Physical Exercises in Hot or Humid Environments

The normal human body temperature is 98.6°F. During vigorous activity, the body produces large amount s of heat, which must be dissipated to keep the body temperature regulated. The body has several ways to dissipate heat. Conduction is the transfer of heat from a hot body to a cold body. Convection is the transfer of heat through the air or other medium. Fans and wind can facilitate heat loss by convection and help regulate temperature. The primary method of cooling is through evaporation of sweat. The chemical process involved in evaporation transfer heat from the body and reduces the body temperature. When conditions are humid, the effectiveness of evaporation is reduced since the air is already saturated with moisture. This is why it is difficult to regulate body temperature when conditions are both hot and humid.

   Maximum sweat rates during physical activity in the heat can approach 1-2 liters per hour. If this fluid is not replaced dehydration can occur. If dehydration is not corrected with water or other fluid-replacement drinks, it becomes increasingly more difficult for the body to maintain normal body temperatures. At some point, the rate of sweating decreases as the body beings to try to conserve its remaining water. It attempts to shunt blood to the skin to transfer excess heat directly to the environment, but this is less effective than evaporation, and various heat-related problems including heat stroke and hyperthermia can result.

   One way to monitor the amount of fluid loss is to monitor the color of your urine. The American College of Sports Medicine indicates that clear (almost colorless) urine produced in large volumes indicates that you are hydrated and ready for activity. Dark yellow urine produced in small volume is a good indicator of dehydration and need for fluid replacement.

   Individuals with good fitness will respond better to activity in the heat than individuals with poor fitness. This is because the ability to sweat improves with training. With regular exposure to the heat, the body becomes conditioned to sweat earlier, to sweat more profusely, and to distribute sweat more effectively around the body. This process of acclimatization makes it easier for the body to maintain a safe body temperature.

   Limit or avoid physical activity in hot or humid environments. The apparent temperature is a combined value determined by both temperature and humidity. When the apparent temperature is below 90°F (32.2°C), exercise is safe for most people. Caution should be used when exercising at apparent temperatures ranging from 90° to 100°F(37.7°C). above 100°F, apparent temperature is the danger zone, and physical activity should be done with extreme care, be limited, or be canceled. Experienced exercisers who have become acclimatized to the heat may be able to perform at higher apparent temperatures than those who are less experienced. However care should be used by all people who perform physical activity in hot and humid environments.

   Replace fluids regularly. Drink water before (2 cups or 16 ounces) and during activity (1 cup or 5-10 ounces every 15 to 20 minutes). After activity, replacing 16 ounces of fluid for each pound of weight lost is a good rule. For exercise lasting more than 1 hour, fluid replacement drinks containing simple carbohydrates (glucose, fructose, or sucrose) and electrolytes are considered beneficial to performance and body cooling. If the concentration of sugars is no more than 4-8 percent, they can replace fluids as quickly as water.

Tips to do exercise in heat and humid condition
  • Gradually expose yourself to physical activity in hot and humid environments. Too much at once is especially dangerous.

  • When possible, do your activity in the morning or evening.

  • Dress properly for exercise in the heat and humidity. Wear white or light colors that reflect rather than absorb heat. Porous clothing allows the passage of air to cool the body. Rubber, plastic, or other nonporous clothing is especially dangerous. A porous hat or cap can help when exercising in direct sunlight.

  • Do not change your wet shirt for a dry one. A wet shirt cools the body better.

  • Rest at regular intervals, preferably in the shade.

  • Watch for signs of heat stress. If signs are present, stop immediately.

  • Take these steps: stop physical activity ; get out of the heat and into the shade; remove excess clothing; drink cool water; immerse the body in cool water; if symptoms of heat stroke are present, seek immediate medical attention; and statically stretch cramped muscles.