Incident cases and incidence rates of heat stroke, by source of report and year of diagnosis, active component, US Armed Forces, 2013–2017. (From Armed Forces Health Surveillance Branch )
In 2016 alone, there were 2163 incident diagnoses of EHI (heat exhaustion and exertional heat stroke [EHS]) among active component service members (incidence rate: 1.79 cases per 1000 person-years [p-yrs]). The overall crude incidence rates of heat stroke and heat exhaustion were 0.38 and 1.41 per 1000 p-yrs, respectively. Subgroup-specific incidence rates of heat stroke were highest among males and service members aged 19 years or younger, Asian/Pacific Islanders, Marine Corps and Army members, recruit trainees, and those in combat-specific and “other” occupations .
Unfortunately, recent efforts to mitigate exertional heat stroke casualties have not proven to reduce injury rates. The annual rate (unadjusted) of cases of heat stroke in 2017 was slightly higher than in 2016 and 58% higher than in 2013. Heat stroke rates in the Marine Corps were 50% higher than in the Army; Army heat injury rates were more than ninefold those in the Air Force and Navy. Lastly, the incidence was 86% higher among males than females .
Military Unique Physiologic Considerations
Human performance is compromised with the extremes of temperature, as well as at depth and at increased altitudes above sea level. Understanding the basic principles of applied physiology that contribute to this performance decrement is critical for the MMO, who is frequently called upon for the proper interpretation of resources and guidelines that discuss environmental stress. Knowledge of physiology enables the MMO to identify and modify those factors that can be mitigated or leveraged for success in the military operational environment while promoting prevention by educating the individual warfighter as well as the unit leadership.
During exercise, the human body acts to dissipate the excess heat generated by skeletal muscle; this requires an intact cardiovascular system that uses blood to transfer heat from the body core to the skin, where the mechanisms for dissipating heat can take effect. However, when the ambient temperature is higher than the body’s core temperature, convection, conduction, and radiation are no longer effective. Environmental conditions also affect evaporative cooling. A water vapor pressure gradient must exist for sweat to evaporate and release heat into the environment. In high humidity (relative humidity >75%) or under occlusive clothing/gear, evaporation becomes ineffective for transferring heat. Thus, in hot and humid conditions, warfighters become susceptible to exertional heat illness.
Limitations on heat dissipation in hot and humid weather are exacerbated during intense exercise by a finite supply of blood that must fulfill multiple functions, including meeting the metabolic demands of active skeletal muscle and transporting heat to the skin surface for cooling. Further complicating matter is the dehydration that develops in most individuals during intense exercise in the heat, which decreases plasma volume. Studies suggest that during intense exercise in the heat, for every 1% of body weight lost from dehydration, there is a concomitant increase in core body temperature of 0.22 °C (0.4 °F) . In other words, other factors being equal, a warfighter who loses only 1% of body weight from dehydration during intense exercise in the heat would be 1 °C cooler compared to a buddy who loses 4% of body weight. This would equate to a temperature difference of approximately 39 °C (102 °F) versus 40 °C (104 °F) at the end of a training session. A number of additional factors influence the rate at which a person’s core body temperature rises during vigorous activity, including fitness level, degree of acclimatization to heat, clothing/equipment, and use of certain types of nutritional supplements that affect physiologic response (e.g., metabolic rate, degree of tachycardia).
During exercise, the body temperature rises in response to the increase in metabolic heat production; a modest rise in temperature is thought to represent a favorable adjustment that optimizes physiologic functions and facilitates heat loss mechanisms as previously described. With compensated heat stress (CHS) , the body achieves a new steady-state core temperature that is proportional to the increased metabolic rate and available means for dissipating heat. Studies in runners describe this mechanism as exercise-induced hyperthermia; acclimatized warfighters and athletes can complete events successfully with significantly elevated core temperatures (103–104°F) while remaining entirely asymptomatic . Uncompensated heat stress (UCHS) results when cooling capacity is exceeded, and the warfighter or athlete cannot maintain a steady temperature. Continued exertion in the setting of UCHS increases heat retention, causing a progressive rise in core body temperature and increasing the risk for severe heat illness.
