This issue of the MSMR provides an annual update on adverse health consequences most often associated with training or operations in high heat environments. Military training and operation environments create a constellation of circumstances that make service members highly susceptible to heat illnesses and their associated morbidities of exertional hyponatremia and exertional rhabdomyolysis. Leaders and medical staff must be attuned to the inherent health risks for operations in a high heat environment, especially for service members who are deconditioned or pushed to the limits of their physical endurance.

The mantra “train as you fight” requires service members to be frequently exposed to harsh environmental conditions. During initial recruit training, large amounts of time are spent outdoors, often in high heat; training installations are generally located in the southern U.S. for perennial use. The environmental stresses of heat and humidity at these installations that are experienced by individuals encumbered with heavy gear, unconditioned for the duration and intensity of the physical activity required during training, combine to create the perfect conditions for heat illness.

The first topic of this MSMR issue, heat illnesses, focuses on heat exhaustion and heat stroke. These conditions present 2 different occasions when the body can no longer rid itself of heat, either generated through activity or absorbed from the environment. Internal body temperature begins to rise during the earlier stage, heat exhaustion, 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 organs begin to fail from heat overload. Heat stroke is distinguished by 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 required.

This issue’s second and third topics, exertional rhabdomyolysis and exertional hyponatremia, are both commonly associated with heat illness, but represent organ damage (rhabdomyolysis) or unintended side effects from over-aggressive rehydration (hyponatremia). Both of these conditions can result in rapid deterioration or death if 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 and implementation of appropriate heat countermeasures. Leaders, as part of their risk assessments, must balance mitigation efforts against the requirements of their operations or trainings. The most effective countermeasures against heat illness include restricting activity to early morning or evening 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 a local heat environment.