Heat Illness

Heat Related Illness In Athletes Exercising In Hot, Humid Environments

Eric E. Coris, MD Head Medical Team Physician, The University of South Florida Department of Athletics Associate Professor, The University of South Florida College of Medicine Director, Division of Primary Care Sports Medicine


In 1980, seventeen hundred people died during a prolonged heat wave in a region under-prepared for heat illness prevention.[1] Dramatically underreported, heat-related pathology contributes to significant morbidity as well as occasional mortality in athletic, elderly, pediatric and disabled populations.

[1-4] Among U.S. high school athletes, heat illness is the third leading cause of death.[2] Significant risk factors for heat illness include dehydration, hot and humid climate, obesity, low physical fitness, lack of acclimatization, previous history of heat stroke, sleep deprivation, medications (especially diuretics or antidepressants), sweat gland dysfunction, and upper respiratory or gastrointestinal illness.

[3-6] Many of these risk factors can be addressed with education and awareness of patients at risk. Dehydration, with fluid loss occasionally as high as 6-10% of body weight, appears to be one of the most common risk factors for heat illness in patients exercising in the heat.[7] Core body temperature has been shown to rise an additional 0.15°C to 0.2°C for every 1% of body weight lost to dehydration during exercise.

[8] Identifying athletes at risk, limiting environmental exposure, and monitoring closely for signs and symptoms are all important components of preventing heat illness.[4-6, 9-11] However, monitoring hydration status and early intervention may be the most important factors in preventing severe heat illness.[9-11]


As the third leading cause of death in U.S. high school athletes,[2] heat illness is a significant concern for all patients exercising in the heat.

The recent high-profile deaths of a collegiate athlete in Florida and a professional athlete in Minnesota have the sports medicine and family medicine communities in a state of “high alert” and searching for the most efficacious methods of preventing such tragedies. Of the five major forms of heat illness described later, only heat stroke is typically a medical emergency.

[12] However, early recognition and effective treatment of the milder forms of heat illness, as well as risk factor awareness, are all crucial to the prevention of heat stroke and potential fatalities associated therewith.

Risk factor recognition is usually not part of routine athlete screening. Other than a rare question on a preparticipation examination form geared mainly toward heat stroke, the athlete, coaching staff, and even the sports medicine team are often not aware of the significant risk factors for heat illness.

In addition to a lack of risk factor awareness, often the athlete will receive inaccurate and inadequate information regarding fluid rehydration practices, medication effects, influence of diet and sleep, and recognizing heat illness symptoms.


Exercising in heat, for any population, places unusual demands on the human body’s thermoregulatory centers. Heat production during exercise is 15-20 times greater than at rest and is sufficient to raise core body temperature 1°C every five minutes if there are no thermoregulatory adjustments.

[13] This generated heat, in addition to ambient heat from the external environment, must be offset by the body’s multiple mechanisms for heat dissipation to avoid significant hyperthermia.[9, 12, 14] These cooling mechanisms include conduction, convection, evaporation, and radiation.

[15] As ambient temperature rises above 20°C, the contributions of conduction, convection, and particularly radiation become increasingly insignificant, with the bulk of heat dissipation in the exercising patient resulting from evaporation as sweat.[10, 15] In hot, dry conditions, evaporation may account for as much as 98% of dissipated heat.

[10] In addition to the influence of dehydration on core body temperature rise, athletic performance suffers considerably with even two to three percent dehydration.[16] Any factor that limits evaporation such as high humidity or dehydration will have profound effects on physiologic function, athletic performance, and risk for heat illness in the exercising patient.[16, 17]

Influence of Hydration

Dehydration of approximately 2-3% of body mass routinely occurs in healthy patients during intermittent high-intensity exercise, especially when the ambient temperature is high.[7] This is particularly evident when thirst is relied upon to trigger fluid intake, which may not occur until the patient is already 5% dehydrated.

