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Heat Related Illness Main

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Other Names

  • Heat Related Illness
  • Exertional Heat Illness


  • This page refers to the spectrum of heat-related illnesses due to exercise or environmentally induced hyperthermia



  • Incidence
    • Among US high school athletes, 1.6 cases per 100,000 exposures[1]
    • Incidence among US Armed Forces in 2017 was 1.41 per 1,000 person years[2]
    • Increase in Emergency Department visits from 2006 to 2010 to 5 per 10,000 visits[3]
  • Prevalence
    • Among US high school athletes, 9000 cases annually[1]
    • In the US Armed Forces in 2017, there were 2,163 cases of heat related illness, 464 cases of heat stroke[2]
  • Mortality
    • #3 cause of death in high school athletes[4]
    • In one study of ED patients, 12% were admitted and the mortality rate was 0.07%[3]
    • According to the National Oceanic and Atmospheric Administration, it is the #1 cause of weather related fatalities annually (need citation)
    • According to the CDC, 8081 deaths were reported between 1999 and 2010, with the majority occurring between May and September[5]
  • Sport
    • The highest incidence is in American Football at 4.5 cases per 100,000 athlete exposures
    • 31 players died from exertional heat stroke between 1995 and 2008[6]


  • General
    • Heat-related illnesses should be thought of as a continuum
    • On one end is heat cramps and at the other is heat stroke
    • Early recognition and prompt treatment are keys to the prevention of morbidity and mortality
  • Heat Wave
    • Rising global temperatures are likely to increase incidence of heat wave[7]
    • Urbanization with development of inner-city heat islands likely contribute
    • According to the U.S. National Weather Service, heat waves kill more people, on average, than any other extreme weather event[8]


  • General
    • Heat Related Illness: general term to describe any illness related to heat exposure or exertional hyperthermia
    • Exertional Heat Illness (EHI): general term to describe any illness related to heat exposure or exertional hyperthermia
  • Minor Pathology
    • Milaria Rubra: Acute inflammation of sweat ducts caused by blockage of pores
    • Heat Edema: Self-limited, mild edema of feet, ankles, and hands
    • Sunburn: reddening, inflammation, blistering and peeling of the skin
    • Heat Tetany: Respiratory alkalosis, extremity/circumoral paresthesias, carpopedal spasm
    • Heat Cramps: Painful, involuntary, spasmodic contractions of skeletal muscles
  • Major Pathology
  • Other
    • Wet Bulb Globe Temperature: measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover
    • Heat Acclimatization: Adaptive physiological and perceived exertion change that occurs with repeated exposure to exercise heat stress
    • Heat Tolerance Testing: Standardized testing to assesss patients tolerance of heat following an EHI
    • Heat Illness Symptom Index (HISI): Proposed to identify mild forms of heat illness based on subjective symptoms
      • Show's promise in an early study in Division 1 football athletes, requires more research[9]


  • Normal
    • Heat stress produces a predictable cascade of cellular events events.
    • Peripheral vasodilation: produce heat loss, shunt blood from the central circulation
    • Sweating and vaporization of the sweat releases heat
    • Sodium loss in sweat can be significant, can play a role in dehydration
    • Cardiopulmonary responses: tachycardia, increased cardiac output, and increased minute ventilation
    • Responses can be impaired by dehydration and excess salt loss
  • Acclimatization
    • Acclimatization to a hotter environment may take several weeks
    • Improved sodium retention, increased renal glomerular filtration rate, enhanced cardiovascular performance occur
    • Help to prevent organ damage
    • Heat shock proteins: serve to assist the cell in tolerating the heat, present in most cell lines
  • Heat exchange
    • Heat exchange occurs via conduction, convection, radiation, and evaporation
    • All methods are dependent on the presence of a heat gradient
    • Heat transfers from a hotter object to a cooler one
    • Loss of this heat gradient by certain environmental conditions can inhibit appropriate thermoregulation
    • Conduction: direct transfer of heat during contact with a cooler object
    • Convection: cooling of the air around the body by way of cooler air passing over the warmer exposed skin
      • Method depends on wind current to bring cooler air to the body (eg, fan)
      • Alternatively, movement of the body through the environment (eg, cycling)
    • Radiation: direct release of heat from a body into the environment[10]
      • Works well if the body temperature exceeds the ambient temperature
      • High ambient temperature reduces heat gradient, does not allow for heat loss from the body to the environment
    • Evaporation: occurs via perspiration[11]
      • Most effective way to release heat
      • Up to 600 kcal/h of heat can be dissipated by this method

