Heat effects the human body in many ways, not the least of which is the effect on what your body needs to do to allow you to exercise in the summer heat. You may have heard of the list of heat-related illnesses that can be caused by exercising in the heat. They can range from heat cramps to the life-threatening heat stroke. Now is the time to review them and discuss their causes and symptoms, as well as treatments, so you can enjoy your summer exercise sessions while limiting the risks of heat-related illnesses.
Heat-related Symptoms and Illnesses
Under normal conditions, your skin, blood vessels and perspiration levels adjust to the heat to create your body’s natural cooling mechanism. To regulate your body temperature, heat gain or loss is controlled by the autonomic nervous system’s alteration of heat flow from the body’s core to the skin via surface blood-flow and sweating. During exercise, your body temperature initially raises to allow the muscles to optimally function (warm up) until heat production reaches an optimal level, then the bodies thermoregulatory system begins to expel heat until production of heat equals loss of heat via sweating and increased blood flow to the skin (Sawka).
The graphic below is an example of the heat forces being forced on the body during exercise and the body’s cooling responses pushing the heat off the body.
Despite this highly effective cooling mechanism, during exercise the muscular system requires an influx of blood to allow the activity to be performed. At this point, the muscles call for more blood, and in normal humans the needs of the muscle outweigh the needs of the skin, so the blood is rushed into the muscles. The body goes into a complicated interplay of cooling the skin via blood and movement created by blood vessels and their need for blood. The large amount of blood needed to perform all of these activities creates a cardiovascular strain which results from the reduction in cardiac output (less blood circulating from the heart to the body), skin and locomotor muscle blood flow decrease, and less oxygen being delivered to the skin and muscles which causes decreased ability to perform the exercise to its fullest potential and a decrease in the body’s cooling ability (Gonzalez-Alonso, 45-53).
This highly organized cooling mechanism is further stressed in the heat by the rapid onset of dehydration and hyperthermia (increased body temperature). If the body is not properly hydrated and cooled, your body will progress through the following stages of heat-illness:
Heat Cramps are painful cramps in the stomach, arm and leg muscles that are caused by not replacing salt and fluids during prolonged and/or intense exercise in the heat.
Heat Syncope is weakness, fatigue and fainting that are caused by too much salt and water loss through perspiration during exercise in the heat. This is normally caused during the first 5 days of adjusting to a new activity, and can be complicated by taking diuretic medications or a previous heat-related illness.
Treatment: Move the person to a cool and shaded area, remove tight clothing, give fluids, begin cooling measures (such as a fan or ice towels), and referral to physician to assess need for fluid/electrolyte replacement or further medical attention.
Heat exhaustion due to water depletion is brought on by heavy sweating and will result in extreme weight loss, and sets in as perspiration decreases, while skin and body temperature increases. Core body temperature typically raises to 104°F.
Treatment: Move the person to a cool and shaded area, remove tight clothing, give fluids, begin cooling measures (such as a fan or ice towels), and referral to physician to assess need for fluid replacement or further medical attention.
Heat exhaustion due to salt depletion is brought on by heavy sweating without sufficiently replacing normal body salts and minerals. Common signs include nausea, vomiting, frequent muscle cramps, and dizziness with a core body temperature rise to 104°F.
Treatment: Give electrolyte fluids, move to a cool and shaded area, apply active cooling measures and refer to a physician for electrolyte replacement and further medical attention, especially if nausea and vomiting occur.
Heat Stroke is the most severe form of heat illness. Heat stroke is medically defined as a core body temperature greater than 104°F. This is a medical emergency that can result in organ failure. Nausea, seizures, and disorientation or confusion are common symptoms and can progress to unconsciousness or coma. Heat stroke can be a progression from heat syncope and heat exhaustion or can occur without any preceding symptoms.
Monitor core body temperature and lower as quickly as possible
Decrease core body temperature through immersion in an ice bath if possible
Remove as much clothing as possible
Apply ice packs to armpits, groin, and neck areas
Continue cooling efforts until Emergency services arrive.
As you can imagine, the best way to deal with these heat related illnesses is to avoid them all together by following these recommendations:
Recommendations to Avoid Heat-Related Illnesses
Make sure you are well hydrated before, during, and after exercise by drinking 24 ounces of non-caffeinated fluid 2 hours before exercise, an additional 8 ounces right before your workout and 8 more ounces every 20 minutes during exercise.
Replace your fluids even if you’re not thirsty
Allow for frequent rest and re-hydration breaks
Weigh yourself before and after exercise to ensure proper fluid replenishment
Wear light-colored, light-weight clothing
Monitor the temperature and avoid the hottest parts of the day
Gradually increase activity over a 7-10 day period in the summer heat to acclimatize to the weather.
Avoid activity on dark colored surfaces in the midday, as the asphalt temperature can be 50° or more higher than the air temperature, which will speed your intolerance to the heat.
Tailor your workout to your fitness level and the outdoor temperature, such as shortening the workout, lowering the intensity, and/or changing the time of day.
Understand your medical risks. People with cardiac and blood pressure issues in their history, the elderly, and children are at risk of a more rapid progression toward heat-related illnesses and a more rapid progression through the illnesses.
Wear sunscreen. A sun burn will decrease your body’s ability to cool itself.
Have a backup plan and change the workout to indoors, such as a walk in the mall, go to the gym, or workout in the pool to avoid the intense heat.
Despite all the above precautions, hot-weather workouts can quickly turn into a heat-related illness, so it is imperative that you remain on watch for the following warning signs of heat related illness:
If these symptoms occur, stop exercising immediately and get to a cool place to hydrate. If the symptoms continue, follow the treatments for the heat-related illnesses above. Remember: If your core temperature approaches 104°F, consult a physician immediately.
Despite the risks associated with exercising in the summer heat, we will all still want to and should be outdoors in the summer. With the proper precautions, summer exercising can be some of the most fun and most productive time to exercise. If you are cautious and aware of heat-related symptoms, you can avoid any heat-related illness and enjoy your summer exercise programs.
Gonzalez-Alonso, J et al. “The Cardiovascular Challenge of exercising in the Heat” J Physiology. 2008 Jan 1; 586(pt 1): 45-53.
Jones, C et al. “Training Load and Fatigue Marker Associations with Injury and Illness: A Systematic Review of Longitudinal Studies.” Sports Med. 2017; 46(5): 943-974
Mayo Clinic Staff. “Heat and exercise: Keeping cool in hot weather.” https://www.mayoclinic.org/healthy-lifestyle/fitness/in-depth/exercsie/art-20048167.
McArdle, WD et al. Exercise Physiology. 4th ed. Williams and Wilkins, Philadelphia. 1996: 501-525.
Sawka, MN et al. “Physiological Responses to Exercise in the Heat” National Academy of Sciences, Washington DC. 1993. Ch 3.