[18] Children, the elderly, and disabled patients are particularly susceptible to this fluid loss and heat accumulation due to decreased sweating ability, increased metabolic heat production, greater surface area to body mass ratio, chronic medical conditions, decreased thirst response, decreased mobility, decreased vasodilatory response, and medication effects.[4, 18, 19] These fluid losses result in decreased circulatory blood volume, blood pressure, sweat production, and stroke volume, and increased vascular resistance leading to decreased skin blood flow, all of which impair heat dissipation.

[20] Heart rate also rises an additional 3-5 beats per minute for every 1% body weight loss due to dehydration.[21] Muscle tissue changes associated with hypohydration include increased glycogen degradation, elevated temperature, and increased lactate levels.

[20, 22] This dehydration predisposes patients to heat exhaustion or more dangerous hyperthermia and heat stroke.[12] It has been demonstrated, however, that active efforts to intervene in this compromised physiological state can be very beneficial. As early as 1944, Pitts et al. demonstrated that fluid ingestion could reduce core body temperature response to exercise in the heat.

[23] Drinking water (compared to no fluid intake) significantly reduced rise in core body temperature, prevented decreased stroke volume and cardiac output, and decreased reduction of plasma volume with exercise.[24] Another study revealed slower core body temperature rise during heat-related exercise when fluid intake was supervised and when it quantitatively replaced sweat losses as opposed to normal “ad libidum” intake.[15]

Risk Reduction

Governing bodies of the sports medicine community have published recommendations for minimizing heat illness risk.[9] Most of the accepted recommendations involve risk factor assessment, exposure control, symptom awareness and perhaps most importantly fluid status monitoring.

However, the risk factors are not routinely assessed, symptom awareness is typically very low, and athletes are often poorly educated regarding fluid rehydration practices. In some athletic cultures, an “old school” mentality persists wherein fluid restriction during athletic training is practiced and deemed necessary to “toughen up” the athletes. Further, athletes obtain much of their hydration practice information from well-meaning but under-informed coaches and teammates. 

Heat illness prevention depends upon an awareness of the myriad of factors related to risk, and heat acclimatization is crucial for athletes not accustomed to exercising in a hot and humid environment.

Such acclimatization leads to an increased sweat rate, decreased electrolyte loss and greater resistance to dehydration.[28] Prior studies have also revealed that acclimated athletes tend to be better “demand” drinkers and thus more effective at voluntarily maintaining hydration status than non-acclimated athletes.[29] Adults typically require between four and seven exercise sessions in the heat, at one to four hours each, to acclimatize adequately.

[25] Children may require slightly more time, as many as eight to ten exercise sessions.[18] Environmental conditions are critical to the development of heat illness. Determination of wet-bulb-globe-temperature (WBGT) and assessing risk of heat illness under given heat and humidity conditions can save lives.

[25] (Table III). WBGT is a standardized index of environmental heat stress that can be obtained using commercially available devices (sling psychrometer or digital instrument). The measure takes into account contributions to heat stress from temperature, radiant heat and humidity. WBGT above 82°F is considered “very high risk.” WBGT between 73-82°F is considered “high risk.” WBGT between 65-73°F is considered “moderate risk,” and WBGT below 65°F is considered “low risk.” Alternatively, risk can be approximated by utilizing a heat illness risk assessment chart (Figure 1) with ambient temperature and relative humidity readings available from local meteorological stations, or cnn.com/WEATHER[9]. Hazardous heat problems can sometimes be avoided by scheduling athletic events during the coolest part of the day or the coolest time of year.

[9] Unfortunately, many events in the southeastern United States occur under moderate to high-risk conditions due to the year-round hot, humid climate. Thus, while temperature extremes can be avoided at times, other methods of lowering risk while exercising in hot climates must often be sought.[15] Such other methods include utilization of shade, air conditioning, frequent breaks, and avoiding the time period between 10:00 a.m. to 6:00 p.m. for the most intense exercise.[15] Proper clothing can also help to minimize the effects of poor environmental conditions. Light-colored, loose-fitting, open-weave clothes allow maximum heat deflection and optimal evaporation.