Risk Factors

  • Demographic
    • Infants (particularly when confined in closed car)[12]
    • Prepubescent children[13]
      • Attributed to: high ratio of surface:mass, immature thermoregulatory system, small blood volume, low sweating rate
    • Elderly
    • Obesity
    • Race: Asian/ Pacific Islander
    • Gender: Male > Female
  • Functional
    • Poor physical fitness
    • Low work efficiency
    • Reduced skin area to mass ratio (eg, large muscle mass, obesity)
  • Environmental
    • Warm or hot weather
    • Unusually hot for region or season
    • Heat wave (defined as >3 d of >32°C (90°F))
    • Increased humidity
  • Medication
    • Diuretics
    • Anticholinergics
    • Beta blockers
    • Antihistamines
    • Antidepressants includig Tricyclic antidepressants
    • Stimulants (amphetamines, cocaine, ecstasy, ephedra)
    • Antiepileptic Drugs
    • ACE inhibitors/ ARBs
    • Decongestants
    • Lithium
  • Drugs
    • Alcohol use
    • Drug use, especially stimulants and amphetamine like drugs
  • Sports
    • American Football
  • Occupations
    • Firefighter
    • Agricultural workers
    • Landscapers
    • Soldiers and Military Personnel
  • Behavioral
    • Overmotivation or pressure from peers and coaches to perform beyond their physiological capability
    • Leadership or organizational structure
  • Training Related
    • Wearing heavy clothes, equipment, or uniforms
    • Inappropriate work to rest ratios
    • Inadequate heat acclimatization for current conditions
    • Heat stress in the previous 1-3 days
    • Poor or under hydration
    • Lack of access to shade, air conditioning
  • Health Conditions
    • Viral or bacterial infections
    • Fever
    • Diarrhea or vomiting
    • Skin disorders (rash, large area of burned skin)
    • Diabetes Mellitus
    • Cystic fibrosis/trait
    • Cardiovascular disease
    • Sunburn (impaired sweating mechanisms can place a patient at risk)
    • Sweat gland dysfunction
    • Xray radiation
  • Congenital
    • Ectodermal Dysplasia
    • Chronic Idiopathic Anhidrosis
    • Malignant Hyperthermia
    • Sickle Cell Trait
  • Unknown
    • Previous heat stroke has been suggested as a risk factor, not currently supported by evidence
  • Not associated/ risk factor

Differential Diagnosis


WBGT levels for modification or cancellation of workouts or athletic competition[14]
  • General
    • Prevention is more effective, easier than treatment
    • Knowledge of signs and symptoms of heat illness for all stakeholders (athletes, parents, coaches, medical staff)
    • Individuals with early signs of illness should not be allowed to participate in physical activity
    • Avoid stimulants (ephedra, etc)
  • During warm weather/ heat wave[15]
    • Stay in air-conditioned homes or other air-conditioned premises (e.g., shopping malls or movie theaters)
    • Use fans
    • Take frequent cool showers
    • Decrease exertion
    • Increase social contact to counteract isolation
  • Training/ Work
    • Acclimatize to changed environmental conditions
    • Match the level of physical exertion to the degree of physical fitness
    • Avoid hot times of the day for training schedules
    • Remove vapor-barrier equipment and clothing that interfere with sweat evaporation
    • Maintain a proper hydration regimen
    • Schedule rest periods during activity
Acclimatization Guidelines for Football[16]
  • Heat acclimatization
    • Definition: Adaptive physiological and perceived exertion change that occurs with repeated exposure to exercise heat stress
    • Improve physical function in heat, reduce physiological strain[17]
    • Recommend 1-2 weeks to induce most physiological adaptations with optimize performance, reduce risk of EHI[18]
    • Accomplished by gradually increasing intensity, duration of exercise during daily heat exposure
Fluid Management During Exertion: Specific Guidelines[16]
  • Hydration
    • Dehydration directly contributes to heat exhaustion[19]
    • Direct link to EHI and EHS have not been established
    • Rehydration strategies are important for athletes to safely perform in warm or hot conditions
    • Best done by combing eating with fluid intake
    • If symptoms of dehydration (dizzy, tachycardic, fatigue, headache), use oral rehydration fluids with electrolytes
    • Recommend 16 oz 1 hour before exertion, then 4-8 oz per 15-20 minutes of exertion
    • Monitor body weight changes, thirst, urine volume or concentration[20]
  • Clothing
    • Uniform wear can decrease the amount of skin surface available for evaporation[21]
    • Clothing should be light colored and oose fitting
    • It should be made from lightweight, open-weave mateial
    • Wearing fewer items of clothing seems to correlate with temperature tolerance
  • Administrative strategies
    • Site leaders should change training sessions by adding longer and more frequent rest breaks
    • This allows heat dissipation and shorter bouts of high intensity exercise duration to decrease heat production
    • Longer rest breaks permit better fluid replacement
    • Factors
      • Heat acclimatization status of participants
      • Fitness and age of participants
      • Intensity and duration of exercise
      • Time of day
      • Clothing or uniform requirements
      • Playing surface for radiant and conductive heat exchange (i.e., grass vs synthetic fields)

