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You are here: Home / HPRC Blog / What is heat acclimatization?

What is heat acclimatization?

published: 06-17-2010 Journal entry icon

For a more detailed analysis on heat acclimatization, click here.

Background

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.

Facts

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.

Caution

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.

References

A.Nunneley, S. (2009). Prevention of Heat Illness Medical Aspects of Harsh Environments, Volume 1: U.S. Army Medical Department's headquarters

Armstrong, L. E. (Ed.) (1998) Encyclopedia of Sports Medicine and Science.

DOD. (2010). Update: Hear Injuries, Active Component, U.S.Armed Forces, 2009. Medical Survillance Monthly Report, Vol.17.

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]

McArdle, W., Katch, F., & Katch, V. (2007). Exercise physiology. Energy, Nutrition & Human Performance (Sixth ed.): Lippincott Williams & Wilkins.

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.