This issue of the MSMR provides an annual update on adverse health consequences most often associated with training or operations in environments with high heat and frequent high humidity. Military training and operational environments create a constellation of circumstances that make service members highly susceptible to heat illnesses and the associated morbidities of exertional hyponatremia and exertional rhabdomyolysis.

The mantra “train as you fight” requires that service members be frequently exposed to harsh environmental conditions. During initial recruit training, large amounts of time are spent outdoors, often in high heat—many military training installations are in the Southern U.S., for perennial use. The environmental stresses of heat and humidity introduced at these installations among individuals, encumbered with heavy gear, who may be unconditioned for the duration and intensity of the physical activity required during training, cohere to create the perfect conditions for heat illness. 

The first topic of this issue, heat illnesses, focuses on heat exhaustion and heat stroke. These conditions represent two different occasions when the body can no longer rid itself of excessive heat generated either from activity or absorbed through the environment. Internal body temperature begins to rise during heat exhaustion, the earlier stage, when affected individuals are generally still aware of their surroundings and can assist in their own care. Heat stroke represents a much more dangerous condition, in which the major organ system begin to fail from heat overload. Heat stroke is clinically characterized by an alteration of consciousness, typically stupor, delirium, lethargy, or unconsciousness. Mortality is a serious risk with heat stroke, and immediate action to cool the body is urgently required.

The topic of this issue’s second and third articles, exertional rhabdomyolysis and exertional hyponatremia, are both commonly associated with heat illness, but represent organ damage (rhabdomyolysis) or unintended side effects from overaggressive rehydration (hyponatremia). Both conditions can result in rapid physical and mental deterioration. Death may result if symptoms are not promptly recognized and treated. While both rhabdomyolysis and hyponatremia have many non-heat-related causes, this issue deals exclusively with cases associated with high levels of exertion. 

These consequences can generally be mitigated, if not fully prevented, by careful environmental risk assessment, implementation of appropriate heat countermeasures, and decisive medical support. Increased awareness by leaders and medical staff of the importance of being alert to the health risks inherent to operations in a high heat environment, especially for service members who are deconditioned or compelled to the limits of their physical endurance, is a critical part of effective prevention approaches. Leaders, as part of their risk assessments, must balance mitigation efforts against the requirements of their operations or training activities.

The most effective countermeasures against heat illness include restricting activity to early morning or evening hours when environmental heat is lower; adherence to work and rest cycles based upon current heat conditions; removal or modification of gear to facilitate heat loss; maintenance of proper hydration levels; maximized physical fitness; and gradual acclimatization to the local heat environment.