Training in the heat substantially increases risk of exertional heat illness, a risk accepted by military units due to the necessity of training in environments similar to those where warfighters may be deployed.¹ Previous work has shown that foot marches (i.e., ruck marches) and timed runs are the activities with highest heat-related illness prevalence.² Severe heat-related illnesses, such as exertional heat stroke, can lead to long recovery times and end organ damage, and constitute a significant threat to force lethality and detriments to deployability.^1,3 While research and guidelines for appropriate conduct of service member activities in hot environments are updated regularly, over 2,500 warfighters continue to be affected by heat-related illnesses annually.⁴

Subject matter experts at the U.S. Army Research Institute of Environmental Medicine conduct research to both prevent heat-related illness and enhance physical performance in the heat, with a particular emphasis on the needs of the warfighter. USARIEM research has led to key developments for enhancing performance along with prevention and treatment strategies for heat-related illnesses.^5-8 The cumulative knowledge from this work leads to guidelines published (among others) in Technical Bulletin, Medical: Heat Stress Control and Casualty Management, which provides resources—including fluid prescription guidelines and work-to-rest ratios—for all flag conditions, as well as details on risk factors to prevent the development of heat illness.⁹

This editorial summarizes the results and conclusions of recent biomedical research on performance and injury risk in the heat, to provide practical considerations for warfighters training in the heat, both for individual warfighters as well as leaders who plan training activities. This editorial describes recent evidence about individualized factors that may influence risk of developing a heat-related illness and strategies to prepare for training in the heat. 

Physiology of Exercise Heat Stress and Illness

Thermoregulation during physical activity in the heat has been extensively reviewed in the literature.^9-12 Environmental heat stress, particularly with exercise, increases risk of heat-related illness.^3,13-15 During exercise heat stress, working skeletal muscle produces heat, with the heart rate elevating to increase cardiac output to accommodate the needs of the working skeletal muscle, as well as the skin, for heat dissipation. The primary heat dissipation mechanisms during exercise are evaporation—of sweat from the surface area of the skin—and convection—from air flow over the skin surface in combination with increased skin blood flow.^16-18

Exertional heat illnesses constitute a spectrum, ranging in severity from heat exhaustion, defined as an inability to continue exercise in the heat, to heat injury, which is defined as heat exhaustion with evidence of end organ damage, and EHS, which is elevated body temperature with altered mental status.^3,19 Altered mental status associated with EHS can also range from change in walking or running gait, to slurred speech, to loss of consciousness.²⁰ Organ damage is also prevalent in EHS cases, with recent evidence suggesting a possible sex difference in level of end organ damage despite apparent similar EHS severity.²¹ Importantly, classic heat stroke differs from EHS: Classic heat stroke more often affects older individuals during heat waves, associated with hot, dry skin and no physical activity, whereas EHS casualties occur more often in younger people sweating due to physical activity.¹⁹

Preparation Strategies and Considerations

Successful prevention of heat-related illness requires preparation well in advance of heat season; these preparations can include heat acclimatization and increased fitness levels for at least one month prior. Heat acclimatization is the most effective means for not only heat stress preparation and decreasing risk of heat-related illnesses, but enhancing performance as well. Heat acclimatization has been extensively investigated, with various protocols demonstrating enhanced performance and thermoregulatory function during heat stress.²² Heat acclimatization is the process of repeatedly exposing the body to heat stress in a controlled manner, with incremental and gradual increases in exercise intensity to allow beneficial adaptations.^23-25 The primary adaptations of heat acclimatization include decreased resting and exercise body temperatures, lower exercise heart rate, increased sweating rate, and increased physical performance capacity.²³ The process can occur over as little as 4-8 days with longer durations prescribed (e.g., 10-21 days) to ensure greatest possible adaptation.^23-25

While heat acclimatization is the most effective preparation strategy, increased fitness from training in cool and temperate environments can also enhance performance during training in the heat, and may reduce risk of heat-related illness.²⁶ Individuals with higher fitness statuses are thought to have preliminary heat adaptations (i.e., partial acclimatization) that are likely due to increases in body temperature during extended exercise, which allow milder adaptations.¹⁰ Endurance training (e.g., running) has been observed to provide the most beneficial adaptations to heat stress.¹⁰ Once training and acclimatization have been achieved, maintenance (i.e., continued exercise or heat exposure) is important to prevent diminishment of those adaptations.²⁷

A key consideration for heat training preparation is awareness of biological and physiological factors that increase risk of heat illness.²⁸ Recent USARIEM research evaluating risk factors for heat illness among groups and individuals has found that higher body mass index, which is associated with a lower body surface area to mass ratio (BSA:mass), increases individual risk for EHS.^7,8 Every 1 unit increase in BMI led to a 3% increase in relative EHS risk.⁷ Additional work from the Uniformed Services University of the Health Sciences confirmed this effect of BMI.