Factors predisposing to heat illness
Lack of acclimatization
Low physical fitness
Extremes of age (infants, elderly)
Genetics (RYR1 mutation, TLR4 polymorphisms)
Metabolic and thermoregulatory disorders: Febrile illness, insomnia/sleep deprivation, hyperthyroidism/thyroid storm, seizures, neuroleptic malignant syndrome, malignant hyperthermia
Disorders of skin and sweating: Extensive rash, sunburn, large areas of burned/scarred/grafted skin, diabetes mellitus, chronic anhidrosis, cystic fibrosis, ectodermal dysplasia
Unknown mechanism: Sickle cell trait
Anticholinergics (e.g., meclizine, tolterodine, atropine)
Antiepileptics (e.g., lamotrigine, topiramate)
Antihistamines (e.g., diphenhydramine, loratadine)
Phenothiazines (e.g., thioridazine, chlorpromazine, promethazine)
Amphetamines (e.g., Ritalin®, Ecstasy [3,4-methylenedioxy-methamphetamine (MDMA)])
Ergogenic stimulants (e.g., caffeine, pre-workout supplements, ephedrine, ephedra)
β-Blockers and calcium channel blockers
Little air movement
Lack of shade
Heavy clothing or gear
Prior compromised heat exposures
Importantly, febrile persons have accentuated elevations in core temperature when exposed to high ambient temperature, physical exercise, or both. Environmental temperature and humidity, medication, and exercise heat stress in turn challenge the cardiovascular system to provide high blood flow to the skin, where blood pools in warm, compliant vessels such as those found in the extremities. When blood flow is diverted to the skin, reduced perfusion of the intestines and other viscera can result in ischemia, endotoxemia, and oxidative stress . In addition, excessively high tissue temperatures (heat shock: >41 ° C [105.8 ° F]) can produce direct tissue injury; the magnitude and duration of the heat shock influence whether cells respond by adaptation (acquired thermal tolerance), injury, or death (apoptotic or necrotic). Heat shock, ischemia, and systemic inflammatory responses can result in cellular dysfunction, disseminated intravascular coagulation, and multi-organ dysfunction syndrome. In addition, reduced cerebral blood flow, combined with abnormal local metabolism and coagulopathy, can lead to dysfunction of the central nervous system. The aforementioned illustrates UCHS, which can severely denigrate the warfighter’s performance, and the process by which UCHS may progress to exertional heat stroke, which puts the individual at risk for significant morbidity and mortality and compromises the unit’s mission.
Military Unique Populations
Analogous to how athletes practice with their teams in order to prepare for game day, warfighters spend significant time training with their units on a day-to-day and month-to-month basis throughout their military careers. Each unit has a specific mission and must maintain readiness to execute that mission in combat operations when called upon. Several aspects of high-risk military training and operations will be discussed to illustrate how each is uniquely associated with heat illness risk.
The task of creating a lethal fighting force out of civilian volunteers drawn from an increasingly sedentary population carries many challenges and risks, among which is the risk of exertional heat illness. Each military component (Army, Navy, Air Force, Marine Corps, Coast Guard) maintains one or more locations dedicated to entry-level training, where civilian recruits are indoctrinated with basic military knowledge and skills. These include marching and drill, physical fitness, basic weapons skills, and proper usage of military gear (e.g., flak vests, helmets, rucksacks, gas masks), among many other tasks. While much of this training is only moderately strenuous, some aspects are quite physically intense or prolonged in nature. With this in mind, the primary risk of heat illness in basic military training stems from the fact that many recruits, just a few weeks or even days prior, were predominantly sedentary and spent most of their time seated indoors. Military training programs generally offer little to no acclimatization period or a progressive nature of training (varies by service and even, to some degree, by individual instructor). However, Air Force Basic Military Training has recently made changes to their physical training program to allow 10 days of acclimatization prior to any maximum effort fitness test.
