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Heat Stress Risks and Exercise Guidelines


Updated June 05, 2014

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Tim Boyle/Getty Images News

Summertime brings a special training and racing caution: heat-stress risks. You want to know what to do before, during and after unusually hot/humid conditions. Know the signs of heat stress, heat exhaustion and heat stroke. It can kill. Know, too, that the higher the heat-index, the slower will be your "racing pace", and that's normal. We're human; we're not super-human.

Our activities are for enhancing our health and fitness, not impairing them or worse. Some macho types think that more suffering in the sun, heat and humidity, even withholding from themselves fluids, is going to toughen them to the conditions. Unless you're training for what you know will be a hot/humid race for which you need to acclimate, avoid training or hard racing in risky conditions.

Factors affecting our risk of heat illness:

  1. Metabolic rate: Higher intensity (or speed) of exercise, the higher our core body temperature becomes over time of exercise. That's why short sprint events in hot weather are much less risky than endurance events. One measure of metabolism is heat (calories) expended.
  2. Temperature:Environmental temps affect the temperature gradient.
  3. Hydration: We must be able to freely perspire to enjoy the cooling effect of evaporation. Dehydration is dangerous!
  4. Humidity: Evaporation rate (therefore, cooling rate) is reduced as the relative humidity rises. See more on humidity below.
  5. Clothing: Our clothes must not hinder the heat-loss mechanism of radiation. We don't want to stop heat from being transferred from our skin surface to the environment.
  6. Fitness: The unfit, and some special populations like children, will not be ready or able to effect the cooling mechanism of adequate perspiration. The obese also will also be less able to enjoy heat-loss, as they wear more naturally insulating adipose tissue like excess clothing.
  7. Medical Risks: General effects of aging. Alcohol or other drug abuse. Chronic illness, such as diabetes or blood-vessel disease. Recent illness involving fluid loss from vomiting or diarrhea.
  8. Acclimatization: Some of us can train ourselves to be more tolerant of heat, by improving our physiological heat-loss mechanisms.

Next page: > Heat Index and Heat Factors to Consider

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