Research at USUHS also observed increased risk of heat-related illnesses in conjunction with both upper and lower respiratory infections.²⁹ Lower respiratory infections (e.g., influenza, bronchitis) have strong effects on EHS risk. Adequate recovery from respiratory infections prior to participation in heat training is particularly important for reducing risk of developing an exertional heat-related illness.²⁹

Decades of research into prevention of heat-related illnesses and EHS have identified many biological, physiological, behavioral, and environmental risk factors, summarized in the table. This evidence can inform decision-making for unit leaders as well as individuals, for appropriate preparation, adjustment, modification, and completion of training exercises and minimization of heat training casualties.

Mitigation Measures

The ability to cool and diffuse heat before or during training activities is paramount for decreasing risk. The Arm Immersion Cooling System is a method used to cool service members prior to, or during, activities of greatest risk.⁶ AICS is most effective if the protocol is properly adhered to, specifically resting the arms (wrists to elbows) in cool ice water for 3-5 minutes.⁶

In addition to cooling before and during activity, readily available treatment remedies are vital for ensuring prompt recognition, arrest, and recovery of heat illness casualties. Cooling modalities range in both practicality and effectiveness. Ice water immersion is the ‘gold standard’ for cooling a heat casualty as quickly as possible³⁰ but requires a significant amount of ice and water, which can only be used for one casualty. Iced sheets is a method with greater practicality but slightly less cooling efficacy. While cooling rates for iced sheets are lower, if used properly—rotated at least every three minutes—they are an effective means of cooling heat casualties.⁵ If iced sheets are not rotated frequently enough, they can trap heat and exacerbate elevated body temperatures.⁵

Injury Identification and Recovery

Prompt identification of a heat casualty can drastically reduce long-term complications, as the amount of time hyperthermic appears to be indicative of the level of damage from the heat illness.³¹ Knowing signs and symptoms including altered mental status, including confusion, change in walking or running gait, slurred speech, and loss of consciousness, are primary examples of AMS.^32,33

Early identification and initiation of cooling can enhance recovery and return-to-duty for heat-related illnesses.³⁴ RTD and recovery from a heat-related illness vary depending on illness severity.³⁴ Many heat exhaustion cases can return to duty and training the following day with appropriate guidance and hydration, while EHS cases can require many weeks.^35,36

EHS cases can range in severity as well, largely dependent on prompt detection, cooling onset, and pre-hospital care.^34,35 Organ damage is common in EHS, with some individuals necessitating organ transplants (usually liver or kidney). Army Regulation 40-501 governs RTD for soldiers and specifies minimum EHS recovery of 10 weeks.³⁷

It is commonly suggested that becoming a heat illness casualty, particularly with EHS, puts an individual at an increased risk for subsequent heat illness, but there is limited research to support this assertion. Full and optimal recovery leads to complete RTD, likely without increased risk for future heat illness.³⁵

Enhanced Prevention and Mitigation Strategies

The prevalence and impacts of heat-related illnesses, on individual warfighter lethality and medical readiness, make continued research efforts for enhanced performance, prevention, recovery, and RTD for heat-related illnesses of continuing importance. Current mitigation efforts at USARIEM include identification of biomarkers of recovery from heat-related illnesses for enhancing RTD, development of practical heat acclimatization strategies that do not require a laboratory nor hot environment, evaluation of inter-individual variability in heat stress as well as physiological and biomarker responses, and enhanced prevention efforts for both individuals and units.

With most military training installations located in the southeastern U.S., training in the heat is a necessity for the U.S. military. It is critical to ensure appropriate preparation and education of risk factors and signs of heat illness and injury for both individual service members and leadership, for their recognition and mitigation of these risks during training events. While heat-related illnesses, including EHS, are not 100% preventable, mitigation of risk factors and adequate preparation can reduce as many as possible, to optimize the health and lethality of our warfighters—both during training and in battlefield environments.