Geography plays an important role in the warfighter’s risk for exertional heat illness. Most military basic training installations lie in hot, humid regions of the USA, such as Ft. Jackson, SC (Army); Ft. Benning, GA, USA (Army); and Joint Base San Antonio—Lackland, TX, USA (Air Force). Marine Basic Training, generally acknowledged as the most physically intense of all enlisted basic training, occurs in similarly hot and humid locations: Marine Corps Recruit Depot (MCRD) Parris Island, on the coast of South Carolina, USA and MCRD San Diego in southern California, USA. Navy and Coast Guard recruits complete their basic training at Naval Station Great Lakes (near Chicago, IL, USA) and Training Center Cape May (New Jersey, USA), respectively, where the climate is more temperate on average than other US basic training sites. But even Great Lakes and Cape May can be quite hot and humid during summer months. Recruits arrive at these training bases from all over the country, many times leaving frigid winter weather to begin training in warmer climates the very next day.
In addition to these factors, the sheer volume of trainees at these installations increases the probability of heat illness in those with a genetic predisposition to heat intolerance. The Department of Defense trains just over 200 K new enlisted recruits annually, including active duty, guard, and reserve components. The Army leads the way with numbers (80–100 K), followed by Navy (40–45 K), Air Force (35–40 K), Marine Corps (30–35 K), and Coast Guard (3–4 K). Among numbers of that scale, it becomes likely that a few will be found with genetic predisposition for heat illness (such as RYR1 gene mutation) or other risk factors, requiring medics to be always at the ready [9, 10].
Finally, there is an element of the traditional mentality or paradigm underlying the training. Each new generation of recruits is trained by a seasoned cadre of instructors who instill the warrior ethos and maintain strict military discipline, conducting training in much the same way they were trained themselves, passing on these military traditions, and ever vigilant to prevent the military from becoming too “soft.” Though there may be operational benefits to this approach, this historical inertia may create resistance to risk reduction interventions (e.g., more gradual progression of training, longer acclimatization periods, more allowance for resting breaks) and promote individual recruit over-reaching.
Most of the considerations related to basic training pertain to officer training as well, with several important differences that will be discussed here. Many military officers are trained at one of the service academies, such as the US Military Academy at West Point (upstate New York, USA), the US Naval Academy (Annapolis, MD, USA), or the US Air Force Academy (Colorado Springs, CO, USA). While each of the academies enjoys a more temperate climate than most of the basic training sites, the risk of heat illness is nonetheless substantial. The overall physical intensity of training in each service academy is difficult to quantify or compare, but none would argue that certain phases of officer training are extremely physically intense. Perhaps the most notable example is that all US military service academies conduct high-intensity training each summer at the very beginning of the academic year with each incoming class of new cadets, though format and content differs. Risk of heat illness could potentially be elevated due to limited time for acclimatization prior to this early phase of training. Additionally, the culture of training and leadership paradigms in these officer training platforms may be more risk tolerant than basic training (if it is possible to generalize). Part of this relatively higher risk tolerance may be well founded, noting generally higher entry-level physical fitness of officer candidates. All service academy cadets must complete a physical fitness assessment prior to entry [11–13], whereas only the Marine Corps and Army conduct physical fitness assessments prior to entry into enlisted basic training [14, 15]. Additionally, officer training at the service academies lasts 4 years, which in theory allows more opportunity for acclimatization, but in reality, as noted above, some of the most physically intense training occurs in the first several weeks after each class of new cadets arrives to begin their freshman year.
Reserve Officer Training Corps (ROTC) programs are offered by most universities and serve as a large proportion of officer accessions. Without question, training at ROTC units is variable by location, some being large and well-reputed programs with intense and well-organized physical fitness curriculum. In contrast, some smaller ROTC programs may be less physically intense (perhaps lowering heat illness risk) but may also have fewer instructors and perhaps less subject matter expertise in physical fitness training or associated medical hazards.
