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Exercising in Hot Weather
 Submitted by CDR Scott Gaustad, USPHS, Therapy Category
It Is Hot Outside!

Summer is officially here and much of the United States is under an excessive heat warning. It is also the time that many amateur, recreational, and elite athletes transition from indoor programs to outdoor warm or hot weather exercise regimens. As the temperature and humidity rise, so does the incidence of environmental heat-related exertional illnesses. Understanding warm and/or hot weather definitions is very important for an athlete or just an exerciser for fitness so that they may better comprehend heat illness, preventative measures, and treatment options if necessary. Of the many relevant heat related definitions, the heat index is one of the most important.

Steadman or Heat Index: The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature. When the heat index is at or over 90 degrees Fahrenheit, extreme caution should be considered before exercising outdoors. A heat index chart can be found at http://www.nws.noaa.gov/om/heat/index.shtml or http://www.nws.noaa.gov/.

Heat Illness: What Is It and How Do You Manage It?

Heat illness or exertional heat illness progresses along a continuum from the mild (heat rash and/or heat cramps and/or heat syncope) through the moderate (heat exhaustion) to the life-threatening (heatstroke). Anyone is susceptible to a heat-related exertional illness. It is very important that the athlete or exerciser understand that the presentation of signs and symptoms associated with heat exertional illness does not necessarily follow this continuum. A dehydrated, non-acclimated or deconditioned individual may right away present with signs and symptoms consistent with heat stroke and not the milder symptoms first.

Heat Cramps are associated with excessive sweating during exercise and are usually caused by dehydration, electrolyte (primarily salt) loss, and inadequate blood flow to the peripheral muscles. They usually occur in the quadriceps, hamstrings, and calves.

Treatment for heat cramps is rehydration with an electrolyte (salt) solution and muscle stretch.

Heat Syncope results from physical exertion in a hot environment. In an effort to increase heat loss, the skin blood vessels dilate to such an extent that blood flow to the brain is reduced causing symptoms of headache, dizziness, faintness, increased heart rate, nausea, vomiting, restlessness, and possibly even a brief loss of consciousness.

Treatment for heat syncope is to sit or lie down in a cool environment with elevation of the feet. Hydration is very important so there is not a possible progression to heat exhaustion or heat stroke.

Heat Exhaustion is a shock-like condition that occurs when excessive sweating causes dehydration and electrolyte loss. A person with heat exhaustion may have headache, nausea, dizziness, chills, fatigue, and extreme thirst. Signs of heat exhaustion are pale and clammy skin, rapid and weak pulse, loss of coordination, decreased performance, dilated pupils, and profuse sweating.

Treatment for heat exhaustion is to immediately stop the activity and properly hydrate with chilled water and/or an electrolyte replacement sport beverage. The exerciser should be cleared by his/her physician before resuming sport or other strenuous outdoor activities.

Exertional Heat Stoke (Hyperthermia) is a life-threatening condition in which the body’s thermal regulatory mechanism is overwhelmed. There are two types heat stroke – fluid depleted (slow onset) and fluid intact (fast onset). Fluid depleted means that the individual is not hydrating at a rate sufficient to function in a heat challenge situation. Fluid intact means that the extreme heat overwhelms the individual even though the fluid level is sufficient. Key signs of heat stroke are hot skin (not necessarily dry skin), peripheral vasoconstriction (pale or ashen colored skin), high pulse rate, high respiratory rate, decreased urine output, and a core temperature (taken rectally) over 104 or 105 degrees Fahrenheit, and pupils may be dilated and unresponsive to light.

Treatment for heat stroke is to move the person to a cool shaded area and reduce the body temperature immediately. If immediate medical attention is not available, immerse the person in a cool bath while covering the extremities with cool wet cloths and massaging the extremities to propel the cooled blood back into the core.

Exercise Induced Hyponatremia (water intoxication) is most commonly associated with prolonged exertion during sustained, high-intensity endurance activities such as marathons or triathlons. In most cases, it is attributable to excess free water intake, which fails to replenish the sometimes massive sodium losses that result from sweating. Symptoms of hyponatremia can vary from light-headedness, malaise, nausea, to altered mental status. Risk factors include hot weather, female athletes/exercisers, poor performance, and possibly the use of nonsteroidal anti-inflammatory medications.