Author Affiliation

Thermal and Mountain Medicine Division, U.S. Army Research Institute of Environmental Medicine

Disclaimer

The opinions or assertions herein are the private views of the authors and are not to be construed as official nor as reflecting the views of the U.S. Army, Department of Defense, Department of Energy, or Oak Ridge Associated Universities and Oak Ridge Institute for Science and Education. Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement nor approval of the products or services of these organizations.

References

1. Périard JD, DeGroot D, Jay O. Exertional heat stroke in sport and the military: epidemiology and mitigation. Exp Physiol. 2022;107(10):1111-1121. doi:10.1113/ep090686  
2. DeGroot D, Henderson K, O’Connor F. Exertional heat illness at Fort Benning, GA: unique insights from the Army Heat Center. MSMR. 2022;29(4):2-7. Accessed Jul. 9, 2025. https://www.health.mil/news/articles/2022/04/01/exertional-heat-msmr  
3. Roberts WO, Armstrong LE, Sawka MN, et al. ACSM expert consensus statement on exertional heat illness: recognition, management, and return to activity. Curr Sports Med Rep. 2021;20(9):470-484. doi:10.1249/jsr.0000000000000878  
4. Maule AL, Kotas KS, Scatliffe-Carrion KD, Ambrose JF. Update: heat exhaustion and heat stroke among active component members of the U.S. Armed Forces, 2020-2024. MSMR. 2025;32(6):4-10. Accessed Jul. 9, 2025. https://www.health.mil/news/articles/2025/06/01/msmr-heat-illness-2025  
5. Caldwell AR, Saillant MM, Pitsas D, et al. The effectiveness of a standardized ice-sheet cooling method following exertional hyperthermia. Mil Med. 2022;187(9-10):e1017-e1023. doi:10.1093/milmed/usac047  
6. DeGroot DW, Kenefick RW, Sawka MN. Impact of arm immersion cooling during ranger training on exertional heat illness and treatment costs. Mil Med. 2015;180(11):1178-1183. doi:10.7205/milmedd-14-00727  
7. Giersch GE, Taylor KM, Caldwell AR, Charkoudian N. Body mass index, but not sex, influences exertional heat stroke risk in young healthy men and women. Am J Physiol Reg Integr Comp Physiol. 2023;324(1):r15-r19. doi:10.1152/ajpregu.00168.2022  
8. Taylor KM, Giersch GE, Caldwell AR, Epstein Y, Charkoudian N. Relation of body surface area-to-mass ratio to risk of exertional heat stroke in healthy men and women. J Appl Physiol (1985). 2024;136(3):549-554. doi:10.1152/japplphysiol.00597.2023  
9. Headquarters, Department of the Army. Technical Bulletin, Medical: Heat Stress Control and Heat Casualty Management. 2022. U.S. Dept. of Defense. Accessed Jul. 9, 2025. https://www.govinfo.gov/content/pkg/GOVPUB-D101-PURL-gpo216616/pdf/GOVPUB-D101-PURL-gpo216616.pdf  
10. Périard JD, Caillaud C, Thompson MW. The role of aerobic fitness and exercise intensity on endurance performance in uncompensable heat stress conditions. Eur J Appl Physiol. 2012;112(6):1989-1999. doi:10.1007/s00421-011-2165-z  
11. Périard JD, Eijsvogels TMH, Daanen HAM. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiol Rev. 2021;101(4):1873-1979. doi:10.1152/physrev.00038.2020  
12. Wendt D, van Loon LJC, van Marken Lichtenbelt WD. Thermoregulation during exercise in the heat. Sports Med. 2007;37(8):669-682. doi:10.2165/00007256-200737080-00002  
13. Gardner JW, Kark JA, Karnei K, et al. Risk factors predicting exertional heat illness in male Marine Corps recruits. Med Sci Sports Exerc. 1996;28(8):939-944. doi:10.1097/00005768-199608000-00001  
14. Howe AS, Boden BP. Heat-related illness in athletes. Am J Sports Med. 2007;35(8):1384-1395. doi:10.1177/0363546507305013  
15. Kenny GP, Wilson TE, Flouris AD, Fujii N. Heat exhaustion. Handb Clin Neurol. 2018;157:505-529. doi:10.1016/b978-0-444-64074-1.00031-8  