[19] Optimizing hydration status is another major factor in improving performance and limiting heat illness in patients exercising in the heat.[8] Hydration issues can be divided into three main components: Pre-exercise, exercise, and post-exercise hydration. Current research supports pre-exercise hydration with 500 ml of fluid two hours prior to exercise, assuming the patient is starting from a “euhydrated” state.

[11] Clearly, mandatory prehydration is more effective than “personal preference” ad libidum consumption.[30] Several studies have assessed the utility of “overhydration” with glycerol prior to activity as a means of providing a fluid “cushion” against dehydration, however such research is still in its early stages.

[30, 31] Exercise hydration, or the consumption of fluids during activity, is extremely important in maintaining a euhydrated state during exercise in the heat.[11] Changes in rectal temperature during exercise are directly related to degree of fluid intake, and water intake equaling sweat loss results in the slowest temperature rise as compared with ad libidum drinking and no water drinking.[23] Unfortunately, most athletes do not readily drink enough water to replace fluid losses. This is due to personal choice, fluid availability, and circumstances surrounding the athletic competition.

[11, 32, 33] Consumption of a minimum of eight ounces of fluid every twenty minutes during activity is recommended to minimize net fluid loss.[19] Ideally, fluid intake should equal net fluid lost both as sweat and insensible loss.[23] Each athlete at practice should follow hydration protocols, and such protocols should be individually modified to optimize fluid status and performance.[34] In order to maintain optimal hydration status, an athlete needs to practice fluid intake just as he or she would practice sport-specific skills. Post-exercise replacement of lost fluid volume and glycogen stores is critical to the optimization of cardiovascular, thermoregulatory and metabolic activity.

[34, 35] Combining carbohydrates with water in fluid replacement may improve intestinal absorption of sodium and water and also replenishes glycogen stores.[36, 37] Ideally, fluid ingestion should equal approximately 150% of weight lost and should occur within six hours after exercise.

[37] Generally, twenty ounces per pound of weight lost will closely approximate the proper fluid replacement.[19] Accurately assessing hydration status and providing feedback to the patient regarding fluid replacement protocols are obviously crucial steps in maintaining the patient’s euhydrated state. Until now, little has been done to ensure accurate assessment of hydration status in patients at highest risk of dehydration and consequential heat illness (e.g. team sport athletes such as football players). Body weight changes, urine color, subjective feelings and thirst have traditionally been used as markers of the need for increased hydration. Some studies have done more elaborate sweat rate determinations.[20, 38] Urine color charts and urine refractometers have been used to identify a patient’s degree of dehydration.

[38] Both of these methods serve merely to categorize athletes by degree of dehydration (i.e. well-hydrated, minimally dehydrated, significantly dehydrated or seriously dehydrated).[15] Bioelectrical impedance analysis (BIA), one form of body composition analysis, may hold significant promise in defining a patient’s actual dehydration level in an effective and practical manner. “BIA provides a reliable estimate of total body water under most conditions.” Although there is some concern for exercise-related fluid shifts leading to measurement error, BIA may prove to be an invaluable adjunct to pre- and post-exercise weight measurement in the accurate assessment of hydration status of exercising patients at highest risk for heat illness.

Knowledge Gap

Despite a relatively large body of knowledge regarding the basic science of heat illness, at present there are significant difficulties in diagnosing its milder forms. Currently, there are no convenient and reliable tools available for use in thoroughly assessing a patient with heat edema, heat cramps, heat syncope, or even heat exhaustion. 

There is also minimal knowledge of the prevalence of heat illness (other than heat stroke) in many high-risk populations, such as young athletes exercising in the heat. A cursory understanding of the risk factors for heat illness and the frequency with which they appear in our high-risk populations does exist, however attempts to screen for these risk factors in our high risk active populations are very limited.


Heat illness, particularly in its milder forms, is an extremely common problem in high-risk populations. The major modifiable risk factor for heat illness is dehydration. Due to excessive demands for heat dissipation in the patient exercising in the heat, the athlete offers a prime opportunity for this relationship between dehydration and heat illness to be further explored. Heat acclimation is also an extremely important modifiable risk factor for heat illness. 

