MAJ. DAVID DEGROOT, PH.D.
U.S. Army Public Health Center (Provisional)
Aberdeen Proving Ground, Maryland
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 Branch indicates that in 2014, more than 200 Soldiers suffered from heat stroke and another 1,200 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 heat stroke. 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. Heat exhaustion is therefore primarily a cardiovascular event caused by exercise and often made worse by dehydration.
Heat injury and heat stroke 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 heat stroke. 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. A recent analysis by the U.S. Army Public Health Center (Provisional) 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 of 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’re 1 percent dehydrated, which is of little concern. However, if you lost 4.5 pounds, you’re 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.
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. Other beverages, including milk, coffee, tea and soft drinks, will also help a Soldier rehydrate; but due to the sugar content of some drinks, they should not be relied upon exclusively. In addition, they should not be put in canteens or hydration systems as they become harder to clean and foster bacteria growth. 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 heat stroke. In reality, a Soldier can have a high (>104 F) core temperature and not be a heat stroke casualty. While high body temperature is suggestive of heat stroke, the presence of central nervous system dysfunction — not core temperature — distinguishes heat stroke 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.
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.
FYI
A wealth of additional information and training materials are available on the U.S. Army Public Health Center (Provisional) Heat Illness Prevention webpage at http://phc.amedd.army.mil/topics/discond/hipss/Pages/HeatInjuryPrevention.aspx as well as in TB MED 507, Heat Stress Control and Heat Casualty Management.