16. Rowell LB. Human cardiovascular adjustments to exercise and thermal stress. Review. Physiol Rev. 1974;54(1):75-159. doi:10.1152/physrev.1974.54.1.75  
17. Rowell LB, Brengelmann GL, Blackmon JR, Twiss RD, Kusumi F. Splanchnic blood flow and metabolism in heat-stressed man. J Appl Physiol. 1968;24(4):475-484. doi:10.1152/jappl.1968.24.4.475  
18. Rowell LB, Brengelmann GL, Murray JA. Cardiovascular responses to sustained high skin temperature in resting man. J Appl Physiol. 1969;27(5):673-680. doi:10.1152/jappl.1969.27.5.673  
19. Bouchama A, Abuyassin B, Lehe C, et al. Classic and exertional heatstroke. Nat Rev Dis Primers. 2022;8(1). doi:10.1038/s41572-021-00334-6  
20. DeGroot DW, O’Connor FG, Roberts WO. Exertional heat stroke: an evidence based approach to clinical assessment and management. Exp Physiol. 2022;107(10):1172-1183. doi:10.1113/ep090488  
21. Goodwin KC, Giersch GE, Murray TA, DeGroot DW, Charkoudian N. Sex differences in biomarkers of end-organ damage following exertional heat stroke in humans. J Appl Physiol (1985). 2024;137(5):1434-1445. doi:10.1152/japplphysiol.00597.2023  
22. McDonald P, Brown HA, Topham TH, et al. Influence of exercise heat acclimation protocol characteristics on adaptation kinetics: a quantitative review with Bayesian meta-regressions. Compr Physiol. 2025;15(3):e70017. doi:10.1002/cph4.70017  
23. Périard JD, Racinais S, Sawka MN. Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scand J Med Sci Sports. 2015;25(suppl 1):20-38. doi:10.1111/sms.12408  
24. Périard JD, Travers GJS, Racinais S, Sawka MN. Cardiovascular adaptations supporting human exercise-heat acclimation. Auton Neurosci. 2016;196:52-62. doi:10.1016/j.autneu.2016.02.002  
25. Racinais S, Sawka M, Daanen H, Périard JD. Heat acclimation. In: Périard JD, Racinais S, eds. Heat Stress in Sport and Exercise: Thermophysiology of Health and Performance. Springer;2019:159-178.  
26. Bedno SA, Urban N, Boivin MR, Cowan DN. Fitness, obesity and risk of heat illness among army trainees. Occup Med (London). 2014;64(6):461-467. doi:10.1093/occmed/kqu062  
27. Daanen HAM, Racinais S, Périard JD. Heat acclimation decay and re-induction: a systematic review and meta-analysis. Sports Med. 2018;48(2):409-430. doi:10.1007/s40279-017-0808-x  
28. Alele F, Malau-Aduli B, Malau-Aduli A, Crowe M. Systematic review of gender differences in the epidemiology and risk factors of exertional heat illness and heat tolerance in the armed forces. BMJ Open. 2020;10(4):e031825. doi:10.1136/bmjopen-2019-031825  
29. Kazman JB, Nelson DA, Ahmed AE, et al. Risk for exertional heat illness among US army enlistees: climate indexes, intrinsic factors and their interactions. Br J Sports Med. 2025;59(4):231-240. doi:10.1136/bjsports-2024-108441  
30. Casa DJ, McDermott BP, Lee EC, et al. Cold water immersion: the gold standard for exertional heatstroke treatment. Exerc Sport Sci Rev. 2007;35(3):141-149. doi:10.1097/jes.0b013e3180a02bec  
31. Adams WM, Hosokawa Y, Casa DJ. The timing of exertional heat stroke survival starts prior to collapse. Curr Sports Med Rep. 2015;14(4):273-274. doi:10.1249/jsr.0000000000000166  
32. Pryor RR, Roth RN, Suyama J, Hostler D. Exertional heat illness: emerging concepts and advances in prehospital care. Prehosp Disaster Med. 2015;30(3):297-305. doi:10.1017/s1049023x15004628  
33. Shapiro Y, Seidman DS. Field and clinical observations of exertional heat stroke patients. Med Sci Sports Exerc. 1990;22(1):6-14.  
34. Koo CJ, Hintz C, Butler CR. Return to duty following exertional heat stroke: a review. Mil Med. 2024;189(5-6):e1312-e1317. doi:10.1093/milmed/usad388  