Proper initiation of preseason practices allowing for increased cardiovascular fitness and acclimatization prior to increasing intensity of workouts is crucial. NATA, GSSI. Current studies at our institution and others seek to further elucidate the incidence of risk factors for heat illness in an active population, determinants of fluid intake, barriers to hydration, the incidence of dehydration with competitive activity in the heat, the prevalence of heat illness in a high risk population, and the relation between dehydration and the milder forms of heat illness. 

In addition, convenient and reliable tools for assessing heat illness symptoms are needed. The relevant objectives of any sports medicine staff should include developing effective prevention and intervention protocols in an attempt to decrease or eliminate episodes of dehydration, heat illness and ultimately heat stroke.


  1. Heat Related Illness and Deaths - United States, 1994-1995. Morbidity and Mortality Weekly Report. 44: p. 465-468
  2. Lee-Chiong TL Jr., S.J., Heatstroke And Other Heat Related Illnesses: The Maladies of Summer. Postgraduate Medicine, 1995. 98: p. 26-36
  3. Wexler RK, Evaluation and Treatment of Heat-Related Illness. American Family Physician, 2002. 65(11): p. 2307-2314
  4. Bar-Or, O., Temperature Regulation During Exercise in Children and Adolescents,, in Perspectives in Exercise Sciences and Sports Medicine, L.D. Gisolfi C, Editor. 1989, Benchmark Press: Indianapolis. p. 335-367
  5. Epstein, Y., Heat Intolerance: Predisposing Factors. Medicine and Science in Sports and Exercise, 1990. 22: p. 29-35
  6. Armstrong LE, D.J., Hubbard RW, Time Course of Recovery and Heat Acclimation: Ability of Prior Heat Stroke Patients. Medicine and Science in Sports and Exercise, 1990. 22: p. 36-48
  7. Galloway, S., Dehydration, Rehydration, and Exercise in the Heat: Rehydration Strategies for Athletic Competition. Canadian Journal of Applied Physiology, 1999. 24(2): p. 188-200
  8. Coyle EF, M.S., Influence of Graded Dehydration on Hyperthermia and Cardiovascular Drift During Exercise. Journal of Applied Physiology, 1992. 73: p. 1340-1350
  9. Armstrong LE, E.Y., et al., American College of Sports Medicine Position Stand: Heat and Cold Illness During Distance Running. Medicine and Science in Sports and Exercise, 1996. 28(12): p. I-x
  10. Armstrong LE, M.C., The Exertional Heat Illnesses: A Risk of Athletic Participation. Med Exerc Nutr Health, 1993. 2: p. 1-35
  11. Convertino VA, A.L., Coyle EF, et al., American College of Sports Medicine Position Stand: Exercise and Fluid Replacement. Medicine and Science in Sports and Exercise, 1996. 28(1): p. I-vii
  12. Bouchama A, K.J., Heat Stroke. New England Journal of Medicine, 2002. 346(25): p. 1978-1988
  13. Nadel ER, R.M., Wenger CB, et al., Physiological Defenses Against Hyperthermia of Exercise. Annals of New York Academy of Science, 1977. 301: p. 98-109
  14. Simon, H., Hyperthermia. New England Journal of Medicine, 1993. 329: p. 483-487
  15. Werner, J., Temperature Regulation During Exercise: An Overview, in Exercise, Heat, and Thermoregulation, L.D. Gisolfi C, Nadel ER, Editor. 1993, Brown and Benchmark: Dubuque. p. 49-77
  16. Armstrong LE, C.D., et al., National Athletic Trainer's Association Position Statement: Fluid Replacement for Athletes. Journal of Athletic Training, 2000. 35(2): p. 212-224
  17. Armstrong LE, C.D., Fink WJ, Influence of Diuretic-Induced Dehydration on Competitive Running Performance. Medicine and Science in Sports and Exercise, 1985. 17: p. 456-461