As a treatment for hyponatremia, new guidelines advise runners to drink only as much fluid as they lose due to sweating during a race. The International Marathon Medical Directors Association recommends that, during extended exercise, athletes drink no more than 31 ounces (or about 800 milliliters) of water per hour. Individuals involved in strenuous exercise in warm or hot weather should consider the sodium (salt) concentration of the beverage being consumed.

How Can You Prevent Exertional Heat Related Illnesses?

The North American Society for Pediatric Exercise Medicine and the National Athletic Trainers’ Association developed an Inter-Association Task Force on Exertional Heat Illnesses position statement, which can be found at http://www.naspem.org/pos%5Fstmts/. Some recommendations on how to prevent exertional heat related illness include:

  • When exercising in high heat and humidity, rest 10 minutes for every hour and change wet clothing frequently.
  • Avoid the midday sun by exercising before 10 a.m. or after 6 p.m., if possible.
  • Use a sunscreen with a rating of SPF-15 or lower dependent upon skin type. Ratings above SPF-15 can interfere with the skin’s thermal regulation.
  • Wear light-weight and breathable clothing.
  • Weigh yourself pre and post exercise. If there is a less than a 2 percent weight loss after exercise, you are considered mildly dehydrated. With a 2 percent and greater weight loss, you are considered dehydrated.
  • During hot weather training, dehydration occurs more frequently and has more severe consequences. Drink early and at regular intervals according to the American College of Sports Medicine. The perception of thirst is a poor index of the magnitude of fluid deficit. Monitoring your weight loss and ingesting chilled volumes of fluid during exercise at a rate equal to that lost from sweating is a better method to preventing dehydration.
  • Rapid fluid replacement is not recommended for rehydration. Rapid replacement of fluid stimulates increased urine production, which reduces the body water retention.
  • Individuals involved in a short bout of exercise are generally fine with water fluid replacement of an extra 8-16 ounces. A sports drink (with salt and potassium) is suggested for exercise lasting longer than an hour, such as a marathon, and at a rate of about 16 to 24 ounces an hour depending upon the amount you sweat and the heat index.
  • Replace fluids after long bouts of exercise (greater than an hour) at a rate of 16 ounces of fluid per pound of body weight lost during exercise.
  • Avoid caffeinated, protein, and alcoholic drinks, e.g., colored soda, coffee, tea.
  • Acclimate to exercising outdoors, altitude, and physical condition. General rule of thumb is 10-14 days for adults and 14-21 days for children (prepubescent) and older adults (> 60 years). Children and older adults are less heat tolerant and have a less effective thermoregulatory system.
  • Educate and prepare yourself for outdoor activities. Many Web sites offer heat index calculations for your local weather conditions. Two Web sites that will calculate the heat index for you are http://www.erh.noaa.gov/box/calculate2.html and http://www.compuweather.com/shared/weather_calculator.htm.

Summer weather does not have to sideline your outdoor exercise regimen. The above suggestions can help you plan and find ways to modify your routine to exercise safely in warm, hot, and humid weather.


Whaley, MH. Appendix E: Environmental Considerations. American College of Sports Medicine Guidelines for Exercise Testing and Prescription, 7th Edition. Baltimore: Lippincott Williams and Wilkens, 2006, pp. 300-304.

Reamy, BV. Environmental Injuries, O’Connor FG, Sallis RE, Wilder RP, Pierre PS (editors): Sports Medicine, Just the Facts, 1st ed. New York, NY, McGraw Hill, 2005, pp. 235-237.

Arnheim DD. Prentice WE (editors). Environmental Considerations. Principles of Athletic Training, 9th ed. New York, NY, McGraw Hill, 1997, pp. 263-271.

The series “Fit For Duty…Fit for Life!”, is a lifestyle-based column that has been provided by the USPHS Dietitian/Nutritionist PAC (D/N-PAC). If you have related topics of interest that you would like to learn more about in future articles, contact CAPT Jean Makie, USPHS, at jean.makie@fda.hhs.gov 
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