B.L.U.F.*

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.

Background

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.

Facts

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.

Cautions

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

Summary and Military Relevance

The Technical Bulletin-Medical 507 provides an established 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 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.

* Bottom Line Up Front

Key Points

• Heat illness and injuries are significant threats to the health and operational effectiveness of military 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 (i.e. 18-28 days). Consequently, personnel who cannot prepare in advance will require an acclimatization period upon arrival at the deployment site.

Background

Heat illness (or heat injury) refers to a continuum of heat-related injuries—heat cramps, heat exhaustion, exertional heat injury, exertional rhabdomyolysis, and heat stroke, in order of severity—and is a significant threat to the health and operational effectiveness of military members and their units [1-3]. In addition, those at the greatest risk for heat injuries are the youngest and most inexperienced Warfighters [4]. Consequently, the United States military has been exceptionally proactive in the prevention, identification, and management of heat illness. Multiple guidelines for the prevention and treatment of heat illness, with a strong command emphasis, have been developed [5]. In particular, Technical Bulletin-Medical (TB Med) 507 provides comprehensive information on heat illness and evidence-based preventive program(s) to protect military personnel.

Symptoms of heat illness

Heat illness is characterized by headache, nausea, vomiting, muscle cramps, and fatigue. Severe heat illness, particularly 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 [6]. Heat stroke could also result in neurological changes such as mental confusion or unconsciousness if the body temperature rises to 105°F (40.5°C) or higher [3,7,8].

Nonetheless, the human body’s response to heat stress is quite resilient if given time to adapt [1]. This improved response of the body to heat stress after a period of heat exposure and regular strenuous exercise is known as “acclimatization.” The process involves physiologic adaptive changes that improve heat tolerance and can be fully achieved after 10 to 14 days of heat exposure, with up to 75% of the adaptation occurring within five days.

Factors affecting acclimatization

Factors that affect heat acclimatization include training status, age, sweating, gender, and body fat as noted below.

Training status. Maximal aerobic power, or VO_2max, generally influences physiologic responses during heat acclimatization. Individuals with a high VO_2max (>60 ml/kg/min) exhibit superior heart rate and rectal temperature responses, and they usually acclimatize to heat faster than those with a low VO_2max (<40 ml/kg/min). However, even a physically active person cannot achieve full heat acclimatization without exposure to environmental heat stress. Athletes who train and compete in hot weather have a distinct thermoregulatory advantage over athletes who train in cool climates and only periodically compete in hot weather [2].

Age. Older persons were previously thought to be less heat tolerant than their younger counterparts. Middle-aged men (>45 yr) were shown to have higher heart rates, higher rectal temperatures, and lower sweat rates during exercise both before and after heat acclimatization than young men. However, recent research that controls for body size and composition, aerobic fitness, hydration, degree of acclimatization, and chronological age has shown little or no age-related decrement with respect to thermoregulatory capacity or heat-stress acclimatization [2].

Sweating and gender. Sweating differs by gender when thermoregulation is considered. Women sweat less than men, although they possess more heat-activated sweat glands per unit skin area. Women start to sweat at higher skin and core temperatures and produce less sweat than men with a comparable heat-exercise load even after an equivalent acclimatization [2]. Interestingly, women and men show equivalent thermoregulation during exercise when controlled for levels of fitness and acclimatization [2].

Body fat level. Fatal heat stroke occurs 3.5 times more frequently in excessively overweight young adults than in individuals of average body weight. An overweight individual also has a smaller ratio of body surface area to body mass for effective sweat evaporation than a smaller person with less body fat [2].

Loss of heat acclimatization

Adaptations acquired during heat acclimatization may revert to baseline levels after only a short period of inactivity (i.e. 18-28 days). The first adaptations to be lost are those that develop first, i.e., heart rate and other cardiovascular changes. The rate of decay is affected by the number of heat exposures per week, the frequency and type of training sessions, and the magnitude of the rise in core body temperature during workouts. Athletes with high fitness usually will maintain heat acclimatization longer than individuals with low fitness [9].

Myths and/or Claims

It is commonly believed that Warfighters who are physically fit and have a high VO_2max do not need adaptation to be heat acclimatized. In fact, no one can be fully acclimatized to heat without exposure to environmental heat stress. However, fit Warfighters do acclimatize faster than those who are less fit.

It is also assumed that older individuals are 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 decrements in one’s ability to adapt to extreme temperatures [2,10,11].

Finally, women have been thought to need a longer period of acclimatization, since they are more vulnerable to heat illness. However, research shows that gender is likely not a factor. Women and men showed equivalent thermoregulation during exercise when fitness levels were accounted for [2,12,13].

Facts

Heat stress resulting in moderate to severe dehydration (> two percent body-weight loss) can impair cognitive function by slowing reaction and decision times [3,14], inducing fatigue [15], and compromising auditory vigilance, target shooting, memory, reasoning, attention, spatial orientation, and time orientation [16].

Military Translation

Heat acclimatization is necessary to prevent or reduce the severity of heat illness for all service members deploying to hot environments. Military units should intensify their physical training in the heat prior to deployment. Personnel who cannot prepare in advance must be allowed a period for acclimatization upon arrival at the deployment site. In either case, acclimatization is most quickly accomplished through daily exercise sessions in the heat that last a minimum of one or, preferably, two hours per day. The exposure must be sufficiently strenuous to produce profuse sweating and a rise in core temperature [17]. Improved tolerance to heat is associated with physiological changes: earlier onset of sweating, lower heart rate, and lower rectal temperature during the same exercise intensity as prior to exposure [18]. Studies indicate that, in contrast to unacclimatized subjects, acclimatized Warfighters suffered no detrimental effects of exertional heat stress on physiologic and cognitive responses, despite almost the same degree of heat stress[19].

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.

Methods.

• 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.

Summary

A Warfighter’s capacity to work in hot conditions can be maximized through preparatory physical conditioning combined with exposure to heat. Accordingly, military units preparing for deployment to hot environments should intensify their physical training in the heat to gradually acclimatize.

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