The Marine Corps Officer Candidates School (OCS) merits separate discussion. This 10-week training program is located at Marine Corps Base Quantico (northern Virginia, USA) and records likely the highest incidence rates per capita of exertional heat stroke of any US military training activity. With an average of roughly 2700 total candidates per year (approx. 2400 graduates), OCS staff have reported 297 total heat strokes over calendar years 2016–2018, for an average just over 99 heat strokes per year. Seasoned OCS physicians attribute this high incidence rate to the extreme physical intensity of training in addition to the hot/humid climate during summer months, when most heat strokes occur .
Special Operations and Other High-Risk Military Populations
Upon completing basic military or officer training, the majority of military personnel proceed to career fields where physical fitness and exercise are largely self-paced and self-guided. However, those who go on to special operations career fields (e.g., Navy SEALs, Army Green Berets, Air Force Pararescue, and Combat Controllers, along with officer counterparts) must complete extremely strenuous training to join and maintain the ranks of these elite warrior athletes. Nearly every conceivable heat illness risk factor is at play in special operations (discussed further below), and these risks do not disappear upon completion of training, as operational missions are often even more demanding.
Other populations in the military also face significant heat stress, and it is not feasible to enumerate all of them due to the number and diversity of military units with their unique missions. Consider heat exposures experienced by the pilot in the cockpit, tank crew on extended missions, and disaster cleanup personnel who must wear full-body chemical protective suits and gas masks for hours at a time, just to name a few.
Military Unique Risk Factors of Heat-Related Illness
Significance of military-specific risk factors
Military-specific risk factors
Gear and equipment
Body armor and gear often weigh 60–100 lbs., driving metabolic workload and barrier to heat dissipation Joint Service Lightweight Integrated Suit Technology (JS-LIST) incapacitates human thermoregulatory mechanisms
70% of military use dietary supplements, of which >10% use high-risk supplements such as pre-workout stimulants or anabolic agents
Intrinsic vs. extrinsic motivation
Training instructors provide powerful extrinsic motivation, while mission or “earning your beret” provides powerful intrinsic motivating forces
Extended duty hours, limited opportunity to sleep. Depression and anxiety may lead to insomnia
Availability of water, palatability of water
Warfighters deploy from home station in temperate climates to arrive in a hot, arid theater of combat; mission may allow little time to acclimatize
Solo training, remote operational missions
Altered mental status may not be detected, with none present to witness and respond. Cooling means not available in remote locations
Large group training and operations with closely shared lodging increase transmission of respiratory and gastrointestinal viruses
Awareness and knowledge level of leadership with respect to heat stress informs their risk tolerance and training paradigms
Military Gear and Equipment
The military gear ensemble varies substantially depending on the unit and mission (especially combat vs. noncombat missions) but begins with the uniform. Combat uniforms typically consist of a durable, long sleeve, mostly-cotton shirt and cargo pants, but more modern combat uniforms feature more functional design and advanced nylon-cotton or other blended fabrics that are lighter, more breathable, ripstop, and in some cases flame-retardant. The combat boot has many varieties and come in hot-weather and cold-weather versions ranging from relatively stiff/heavy to flexible/lightweight construction. In addition to the uniform, combat personnel must wear body armor (tactical helmet, flak vest), eye protection, and duty gloves. Depending on the mission, most warfighters must also carry weapons and essential gear (radios, batteries, food/water, first aid kits, etc.). The metabolic workload of carrying the body armor and gear (often weighing 60–100 lbs.) is momentous, and these items also constitute a significant barrier to heat dissipation via sweat evaporation, convection, and radiation. However, of all gear and uniform items, perhaps the greatest heat stress comes with the Joint Service Lightweight Integrated Suit Technology (JS-LIST) . This is a full-body suit with gas mask, which provides a complete, impermeable barrier to chemical, biological, radiological, and nuclear (CBRN) warfare agents . It is used by all military services, and anyone who has performed any type of physical activity while wearing it (or similar gear) understands the extreme heat stress it brings. Sweat evaporation, the primary means of human thermoregulation, is basically impossible and completely ineffective. Heat dumping through convection, radiation, and evaporation with respiration is also severely impaired.