  18. Anderson SJ, G.B., Johnson MD, et al., American Academy of Pediatrics, Committee on Sports Medicine and Fitness: Climactic Heat Stress and the Exercising Child and Adolescent. Pediatrics, 2000. 106(1): p. 158-159
  19. Barrow MW, C.K., Heat-Related Illness. American Family Physician, 1998. 58(3): p. 749-756
  20. Murray, R., Dehydration, Hyperthermia, and Athletes. Journal of Athletic Training, 1996. 31: p. 248-252
  21. Mack G, N.E., Nose H, Role of Cardiopulmonary Baroreflexes During Dynamic Exercise. Journal of Applied Physiology, 1988. 65: p. 1827-1832
  22. Dillo P, H.M., et al., Effect of Fluid Ingestion on Muscle Metabolism During Prolonged Exercise. Journal of Applied Physiology, 1996. 80: p. 363-366
  23. Pitts GC, J.R., Consolazio FC, Work in the Heat as Affected by Intake of Water, Salt, and Glucose. American Journal of Physiology, 1944. 142: p. 253-259
  24. Hamilton MT, G.-A.J., Montain SJ, Fluid Replacement and Glucose Infusion During Exercise Prevents Cardiovascular Drift. Journal of Applied Physiology, 1991. 71: p. 871-877
  25. Mellion MB, S.G., Safe Exercise in the Heat and Heat Injuries, in The Team Physician's Handbook, W.W. Mellion MB, Shelton GL, Editor. 1997, Hanley and Belfus: Philadelphia p. 151-165.
  26. Richards DR, R.P., et al., Management of Heat Exhaustion in Sydney's Sun City-to-Surf Fun Runners. Medical Journal of Australia, 1979. 2: p. 457-461
  27. Costrini, A., Emergency Treatment of Exertional Heat Stroke and Comparison of Whole Body Cooling Techniques. Medicine and Science in Sports and Exercise, 1990. 22: p. 15-18
  28. Armstrong LE, M.C., The Induction and Decay of Heat Acclimatization in Trained Athletes. Sports Medicine, 1991. 12: p. 302-312
  29. Murray, R., Nutrition for the Marathon and Other Endurance Sports: Environmental Stress and Dehydration. Medicine and Science in Sports and Exercise, 1992. supplement: p. 319-323
  30. Rico-Sanz J, F.W., et al., Effects of Hyperhydration on Total Body Water, Temperature Regulation and Performance of Elite Young Soccer Players in a Warm Climate. International Journal of Sports Medicine, 1995. 17: p. 85-91
  31. Casa DJ, W.J., et al., Influence of a Pre-Exercise Glycerol Hydration Beverage on Performance and Physiological Function During Mountain Bike Races in the Heat. Journal of Athletic Training, 1999. 34(supplement): p. 35
  32. Greenleaf, J., Problem: Thirst, Drinking Behavior, and Involuntary Dehydration. Medicine and Science in Sports and Exercise, 1992. 24: p. 645-665
  33. Broad EM, B.L., et al., Body Weight Changes and Voluntary Fluid Intakes During Training and Competition in Team Sports. International Journal of Sports Nutrition, 1996. 6: p. 307-320
  34. Gisolfi CV, D.S., Guidelines for Optimal Fluid Replacement Beverages for Different Athletic Events. Medicine and Science in Sports and Exercise, 1992. 24: p. 679-687
  35. Maughan RJ, S.S., Recovery From Prolonged Exercise: Restoration of Water and Electrolyte Balance. Journal of Sport Science, 1997. 15: p. 297-303
  36. Murray, R., The Effects of Consuming Carbohydrate-Electrolyte Beverages on Gastric Emptying and Fluid Absorption During and Following Exercise. Sports Medicine, 1987. 4: p. 322-351
  37. Fallowfield JL, W.C., Carbohydrate Intake and Recovery From Prolonged Exercise. International Journal of Sports Nutrition, 1993. 3: p. 150-164
  38. Armstrong, L., Keeping Your Cool in Barcelona: The Effects of Heat, Humidity, and Dehydration on Athletic Performance, Strength, and Endurance. United States Olympic Committee, Colorado Springs, Colorado, 1992(1992): p. 1-29