GABRIELLE E.W. GIERSCH, Ph.D.
Thermal and Mountain Medicine Division
U.S. Army Research Institute of Environmental Medicine
Natick, Massachusetts

Military training in the heat impairs physical performance and can lead to the development of heat-related illnesses, which impact over 2,500 uniformed personnel each year. Heat illnesses range in severity from heat exhaustion (least severe) to exertional heat stroke (EHS) (most severe). Heat exhaustion is an inability to continue activity/training in the heat, while EHS is elevated body temperature with altered mental status and is potentially fatal.

The U.S. Army Research Institute of Environmental Medicine conducts research to prevent heat-related illness while enhancing physical performance in warm-weather conditions. Specifically, Technical Bulletin (Medical) 507: Heat Stress Control and Casualty Management provides resources — including fluid prescription guidelines and work-to-rest ratios — for all flag conditions. Additionally, we have evaluated risk factors for heat illness across groups and individuals (Giersch et al., 2022; Taylor et al., 2024).

With the heat season already under way in 2025, this article provides best practices at the individual and unit levels to prepare for training in the heat. Specifically, we describe recent evidence about individualized factors that may influence the risk of developing heat-related illness and strategies for preparing for training in the heat.

Recent Evidence: Individualized Risk Factors

Body mass index (BMI). A higher BMI leads to significantly increased risk for developing EHS. For every one unit increase in BMI, there was a 3 percent increase in relative risk of becoming an EHS casualty (Giersch et al., 2023).

Respiratory infection. There is an increased risk of all heat-related illnesses in conjunction with both upper and lower respiratory infections. Additionally, lower respiratory infections (e.g., the flu, bronchitis, etc.) had a very strong impact on the risk of EHS, making appropriate recovery from respiratory infections prior to participation in heat training particularly important (Kazman et al., 2025).

Advanced Planning (one-plus month(s))

Heat acclimatize. Allowing your body to systematically adapt to heat stress is the best tool for preventing heat-related illness and enhancing performance during training (Periard et al., 2015). Heat acclimation can take place over eight to 21 days. The process includes gradual exposures to training in the heat with potentially slow and steady increases in exercise intensity. Primary adaptations that come from heat acclimation are enhanced performance, lower body temperature, lower heart rate and increased sweating (Pandolf et al., 1988; Periard et al., 2016).

Fitness. Increasing fitness status by training in cool and temperate environments can drastically enhance performance during training in the heat and may reduce risk of heat-related illness. Individuals with a higher fitness status are often thought to have preliminary heat adaptations (i.e., be partially acclimatized). Increasing and maintaining fitness is a great long-term tool to best prepare for the heat season.

Immediate Planning (within 24-48 hours)

Individuals:

Remain hydrated. Starting exercise or training while hydrated and maintaining hydration during activity can help reduce the risk for developing a heat-related illness, with hydration increasing body temperature during training in the heat and leading to increased rate of body temperature increases (Giersch et al., 2021; Sawka et al., 2001). Importantly, while remaining hydrated helps to reduce the risk for developing heat illnesses, it will not make individuals immune to heat illness development and is only one of the risk factors to consider for risk mitigation. Additionally, avoiding alcohol in the 48-72 hours prior to training can help maintain appropriate hydration status and reduce alcohol-induced fatigue or symptomology during training.

Sleep. Getting adequate amounts of sleep can ensure you’re best prepared for the training ahead and help to reduce the risk of developing heat-related illnesses. Sleep deprivation is often characterized as a risk factor for heat-related illness.

Avoid energy drinks and supplements. Energy drinks with high amounts of caffeine and supplements — such as pre-workout — are not regulated by the Food and Drug Administration and may contain ingredients that are harmful and can exacerbate elevated body temperature during exercise and heat stress. They also may influence the risk of developing a heat-related illness.

Understand your circumstances. Individuals who are particularly motivated to push themselves harder can drive themselves to the point of developing a heat-related illness. Additionally, if you haven’t slept well or are feeling under the weather (e.g., are fighting a respiratory infection), be aware that this may increase your risk, so take it easier when you can.

Pre-cool and shade. If possible, staying in the shade during activity or pre-cooling immediately before exercise can reduce elevated body temperatures during exercise. The Army Immersion Cooling System (AICS) can be used as a good tool for this (resting forearms in the water for up to three to five minutes).

Unit Leaders:

Be prepared to cool. Know where the closest opportunity for cooling is located (may be a cooling deck with cold-water immersion or in coolers with iced sheets). If you are using iced sheets, they should be rotated at least every three minutes, if not more frequently. If sheets are not rotated frequently enough, they can trap heat and exacerbate elevated body temperatures (Caldwell et al., 2022).

Know your unit. Be aware of the general BMI, fitness and motivation of the individuals in your units. All those factors can increase the risk of developing a heat-related illness. Be prepared to coach and intervene where necessary.

Know the flag conditions. Wet-bulb globe temperature (WBGT) is a composite temperature that takes humidity, ambient temperature and solar radiation (from the sun) into account. These flag conditions govern work-to-rest ratios and fluid consumption guidelines (Technical Bulletin, Medical Heat Stress Control and Heat Casualty Management, 2022). In higher WBGTs, the risk for heat-related illness increases.

Know the activity-specific risks. Training with a ruck/load while in the Army Combat Uniform can lead to greater heat production and heat storage. Previous work from the U.S. Army Heat Center at Fort Benning, Georgia, has also shown that a majority of heat illnesses occur during ruck marches and timed runs (DeGroot et al., 2022). During these events, the likelihood of individuals developing heat-related illnesses is elevated.

Conclusion

Training in the heat is important to prepare Soldiers for combat operations in hot environments. While we are never going to be able to prevent 100 percent of heat-related illnesses, we can prepare for training in the heat to mitigate risk and prevent as many incidents as possible. Integrating recent research and evidence, as well as preparation by individuals and unit leaders, can contribute toward the goal of fewer heat-related illnesses.

References

• Caldwell, A. R., Saillant, M. M., Pitsas, D., Johnson, A., Bradbury, K. E., & Charkoudian, N. (2022). The effectiveness of a standardized ice-sheet cooling method following exertional hyperthermia. Military Medicine, 187(9-10), e1017-e1023.
• DeGroot, D., Henderson, K., & O’Connor, F. (2022). Exertional heat illness at Fort Benning, GA: unique insights from the Army Heat Center. Msmr, 29(4), 2-7.
• Giersch, G., Garcia, C., Stachenfeld, N., & Charkoudian, N. (2022). Are there sex differences in risk for exertional heat stroke? A translational approach. Experimental Physiology.
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• (2022). Technical Bulletin, Medical: Heat Stress Control and Casualty Management. Army Publications Directorate.