How can heat acclimatization prevent heat illness?

Heat acclimatization is necessary to prevent or reduce the severity of heat illness. Acclimatization is most quickly accomplished through daily exercise sessions in the heat that last a minimum of one or, preferably, two hours per day.

Key Points

  • Heat illness and injuries are significant threats to the health and operational effectiveness of Service Members and their units.
  • Severe heat illness—in particular, heat stroke—may cause irreversible damage to the heart, lungs, kidneys, and liver, which could lead to cardiovascular disease (CVD), ischemic heart disease (IHD), chronic liver damage, and/or renal failure.
  • Heat illness/injury impacts the cognitive functions of reaction and decision time, vigilance, errors of omission, target shooting, memory, reasoning, attention, spatial orientation, and time orientation.
  • The U.S. Armed Forces are proactive in the prevention, identification, and management of exertional heat illness; for example, TB MED 507 provides evidence-based preventive programs to protect military personnel from heat stress.
  • Heat acclimatization is fully achieved after 10 to 14 days of heat exposure, with up to 75% of adaptation being achieved within five days.
  • The physiologic adaptations include: improved sweating, fluid balance, lowered metabolic rate and heart rate, increased stroke volume and plasma volume, and reduced electrolyte loss when working at the same workload as before acclimatization.
  • Factors that affect acclimatization are: training status, age, sweat rate, gender, and body-fat level.
  • Heat acclimatization degrades after a period of inactivity (that is, 18–28 days). Consequently, personnel who cannot prepare in advance will require an acclimatization period upon arrival at the deployment site.


Heat-related injuries are significant threats 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 a collection of inside adjustments to the outside environment that improve heat tolerance.

Myths and Claims

First: It is commonly believed that Warfighters who are physically fit do not need to be heat acclimatized. Even physically active individuals cannot be fully acclimatized without exposure to environmental heat stress, although they will probably acclimatize to heat faster than less fit people. Second: Older persons were previously thought to be less heat tolerant than their younger counterparts. Research that controls for body size and composition, aerobic fitness, hydration, degree of acclimatization, and chronological age shows little or no age-related difference in one’s ability to manage or acclimatize to extreme temperatures. Last: Women were also thought to need longer acclimatization, since they are more vulnerable to heat illness. Women and men, in fact, show equivalent reactions to heat during exercise when controlled for levels of fitness and acclimatization.


Heat illness remains a major health issue within the Armed Forces. The human body’s response to heat stress is quite durable if given several weeks for adaptation to occur. This adaptation is a process that is fully achieved after 10 to 14 days of exposure to heat, but 75% of the adaptation usually can be achieved within five days.


Heat acclimatization adaptations may vanish after only a few days or weeks of inactivity (that is, 18–28 days). The first adaptations to decay are those that develop first: heart rate and other cardiovascular variables.

Heat acclimatization strategies

Many strategies can be used to acclimatize to extreme heat. All involve working out or exercising in a warm environment. Reproduced here are the heat acclimatization strategies as published in TB Med 507 (Table 3-2.):

Mimic the deployment climate.

Ensure adequate heat exposure by—

  • Invoking sweating.
  • Using exercise and rest to modify the heat strain.
  • Having 4 to 14 days of heat exposures.
  • Maintaining the daily exposure of at least 100 minutes.

Start early (1 month before deployment).

  • Performance benefits may take longer than physiological benefits.
  • Be flexible with training.
  • Build confidence.
  • Pursue optimum physical fitness in the current climate.


  • Pre-deployment: Climate controlled room or hot weather.
  • Integrate with training by adding acclimatization sessions; inserting acclimatization with training; and alternating acclimatization days with training days, and no detraining.

On arrival.

  • Start slowly and reduce training intensity and duration and limit heat exposure.
  • Increase heat and training volume as tolerance permits.
  • Acclimatize in heat of day.
  • Train in coolest part of day.
  • Use work/rest cycles or interval training.
  • Be especially observant of salt needs for the first week of acclimatization.


The recommended heat acclimatization strategies in TB MED 507 provide an established preventive program to protect military personnel from heat stress and associated adverse health effects. They mimic the deployment climate and ensure adequate heat exposure (by invoking sweating, using exercise and rest to modify heat strain, having four to 14 days of heat exposure, and maintaining the daily exposure of at least 100 minutes). Heat acclimatization should start at least one month before deployment, alternating acclimatization with training. The process should start slowly and build by increasing heat exposure and training level as tolerance permits. Use work-rest cycles or interval training, and be especially careful with fluid and electrolyte needs during the first week of acclimatization.

A Warfighter’s capacity to work in hot conditions can be maximized through preparatory physical conditioning. Military units preparing for deployment to a hot climate should intensify their physical training in the heat in order to gradually increase their state of heat acclimatization. Personnel who cannot prepare in advance should have a period for acclimatization on arrival at the deployment site.

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