Altitude Illness

 Altitude Effects on the Human Body

Last Updated: July 30, 2018
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Some of the most common effects of altitude exposure on the human body include:

Reduced physical performance: Soldiers cannot maintain thesame physical performance at altitude as they can at sea level, regardless of their fitness level. Countermeasures include ensuring acclimatization, adjusting work rates and load carriage, planning frequent rests during work and exercise, and planning and performing physical training programs at altitude.

Psychological Effects: Altitude exposure may result in changes in senses (e.g., vision, taste), mood, and personality. These effects are directly related to altitude and are common at over 3,048m. Some effects occur early and are temporary while others may persist after acclimatization or even for a period of time after descent.

      • Vision is generally the sense most affected by altitude exposure. Dark adaptation is significantly reduced, affecting
        Soldiers as low as 2,438m and can potentially affect military operations at altitude. 
      • Mental effects most noticeable at very high and extreme altitudes include decreased perception, memory, judgment, and attention. 
      • Alterations in mood and personality traits are common during altitude exposures.

Dehydration: Dehydration is a very common condition in Soldiers at altitude. Causes include perspiration/sweating, vomiting, increased breathing, and diminished thirst sensation. Dehydration decreases physical performance, increases symptoms of altitude illness, and may increase risk of developing cold injuries. 

Nutrition: Poor nutrition can severely impact military operations and contribute to illness or injury, decreased performance and poor morale. At high elevations dulled taste sensations (making food undesirable), nausea, or lack of energy can decrease the motivation to prepare or eat meals. Poor eating habits may also lead to constipation, aggravation of hemorrhoids, and undesired weight loss.


 Meters (m) / Feet (ft)

 Effects of Acute Altitude Exposure


Sea Level – 1200m / 4000 ft



1200-2400m / 4000-7870 ft

Mild altitude illness and decreased performance may occur


2400-4000m / 7870-13,125 ft

Altitude illness and performance decrements are more common and greater

Very High

4000-5500m / 13,125-18,000 ft

Altitude illness and decreased performance is the rule


5500m / 18,000 ft - and higher

With acclimatization, humans can function for short periods of time

Products to Avoid at Altitude

  • Tobacco smoke interferes with oxygen delivery in the body and increases the amounts of carbon monoxide (CO) in close spaces. The irritant effect of tobacco smoke can narrow the airways and interfere with breathing.
  • Alcohol impairs judgment and perception, depresses respiration, causes dehydration, and increases susceptibility to cold injury.
  • Caffeine from coffee and other sources may improve physical and mental performance. But it should be consumed in moderation.


Altitude acclimatization eliminates altitude illness and allows Soldiers to achieve the maximum physical work performance possible. Once acquired, acclimatization is maintained as long as the Soldier remains at altitude, but is lost over several days upon return to lower elevations. Exposure to higher altitudes requires further acclimatization.

For most Soldiers at high to very high altitudes, 70-80 percent of the respiratory component of acclimatization occurs in 7-10 days; 80-90 percent of overall acclimatization generally occurs in 14-30 days; and maximum acclimatization may take months or years.

Two Ways to Achieve Acclimatization

  • Staged Ascent: Soldiers ascend to a moderate altitude and remain there for 4 days or more to acclimatize before ascending higher. When possible, Soldiers should stop at several altitudes to allow a greater degree of acclimatization.
  • Graded Ascent: Slow ascents allow partial acclimatization. To reduce the risk of altitude illness, have Soldiers spend one or two nights at moderate altitude (1200-2400m). At altitudes above 2400m, Soldiers should sleep no more than 300m above the previous night's sleeping altitude. 

A combination of staged and graded ascent is the safest and most effective way to prevent altitude illnesses.


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