Anyone who has lived in or visited Florida during the summer months has experienced how taxing exercise, or any additional physical exertion for that matter (i.e. yard work), can be. With temperatures reaching the nineties most days in July, it is essential do everything in one’s power to help the body adjust to the grueling temperatures and prevent heat-related illness. It is also advantageous to familiarize oneself with the common heat illnesses that may occur, if necessary precautions are not taken when exerting oneself in the heat.
The two major physiological concerns with regard to exercise in the heat are dehydration, losing more water than is imbibed, and hyperthermia, elevated internal body temperature. Both dehydration and hyperthermia can become medical emergencies if not treated in the early stages.
Risk of hyperthermia increases when exercising in the heat because the body faces a difficult task of trying to cool the internal temperature of the body while the external ambient temperature is, on a typical summer day, just as hot or hotter than the temperature of the body. The body has a protective mechanism of triggering sweat glands to release water and electrolytes onto the surface of the skin when the muscles are exercised (active muscles create metabolic heat thus raising the body’s core temperature). The skin’s blood vessels also naturally dilate shunting blood to the skin, transferring heat from the core, to the skin and to the air. The water in sweat absorbs heat from skin and the water is evaporated, transferring body heat to the air. (The evaporation of sweat occurs more slowly in humid environments making it more difficult for the body to cool.) This naturally protective loss of body water in the form of sweat can lead to dehydration.
Dehydration occurs when the body’s water output exceeds input. The body has three methods of water intake, fluids, food water and metabolic water (water created during cellular processes) The body eliminates water in four ways: urination (1.1 L/day), elimination (0.1 L/day), breathing (0.2 L/day), and sweating (0.6 L/day). During exercise the body increases the breathing rate and sweat rate, therefore increasing the water lost by those two methods. Not to mention, our bodies use water for almost all cellular processes that keep our bodies functioning normally, which is discussed in more detail in our Spotlight on Nutritionarticle.
Tips for Preventing Dehydration and Hyperthermia
  • The key to preventing dehydration is to adequately hydrate the body.
    • “The Institute of Medicine determined that an adequate [fluid] intake (AI) for men is roughly 3 liters (about 13 cups) of total beverages a day. The AI for women is 2.2 liters (about 9 cups) of total beverages a day.” (Mayo)
    • Measure body weight before and after exercise and “drink at least 1 pint of fluid for each pound of body weight lost.” (Thompson) This is in addition to normal water consumption.
    • Avoid dehydrating beverages such as coffee, tea, carbonated drinks and alcohol.
    • Maintain daily sodium intake of 1,500-2,300 mg per the USDA’s dietary guidelines.
    • Always carry a water bottle with you to help you consistently drink throughout the day.
  • Take rest breaks during long exercise bouts.
  • Gradually progress to longer exercise sessions in the heat to allow the body to acclimatize.
    • The first session may be as short as 10-15 minutes.
  • Only exercise when adequately nourished and rested.
  • Limit intense exercise to cooler hours of the day.
  • Exercise in the shade if possible.
  • Wear loose-fitting, lightweight and porous clothing. (The less clothes you can wear, within reason, the better.)
  • Avoid or reduce exercise in the heat if experiencing sleep loss, infectious illness, fever, diarrhea, vomiting, carbohydrate depletion or if you are taking certain medications or abusing alcohol or other drugs.
Exertional Heat Illnesses
Dehydration is a contributing risk factor for exertional heat illnesses including: heat exhaustion, heat cramps, heat syncope and heatstroke. Other contributing factors include but are not limited to: inadequate physical fitness, excess adiposity, improper clothing, protective pads, incomplete heat acclimatization, illness or certain medications.
  1. Heat exhaustion is the inability to continue exercise in the heat. Symptoms include prominent fatigue and progressive weakness without hyperthermia. (Other symptoms include: low blood pressure, elevated heart rate and respiratory rate, wet and pale skin, headache, dizziness, decreased muscle coordination, chills, nausea, vomiting, and diarrhea.) Treatment is re-hydration by oral fluids or intravenous fluids if the subject is unable to swallow, is unconscious and is losing fluid (experiencing vomiting or diarrhea).
  2. Heat cramps generally occur in the abdominal or limb muscles, usually during sports like football, tennis or distance running. Causation generally includes factors such as muscle fatigue, water loss, and significant sweat sodium. The main symptom is feeble, localized, wandering spasms that may progress to debilitating cramps. Treatment commonly includes rest, prolonged stretching, and dietary sodium chloride (1/4 t. table salt mixed with 1/2 liter of water).
  3. Heat syncope (fainting) is caused by standing for long periods or when stopping long, difficult bouts of upright exercise. It results from maximal blood vessel dilation in the skin leading to a decline in blood pressure and insufficient oxygen to the brain. Symptoms include slowed heart and breathing rate, pale skin, sensations of weakness, tunnel vision, vertigo or nausea before fainting. One can help guide a fainting person to the ground to prevent injury, but NEVER stop a fainting person from lying down. Fainting is the body’s natural way of getting more oxygen to the brain by increasing blood flow to the upper part of the body. 
  4. Heatstroke, the most severe form of heat illness, is caused by hyperthermia and results in a core body temperature above 104 degrees Fahrenheit. Heatstroke is connected to central nervous system disturbances and multiple organ system failure and is a LIFE-THREATENING medical emergency. Symptoms include disorientation, dizziness, irrational behavior, apathy, headache, nausea, vomiting, hyperventilation, and wet skin. Treatment is to immediately seek medical attention for whole-body cooling with cold-water and ice-water immersion therapy.

If you or someone you know is experiencing symptoms of heatstroke, seek immediate medical attention.


Questions to Evaluate Readiness to Exercise in a Hot Environment
Adults should ask the following questions to evaluate readiness to exercise in a hot environment. Corrective action should be taken if any question is answered “no.”
  1. Have I developed a plan to avoid dehydration and hyperthermia?
  2. Have I acclimatized by gradually increasing exercise duration and intensity for 10 to 14 days?
  3. Do I limit intense exercise to the cooler hours of the day?
  4. Do I avoid lengthy warm-up periods on hot/humid days?
  5. When training outdoors, do I know where fluids are available, or do I carry water bottles in a belt or backpack?
  6. Do I know my sweat rate and the amount of fluid that I should drink to replace body-weight loss?
  7. Was my body weight this morning within 1% of my average body weight?
  8. Is my 24-hour urine volume plentiful?
  9. Is my urine pale yellow or straw colored?
  10. When heat and humidity are high, do I reduce my expectations, my exercise pace, the distance, and/or the duration of my workout or race?
  11. Do I wear loose-fitting, porous, lightweight clothing?
  12. Do I know the signs and symptoms of heat exhaustion, exertional heatstroke, heat syncope, and heat cramps?
  13. Do I exercise with a partner and provide feedback about his/her physical appearance?
  14. Do I consume adequate salt in my diet?
  15. Do I avoid or reduce exercise in the heat if I experience sleep loss, infectious illness, fever, diarrhea, vomiting, carbohydrate depletion, some medications, alcohol, or drug abuse?
Knowing how to prevent dehydration and hyperthermia can help make exercising in the heat more enjoyable and easier on the body.  Be sure to take care of yourself by taking necessary steps to prevent heat illness while you enjoy the summer sunshine.
Mayo Clinic Staff. Nutrition and Healthy Eating. “Water: How much should you drink every day?.” 12 Oct 2011.   
Thompson, Walter. ACSM’s Guidelines for Exercise Testing and Prescription. Lippincott Williams & Wilkins, 8th ed. 2010. p. 194-198.