is totally hostile to human life. Unprotected exposure to the extremely low temperatures and pressures found at these high altitudes can quickly result in death. An example of how dangerous this area can be is found at 63,000 feet (Armstrong’s line). The barometric pressure at this altitude is only 47 mm/Hg, which equals the partial pressure of water.

Although mountain elevations do not exceed 30,000 feet above sea level, elevations between 10,000 feet and 29,029 feet (Mt Everest) above sea level are extreme environments to perform in.

At 10,000 feet above sea level the reduced atmospheric pressure environment, 10.1 psi, is sufficient to reduce oxygen saturation in the blood from a 100% to a 90% level. At 18,000 feet, 7.34 psi, the reduced atmospheric pressure environment allows only a 71% blood oxygen saturation level. It is this reduction in pressure (or in other words, the less dense air) that causes hypoxia, a condition during which the pressure in the blood is not sufficient enough to deliver oxygen to the brain. However the normal healthy human body can general quickly adapt for a short time at altitudes between 10,000 feet and 13,000 feet and acclimatize (typically needs 24 hours or more) to stay for longer durations at elevations 10,000 feet above sea level.

There is considerable variability between individuals and between populations in their ability to acclimatize to the reduced atmospheric pressure environmental stresses found at elevations higher than 10,000 feet above sea level. Exertion, lack of time to adapt and many acute medical problems may cause additional difficulty as elevation is gained. It is generally accepted that acclimation is impossible above 26246 feet.

At 18,000 feet atmospheric pressure is 7.34 psi and acclimation is critical to gaining higher elevation. The atmospheric pressure environment found at the highest elevation existing on planet Earth (Mt Everest-29, 029 ft /8.848 m above sea level) is about 4.6 psi. It is this reduction in pressure (or in other words, the less dense air) as elevation is gained that causes hypoxia, high altitude pulmonary edema, cerebral edema and many other life threatening physiological problems.

The respiratory and circulatory systems of the human body have the capability to adjust and function in a variety of environments. But, the body has its limitations particularly when other environmental adversities factors such as cold (hypothermia), very low in humidity (dehydration), fatigue (inadequate sleep), and prolonged moderate to heavy physically demanding activity are present. If the change is too abrupt, then these systems can't adjust quickly enough and the body will suffer the affects. Preexisting Medical Conditions can and to compromise the bodies capability to adjust.

Human performance capacity to physiologically adapt (cope) has anomalies in that tolerance to atmospheric pressure varies significantly in environments above 10,000 feet above sea level (different people will acclimatize at different rates). Unfortunately acclimatization can never be absolute and thus is never able to completely abolish the ill-effect of hypoxia exposure leading to high altitude pathologies.

Acclimatization to high altitudes requires changes in all body systems involved in the uptake of oxygen into the body, the transport of that oxygen to the tissues, and the unloading of that oxygen at the tissues. Acclimatization can never be absolute and thus is never able to completely abolish the ill-effect of hypoxia exposure leading to high altitude pathologies.

Successful acclimatization rarely results in the same level of physical and mental fitness that was typical of altitudes close to sea level. The overall measure of physical performance: VO2 max drops 3% per thousand feet of altitude gain, starting at around 5000 ft.

As a general mountaineering rule altitudes elevations above 18,000 feet are considered are considered extreme altitudes. Although with acclimatization humans can function for short periods of time above 18,000 feet attempts to acclimatize beyond 17,000 feet more often results in a degradation of the body greater than the benefits gained. The term ‘acclimatization’ is probably not appropriate for altitudes above 26246ft/8000m.

U.S. military forces do not routinely train in high altitude mountainous terrain. Therefore, extensive preparations are needed to ensure individual and high altitude mountaineering team effectiveness. Being physically and psychologically conditioned and acclimatization is critical in accomplishing high altitude mountaineering operations.