Comorbidity Considerations


  • General
    • Obesity is associated with increased risk of HRI[22]
    • Increased body mass results in reduced surface area necessary for heat dissipation via evaporation
    • Metabolic exothermia increases with body weight
    • No specific restriction are neaded for obese idividuals
    • However, they should be monitored closely for signs of heat illness

Sickle Cell Trait

  • General
    • Stressors, including exercise and heat illness, predispose HgbS patients to a sickling crisi
    • There are reports of death in SST athletes related to exertional heat stroke[23]
    • Risk is increased by dehydration, extreme heat and exercise at altitude
    • Patients with HgbS should be monitored closely to maintain hydration, especially in heat or at altitude
  • Note: Sickle Cell Disease patients are unlikely to participate in athletics

Return To Play

See Also


  1. 1.0 1.1 Kerr ZY, Casa DJ, Marshall SW, Comstock RD. Epidemiology of exertional heat illness among U.S. high school athletes. Am J Prev Med. 2013;44(1):8–14.
  2. 2.0 2.1 Armed Forces Health Surveillance Bureau. Update: heat illness, active component, U.S. Armed Forces, 2018. MSMR. 2018;25(4):6–10.
  3. 3.0 3.1 Hess JJ, Saha S, Luber G. Summertime acute heat illness in U.S. emergency departments from 2006 through 2010: analysis of a nationally representative sample [published correction appears in Environ Health Perpsect. 2014;122(11):A293]. Environ Health Perspect. 2014;122(11):1209–1215.
  4. Jardine DS. Heat illness and heat stroke [published correction appears in Pediatr Rev. 2007;28(12):469]. Pediatr Rev. 2007;28(7):249–258.
  5. CDC Infographic: https://www.cdc.gov/pictureofamerica/pdfs/picture_of_america_heat-related_illness.pdf
  6. Mueller, F., and R. Cantu. "Catastrophic sports injury research: twenty-sixth annual reports." (2008).
  7. Kravchenko, Julia, et al. "Minimization of heatwave morbidity and mortality." American journal of preventive medicine 44.3 (2013): 274-282.
  8. NOAA, NWS. "Weather fatalities 2017." (2018).
  9. Coris, EE , Walz, SM , Duncanson, R. , Ramirez, AM , Roetzheim, RG Heat illness symptom index (HISI): a novel instrument for the assessment of heat illness in athletes. South Med J.2006; 99:340–345.
  10. Gaffin, SL , Moran, DS Pathophysiology of heat-related illnesses. In: Auerbach, PS , ed. Wilderness Medicine. 4th ed. St Louis, Mo: Mosby; 2001:240–281.
  11. Glazer, JL Management of heatstroke and heat exhaustion. Am Fam Phys.2005;71:2133–2140.
  12. Booth, John N., et al. "Hyperthermia deaths among children in parked vehicles: an analysis of 231 fatalities in the United States, 1999–2007." Forensic science, medicine, and pathology 6.2 (2010): 99-105.
  13. Heat-related deaths — United States, 1999–2003. MMWR Morb Mortal Wkly Rep 2006;55:796-798.
  14. Roberts, William O., et al. "ACSM expert consensus statement on exertional heat illness: recognition, management, and return to activity." Current sports medicine reports 20.9 (2021): 470-484.
  15. Hajat, Shakoor, Madeline O'Connor, and Tom Kosatsky. "Health effects of hot weather: from awareness of risk factors to effective health protection." The Lancet 375.9717 (2010): 856-863.
  16. 16.0 16.1 Howe, Allyson S., and Barry P. Boden. "Heat-related illness in athletes." The American Journal of Sports Medicine 35.8 (2007): 1384-1395.
  17. Taylor NA. Human heat adaptation. Compr. Physiol. 2014; 4:325–65.
  18. 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.
  19. Sawka MN, Young AJ, Latzka WA, et al. Human tolerance to heat strain during exercise: influence of hydration. J. Appl. Physiol. 1992; 73:368–75.
  20. Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Compr. Physiol. 2014; 4:257–85.
  21. Kulka, TJ , Kenney, WL Heat balance limits in football uniforms. Physician and Sportsmedicine.2002;30:29–39.
  22. Wyndham, CH Heat stroke and hyperthermia in marathon runners. Ann NY Acad Sci.1977;301:128–138.
  23. Pretzlaff, RK Death of an adolescent athlete with sickle cell trait caused by exertional heat stroke. Pediatr Crit Care Med.2002;3:308–310.
Created by:
John Kiel on 13 June 2019 05:29:54
Last edited:
31 August 2022 15:22:33