LT. COL. DAVID DEGROOT, PH.D.
Heat Center Director
Martin Army Community Hospital
Fort Moore, Georgia
The very nature of our profession as Soldiers — training outdoors, wearing uniforms and carrying equipment — practically guarantees we will be exposed to heat stress. The latest data from the Armed Forces Health Surveillance Division indicates that in 2022, more than 200 Soldiers suffered from heat stroke and another 1,000 suffered from other less severe forms of heat illness that required medical attention and led to lost duty time. It may be unrealistic to hope for zero heat illnesses, but through proper training, education and preparation, we can minimize the number of Soldiers who suffer from a serious or even fatal heat illness.
The spectrum of heat illnesses includes dehydration, heat cramps, heat exhaustion, heat injury and exertional heat stroke (EHS). Dehydration results when body fluid losses from sweating and urination exceed fluid intake. The cause of muscle cramps that occur during heat exposure is unknown, though electrolyte loss and/or dehydration likely contribute. During exercise in the heat, there is very high demand for blood flow to the exercising muscles and skin for heat dissipation. When this demand exceeds the pumping capacity of the heart, heat exhaustion may occur. Therefore, heat exhaustion is primarily a cardiovascular event caused by exercise and often made worse by dehydration. Heat exhaustion is not associated with any organ damage and return to duty can occur just a few days later.
Heat injury and EHS are the most severe heat illnesses. Heat injury is characterized by organ (liver, kidney) and tissue (muscle) damage resulting from strenuous exercise and heat stress. When profound central nervous system dysfunction also occurs, heat injury has progressed to EHS. Common signs include loss of consciousness, combativeness and/or altered mental status. If not properly treated with aggressive cooling, heat stroke is potentially fatal.
The following items are some facts about working in the heat, as well as some commonly held beliefs that are simply not supported by facts.
Fact: Acclimatization to the heat is extremely important and represents what might be the most important thing Soldiers and leaders can do to prepare. Acclimatization results from moderate exercise in the heat; and while full acclimatization may take up to two weeks of two hours per day of exposure, most of the changes occur within the first five to seven days. Heat acclimatization causes body core temperature to be lower at rest and at a given exercise intensity. Sweating starts sooner and reaches a higher rate, so evaporative heat loss is increased. While acclimatization causes sweat to become more dilute (less salty), the increased sweat rate will increase fluid replacement needs. Unit leaders should plan time for Soldiers to heat acclimatize before engaging in higher-intensity activities.
Fiction: Heat illnesses only occur during the summer months, or the “heat season.” While Soldiers and leaders at all levels are correct to expect increased heat stress during the summer, due to the clothing we wear, loads we carry and intensity at which we work, heat illness risk is present year-round. An analysis by the then-U.S. Army Public Health Center (now the Defense Centers for Public Health-Aberdeen) indicates that about 18 percent of all heat illnesses occur outside the heat season and there was not a single week during the calendar year when there was not a heat illness, including heat stroke. At some locations, 30 percent of all heat illnesses occurred outside the heat season. It does not have to be hot for a Soldier to become a heat casualty.
Fact: Proper fluid replacement is important for preventing heat illness. Dehydration is associated with increased cardiovascular strain, lower sweat rate, lower skin blood flow and reduced exercise performance. When sweat rate and skin blood flow are reduced, heat transfer from the body to the environment is reduced, resulting in an increased core temperature. To estimate how dehydrated you are, step on a scale before and after exercise. If you weigh 150 pounds and lost 1.5 pounds during exercise, you are 1 percent dehydrated, which is of little concern. However, if you lost 4.5 pounds, you are 3 percent dehydrated. When dehydration exceeds 2 percent of body weight, physiological strain and risk of becoming a heat casualty increase.
Fiction: Fluid replacement is the only thing that is important for preventing heat illness. Data from the U.S. Army Research Institute of Environmental Medicine shows that only 17 percent of heat stroke cases were associated with dehydration. The reality is there are many contributing factors, including dehydration, as well as a Soldier’s acclimatization status, physical fitness, medication and/or dietary supplement usage, and if they’ve recently experienced a viral infection (cold or flu). Focusing solely on fluid replacement may cause Soldiers to overlook other equally important risk factors. Every EHS casualty is associated with multiple risk factors. Even though the Soldier is adequately hydrated, other risk factors still matter!
Fact: Drinking water is preferable for rehydration. Sports drinks are effective but often not necessary, as long as Soldiers are also eating their meals, which typically contain enough electrolytes to replace those lost from sweating. Drinking water and fully consuming meals will be sufficient to replace fluid and electrolyte losses.
Fiction: When a Soldier is too hot, he or she has exertional heat stroke. In reality, a Soldier can have a high (>104 °F) core temperature and not be an EHS casualty. While high body temperature is suggestive of EHS, the presence of central nervous system (CNS) dysfunction — not core temperature — distinguishes EHS from less severe forms of heat illness. The Soldier may display confused, combative, irrational or aggressive behavior, or may pass out. These are all strong indicators the Soldier is experiencing heat stroke and requires immediate medical attention and rapid cooling. If a Soldier is displaying signs of CNS dysfunction during or immediately following high-intensity work or exercise in the heat, the presumptive diagnosis is EHS until proven otherwise.
Conclusion
The risk of becoming a heat casualty exists year-round. By maintaining a high degree of physical fitness, proper body weight, acclimatizing to the heat and rehydrating appropriately, we can each do our part to minimize the risk.