Keeping Cool to Avoid Heat Injury

Keeping Cool to Avoid Heat Injury

Keeping Cool to Avoid Heat Injury

Endurance & Ultra Marathon Blog Post

Author: Dr. Aaron Reilly, DO | Medical Director


Heat injury is a constant looming threat in the desert environment of the Southwest. The best way to combat heat injury is to be aware of how your body responds to heat, and to prepare well in advance.


The equation is simple: heat loss has to equal heat generation. General metabolic processes in the body generate heat, however with intense physical activity heat production can be up to 20x greater. Our bodies lose heat by two primary mechanisms: evaporation (heat liberation by liquid changing into a vapor), and radiation (heat transfer from a warm object into cooler air).


Evaporation occurs by sweating, and radiation occurs by shunting blood from the body’s core out to the skin. Under normal conditions, these mechanisms work quite well, however there are several environmental and individual factors that can reduce heat loss and lead to heat accumulation.


The environmental factors that affect heat loss are related to humidity and temperature. As humidity reaches 100%, sweat evaporation is impaired as the air has “no room” to accept vaporized sweat molecules. In regards to radiation, as the ambient temperature gets closer to body temperature (~35°C/95°F), the heat transfer gradient equalizes. Therefore, when the environmental temperatures increases above body temperature, the heat gradient reverses and favors heat transfer to the body.


There are a multitude of individual factors that lead to heat accumulation. Skin conditions that impair the body’s ability to sweat increase the risk of heat injury. In addition, anything that decreases the body’s ability to shunt blood to the skin, such as poor overall fitness, certain medications, heart conditions, and dehydration, can impair heat loss. Even substances such as performance enhancers, caffeine, and psychiatric medications can increase metabolic heat production.

Ultimately, the biggest individual risk factor for heat injury is a previous episode.

So what is the best way to prevent heat injury? Prepare well in advance. It is well established that heat acclimation training helps both reduce heat injury and increase performance in a hot environment.


A general program is to perform moderate exercise in an environment that mimics the race locale, 90 minutes per day for 14 days. This has been shown to increase both sweating and shunting efficiency, and also develops heat shock proteins, which protects your body against heat injury.


Additional practices to reduce the risk of heat injury during exercise include having a safe hydration plan (see blog post on Staying Hydrated, the safe way), regulating exercise intensity, and periodic rest in the shade as needed. Clothing that protects from solar radiation or using damp cloths can also help keep you cool.


In summary, prepare in advance for exercise in the heat by performing heat training. Stay adequately hydrated, and take rests as needed to allow the body to cool. Make sure to document any previous history of heat injury, as well as all of your medications on your medical form so that we can effectively counsel you in regards to your risk.


Have thoughts? Leave a comment below or feel free to email with any questions.

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