Above 1,200 meters in altitude, low oxygen conditions can lead to hypoxic stress, triggering a series of changes in the tissues of people who ascend from lower altitudes. The effects on the body are collectively referred to as altitude illness, but this can range from mild to severe or even life-threatening conditions.
In general, the higher the elevation the greater the risk of more severe effects:
| Altitude | Meters (m) / Feet (ft) | Effects of Acute Altitude Exposure |
|---|
| Low | Sea Level - 1200m / 4000 ft | None |
| Moderate | 1200-2400m / 4000-7870 ft | Mild altitude illness and decreased performance may occur |
| High | 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 |
| Extreme | 5500m / 18,000 ft - and higher | With acclimatization, humans can function for short periods of time |
Altitude exposure may also increase risk of other environmental injuries (e.g., weather-related injuries) or worsen pre-existing medical conditions.
Military high-altitude activities aim to maximize the physical and cognitive performance of service members through reducing risk of altitude illness.
HOW TO PREVENT ALTITUDE ILLNESS
1. Avoid these products prior to and during high altitude exposures:
- Tobacco 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 should be consumed in moderation.
2. Acclimatize
A gradual increase to higher altitude(s) and the amount of physical activity allows the body tissues time to adapt to the low oxygen environment – this is known as “acclimatization.”
Service members are able to perform duties for the altitude to which they are properly acclimatized.
How to achieve acclimatization?
A combination of two types of ascent (staged and graded) is the safest and most effective way to achieve acclimatization and prevent altitude illnesses.
- Staged Ascent: Soldiers ascend to a moderate altitude and remain there for 4 days or more to acclimatize before ascending higher. When possible, service members 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, service members should sleep no more than 300m above the previous night's sleeping altitude.
How long does acclimatization take?
For most service members trained at 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.
How long does acclimatization last?
Once acquired, acclimatization is maintained as long as a person remains at that altitude. Acclimatization is lost over several days upon return to lower elevations. Exposure to higher altitudes requires further acclimatization.
3. Check for pre-existing conditions
- Medical conditions: Heart and coronary artery conditions, diabetes, respiratory conditions such as chronic pulmonary disease, severe asthma, or preexisting hypoxemia, obstructive sleep apnea, or sickle cell trait may increase a person's risk of altitude illness or their medical condition may be exacerbated by high altitude.
- Prior altitude illness: Anyone who has previously experienced altitude illness may be more susceptible in the future.
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