Use of dietary supplements is rampant in the military, with approximately 70% of servicemembers using some form of dietary supplement regularly versus 50% of civilians. Most of these supplements are low-risk, such as protein, amino acids, and multivitamins, but at least 10% of military personnel (higher in certain subgroups) use high-risk supplements such as pre-workout stimulants or anabolic agents. There is minimal regulation or oversight of the dietary supplements industry, and product safety does not have to be proven before marketing and sale to consumers, raising serious concerns about potential harms. In response to serious adverse events (including multiple deaths) among servicemembers, the DoD has led the way by banning the sale of supplements containing certain ingredients (e.g., ephedra, ephedrine alkaloids, 1,3-dimethylamylamine, or DMAA) on military bases; these substances were later banned by the US Food and Drug Administration (FDA) . Nonetheless, supplement manufacturers continue to sell products containing prohibited or dangerous substances through dishonest labeling and other means. Serious adverse events continue to occur among servicemembers, including heat illness, rhabdomyolysis, and liver/kidney injury, sometimes leading to hospitalization or even death. Efforts to educate warfighters, military leaders, and lawmakers must continue.
Intrinsic Versus Extrinsic Motivation
The protective effects of fatigue may at times be overridden by strong motivation to train and excel, which may be intrinsic or extrinsic. Classically, instructors in basic training (commonly known by the Army term “Drill Sergeant”) and officer training are passionate, vocal, and strict, providing a strong extrinsic motivating force. Conversely, many trainees have more powerful intrinsic motivation. The classic example of this is special operations training, where many trainees have an intense desire to successfully complete training and attain status as an elite special operator (e.g., SEAL, Green Beret, Pararescueman), despite very high attrition rates due to high-intensity training.
Military training and operations may at times involve extended hours, very early mornings, and limited opportunity to sleep. Some special operations training involves “stress inoculation” with 24-hour training exercises or longer, mimicking combat operations that may involve round-the-clock tempo. In other instances, military personnel may suffer from insomnia related to anxiety, depression, and/or excessive use of caffeine and other stimulants. When physical exertion occurs in the setting of sleep deprivation, the risk of heat illness is increased.
Overall, most military units are very cognizant of the risks of dehydration and offer ample opportunity to hydrate. Dehydration remains a challenge, however, not so much due to availability of water but sometimes palatability of water (in some areas, the only water source is a large mobile water tank/trailer or “water buffalo,” often left out in the sun causing the water to heat up).
For various reasons, military training and operations often allow little to no time to acclimatize. Deployment is a good example, with warfighters leaving home station in potentially cool, temperate climates to arrive in a hot, arid theater of combat.
Solo Training/Remote Operational Missions
Military survival training, known as SERE (Survival, Evasion, Resistance, and Escape), includes many components of physically arduous and prolonged exertion, and some of this is done as a solo wilderness survival exercise. Solo training presents the threat that if one begins to succumb to heat stress, altered mental status may not be detected until it is too late, with none immediately present to witness and respond. At least one such case resulted in death due to probable exertional heat stroke in recent years. Remote operational missions present a similar difficulty, except that although heat illness may be identified, definitive cooling capability may be inaccessible. Furthermore, warfighters who become isolated from their unit in hostile territory may be required to survive and evade capture while navigating back to safer zones or to an optimal location for pickup by rescue teams. This may involve significant heat exposure and dehydration. If the isolated warfighter is solo, the risk of heat illness would be significantly higher.
Military training often occurs in large group settings, with personnel frequently housed in large barracks with shared bathroom facilities. These closely shared living and lodging situations increase transmission of respiratory and gastrointestinal viruses, which often results in fever, fatigue, and decreased ability to compensate for heat stress.
Paramount in heat illness prevention, leadership factors often distinguish units that have few heat casualties from those that have many. The awareness and knowledge level of leadership with respect to heat stress informs their risk tolerance and training paradigms. Sensible policy and well-trained personnel provide a very effective safety net against many threats, including heat illness.