Filed under: Environment
Heat-related injuries are a threat to Warfighters, even those in top physical condition, deployed to extreme environments. Heat acclimatization is necessary to ensure that the health and performance of Warfighters is not compromised to a dangerous degree when exposed to heat stress.
Take it slow. For unacclimatized Warfighters, physical exertion should be limited in intensity and time. Allow 9-14 days of progressive heat exposure and exertion—more for less-fit Warfighters, less for more-fit Warfighters.
Don’t overdo it; don’t underdo it! Heat acclimatization requires exposure at least two hours per day (can be two one-hour segments) while engaged in a cardiovascular exercise (which should increase in intensity each day of the acclimatization period).
It’s all relative. The level of heat acclimatization achieved is relative to the exertion normally expended by the Warfighter. If light exertion is the norm, the level of heat acclimatization after two weeks will match that. If more strenuous exertion is called for, additional acclimatization and possibly improved fitness is required.
Work smart. If Warfighters must perform physical work during the acclimatization period, take advantage of the cooler hours during the morning, evening, and night.
Stay hydrated. Adequate water is essential. Heat acclimatization increases sweating and, therefore, water requirements. Dehydration rapidly degrades safety and performance, even for those who are already heat acclimatized or in top physical condition.
For a more detailed look at heat stress and acclimatization, read HPRC’s reports on managing heat exposure.
Men and women in the military who operate in and around open water need protection against drowning and immersion syndrome. Quick response to cold-water immersion is important because of its immediate impact on the body. Immersion syndrome could cause cessation of breathing and/or cardiac arrest in certain people. Cold water causes a loss of heat much faster than cold air does and can quickly cause performance impairments such as not being able to fasten a life jacket or other safety equipment. Making an effort to tread water or swim only increases the body’s heat loss and hastens the onset of hypothermia.
A person alone can extend survival time by using body posture that covers areas especially vulnerable to heat loss. Those in the water should avoid movement and use the “heat-escape lessening posture” (H.E.L.P.). This posture minimizes the exposure to cold water of the individual’s groin and chest because the arms are folded across the chest and pressed to the sides and the knees are drawn up with the legs crossed at the ankles, creating a fetus-like position (see figure 13-6 in the article linked above for an illustration). Note that this technique does require the use of a personal floatation device that allows the knees to be drawn up.
If more than one person is involved, the “huddle position” should be used to reduce heat loss. In this position, individuals should press together their chests, abdomens, and groins. In addition to conserving each individual’s body heat, the huddle position helps prevent the swimmers from becoming separated before they are rescued, provides a larger rescue discovery target, improves morale, reduces shock and panic, and may reestablish a chain of command.
These survival techniques require practice and the use of personal flotation devices. Treading water in the H.E.L.P. position in heavy seas is a major challenge, and the huddle position requires that all participants be able to tread water, so being fit and prepared is essential.
Heat-related injuries are a significant threat to the health and operational effectiveness of military members and their units. The human body’s response to heat stress is quite resilient if given several weeks for adaptation to occur. This process, called acclimatization, involves internal adjustments, in response to the outside environment, which improve heat tolerance. This adaptation can be fully achieved after 10 to 14 days of exposure to heat, but two-thirds, or even 75 percent, of the adaptation takes place within five days.
Myths and/or Claims
1) It is commonly believed that warfighters who are physically fit do not need to be heat acclimatized.
2) It is also assumed that older individuals are less heat tolerant than their younger counterparts.
3) Women are thought to need longer acclimatization time, since they are more vulnerable to heat illness.
1) Though fit warfighters acclimatize faster than less fit warfighters, a physically active person cannot be fully acclimatized without exposure to environmental heat stress.
2) Age has no effect on acclimatization. Research that controlled for body size and composition, aerobic fitness, hydration, degree of acclimatization, and chronological age showed little or no age-related decrements in one’s ability to manage or acclimate to extreme temperatures.
3) Nor does gender appear to be a factor: women were thought to need longer acclimatization, since they are more vulnerable to heat illness. However, women and men show equivalent thermoregulation during exercise when levels of fitness and acclimatization were controlled.
4) Heat-related injuries such as exertional heat illness remain a major cause of illness and occasional fatalities within the Armed Forces. However, as mentioned earlier, the human body’s can be resilient to heat stress if given several weeks to adapt.
Heat acclimatization adaptations may vanish after only a few weeks of inactivity (i.e., 18-28 days). The first adaptations to degrade are those that develop first: heart rate and other cardiovascular variables.
Summary for Military Translation
Studies have shown that acclimated soldiers suffer no detrimental effects of exertional heat stress, despite almost the same degree of heat strain. The Technical Bulletin-Medical 507 provides an evidence-based preventive program to protect military personnel from heat stress and associated adverse health effects. The recommended heat acclimatization strategies are to mimic the deployment climate, ensure adequate heat stress (i.e. by invoking sweating, having 4 to 14 days of heat exposure, and maintaining the daily duration of at least 100 minutes). It is also suggested that heat acclimatization start at least one month before deployment; and upon arrival, acclimatization should start slowly and build up by increasing heat and training volume as tolerance permits.
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Armstrong, L. E. (Ed.) (1998) Encyclopedia of Sports Medicine and Science.
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Lugo-Amador, N. M., Rothenhaus, T., & Moyer, P. (2004). Heat-related illness. Emerg Med Clin North Am, 22(2), 315-327, viii. doi: 10.1016/j.emc.2004.01.004S0733862704000057 [pii]
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Radakovic, S. S., Maric, J., Surbatovic, M., Radjen, S., Stefanova, E., Stankovic, N., et al. (2007). Effects of acclimation on cognitive performance in soldiers during exertional heat stress. Mil Med, 172(2), 133-136.
USACHPPM. (2003). Heat stress control and heat casualty management.