Know the facts about heat-related illness

Originally published in The Ottawa Citizen May 27, 2003
Original Title: Don’t sweat the small stuff

You can prevent heat-related illness with an understanding of how summer heat affects your body.

First, the biology lesson. The body has four ways to dissipate heat: conduction, evaporation, radiation and convection.

Conduction is the transmission of heat through a substance like blood, water or other tissues. Muscles that are warm from exercise can dissipate heat directly to the skin surface.

Blood can absorb great quantities of heat from the muscles and other tissues. It will return to the heart and then circulate to the small blood vessels in the skin. During exercise, the blood vessels dilate to allow greater quantities of blood to transfer heat to the skin surface.

The skin will radiate heat into the surrounding air and environment just like a space-heater. Sweat on the skin surface can absorb the heat and evaporate to reduce body temperature.

As the air warms around the body, it will rise. Cooler air moves in to replace it and absorbs body heat. This cycle is called convection and explains why fans help cool us.

Each mechanism works best within a specific temperature range. At temperatures less than 20C, radiation, convection and conduction will dissipate most generated body heat. Above 20C, sweat evaporation is the primary means of dissipating heat.

Children do not sweat as much as adults and produce more heat for the same level of activity. They need to generate greater levels of heat before they do sweat.

Overweight individuals do not dissipate heat as well as those of normal weight. The elderly have a decreased thirst response, and a reduced ability to circulate blood to the skin surface. Their blood vessels do not dilate as well as younger adults. Some medications can contribute to the risk of heat illness.

As temperature and humidity increase, evaporation becomes less effective. On a hot city day, core body temperatures increase because of radiant heat from the sun and hot concrete surfaces.

Evaporation accounts for 85 per cent of heat loss during vigorous exercise (a 70 kilogram athlete can lose one to two litres of sweat per hour).

Failing to replace water and salt loss further compromises conduction and evaporation.

Adapting to the effects of heat during exercise over a specific time is termed acclimatization — adapting to the increased demand to dissipate heat. Fluid replacement is essential for this process to work.

There are five types of heat-related illness. From mild to severe they are:

  1. heat swelling (edema);
  2. heat cramps;
  3. fainting from heat (heat syncope);
  4. heat exhaustion;
  5. heat stroke.

Heat edema occurs in people who have not undergone acclimatization. Fluid leaks out into the tissues of the feet especially when standing for prolonged periods. To reverse the process, elevate your legs.

Heat cramps are painful abdominal, arm or leg muscle spasms occurring when too much salt and water is lost. This is a warning sign of pending heat exhaustion. Drinking water, juice or sport drinks and eating salty foods will relieve the cramps.

Fainting is a risk if there is no cool-down period after exercise. Blood pressure can drop when quickly transferring from a sitting to standing position. Lay flat and elevate your legs to reverse this condition.

Heat exhaustion occurs with excessive sweating in a hot humid environment. Body fluid volume is lost. The core body temperature increases from 38C to 40.5C. Symptoms include profuse sweating, fatigue, headache, dizziness, visual disturbances, lack of appetite, nausea, vomiting, vertigo, chills, muscle weakness, rapid heart rate, low blood pressure and skin flushing.

Move a person with heat exhaustion to a cool area. Applying cool water-soaked cloths helps. Elevate the legs. Those who are alert need one litre of oral fluid replacement per hour for two to three hours. If the person is disoriented or unresponsive, a trip to the emergency room and a thorough medical examination is in order.

Heat stroke is the most severe form of heat-related illness. Body temperature exceeds 40.5C and leads to multi-organ damage and failure. Altered mental status is a critical determinant of heat stroke. This medical emergency needs prompt evaluation and treatment.

Preventing heat-related illness is straightforward:

  • Stay in air-conditioned surroundings if possible.
  • Drink lots of water before, during and after any outdoor activity.
  • Avoid drinks with caffeine or alcohol. You will lose fluid via urination.
  • Increase the amount of time you spend outdoors every day little by little.
  • Take frequent rest breaks while outdoors on hot days.
  • Avoid direct sunlight and stay in the shade when possible.
  • Wear light-coloured, loose-fitting, open-weave clothes.
  • Avoid activities that require using a helmet.
  • Try scheduling activities or workouts early in the morning or late evening. Avoid heavy outdoor activity between 10 a.m. and 6 p.m.

Enjoy your summer.


© Dr. Barry Dworkin 2003

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