35. O’Connor FG, Casa DJ, Bergeron MF, et al. American College of Sports Medicine roundtable on exertional heat stroke–return to duty/return to play: conference proceedings. Curr Sports Med Rep. 2010;9(5):314-321. doi:10.1249/jsr.0b013e3181f1d183  
36. O’Connor FG, Williams AD, Blivin S, et al. Guidelines for return to duty (play) after heat illness: a military perspective. J Sport Rehabil. 2007;16(3):227-237. doi:10.1123/jsr.16.3.227  
37. Headquarters, Department of the Army. Army Regulation 40–501, Medical Services: Standards of Medical Fitness. 2019. U.S. Dept. of Defense. Accessed Jul. 9, 2025. https://dacowits.defense.gov/portals/48/documents/general%20documents/rfi%20docs/june2023/usa%20rfi%202a_ar%2040-501-1.pdf?ver=pgbcqptj3cj0i0dvinb-mq%3d%3dhttps://dacowits.defense.gov/portals/48/documents/general%20documents/rfi%20docs/june2023/usa%20rfi%202a_ar%2040-501-1.pdf?ver=pgbcqptj3cj0i0dvinb-mq%3d%3d  
38. Carter R, Cheuvront SN, Williams JO, et al. Epidemiology of hospitalizations and deaths from heat illness in soldiers. Med Sci Sports Exerc. 2005;37(8):1338-1344. doi:10.1249/01.mss.0000174895.19639.ed  
39. Wallace RF, Kriebel D, Punnett L, et al. The effects of continuous hot weather training on risk of exertional heat illness. Med Sci Sports Exerc. 2005;37(1):84-90. doi:10.1249/01.mss.0000150018.90213.aa  
40. Roberts KE. Mechanism of dehydration following alcohol ingestion. Arch Intern Med. 1963;112(2):154-157. doi:10.1001/archinte.1963.03860020052002  
41. Sawka MN, Montain SJ, Latzka WA. Hydration effects on thermoregulation and performance in the heat. Comp Biochem Physiol A Mol Integr Physiol. 2001;128(4):679-690. doi:10.1016/s1095-6433(01)00274-4  
42. Montain SJ, Latzka WA, Sawka MN. Control of thermoregulatory sweating is altered by hydration level and exercise intensity. J Appl Physiol (1985). 1995;79(5):1434-1439. doi:10.1152/jappl.1995.79.5.1434  
43. Sawka MN, Latzka WA, Matott RP, Montain SJ. Hydration effects on temperature regulation. Int J Sports Med. 1998;19(suppl 2):s108-s110. doi:10.1055/s-2007-971971  
44. Armstrong LE, Casa DJ, Millard-Stafford M, et al. American College of Sports Medicine position stand. exertional heat illness during training and competition. Med Sci Sports Exerc. 2007;39(3):556-572. doi:10.1249/mss.0b013e31802fa199  
45. DeGroot DW, Ruby B, Koo A, O’Connor FG. Far from home: heat-illness prevention and treatment in austere environments. Wilderness Environ Med. 2025:10806032251332283. doi:10.1177/10806032251332283  
46. Nelson DA, Deuster PA, O’Connor FG, Kurina LM. Timing and predictors of mild and severe heat illness among new military enlistees. Med Sci Sports Exerc. 2018;50(8):1603-1612. doi:10.1249/mss.0000000000001623  
47. Sawka MN, Gonzalez RR, Pandolf KB. Effects of sleep deprivation on thermoregulation during exercise. Am J Physiol. 1984;246(1 pt2):r72-r77. doi:10.1152/ajpregu.1984.246.1.r72  
48. Lalli K, Charkoudian N, Moreh Y, DeGroot DW. The role of motivation to excel in the etiology of exertional heat stroke. J Spec Oper Med. 2024;24(2):28-33. doi:10.55460/4tiv-hqlo  
49. Havenith G, Fiala D. Thermal indices and thermophysiological modeling for heat stress. Compr Physiol. 2015;6(1):255-302. doi:10.1002/cphy.c140051  
50. Hosokawa Y, Adams WM, Belval LN, et al. Exertional heat illness incidence and on-site medical team preparedness in warm weather. Int J Biometeorol. 2018;62(7):1147-1153. doi:10.1007/s00484-018-1517-3  
51. Wallace RF, Kriebel D, Punnett L, et al. Risk factors for recruit exertional heat illness by gender and training period. Aviat Space Environ Med. 2004;77(4):415-421.