Guidelines for exercising in heat and humidity have been developed for road races, but these guidelines can be applied to any strenuous physical activity performed outdoors during warm weather. Ambient conditions are considered safe when the temperature is below 70 °F and the humidity is below 60%. Caution should be used and people sensitive to heat and humidity should reconsider exercising when the temperature is greater than 80 °F or the humidity is over 60%. People who are trained and heat acclimated can continue to exercise in these conditions, but they should be aware of the potential hazards and take precautions to prevent heat illness.
The keys to exercising without incident in hot weather are acclimating to the heat and maintaining the body’s normal fluid level. Acclimation to heat is characterized by physiological adjustments that occur naturally from repeated exposure to exercise in the heat. Acclimation includes the early onset of sweating, an increase in the rate of sweating, and the reduction of sodium in sweat. These adjustments result in less cardiovascular strain and a lower body temperature for a specific amount of exercise. Most healthy people become fully acclimated to heat in 10 to 14 days. The main consequence of dehydration (excessive fluid loss) is a reduction in blood volume. This results in sluggish circulation, which decreases the delivery of oxygen to the exercising muscles. Lowered blood volume results in less blood that can be sent to the skin to remove the heat generated by exercise. If too much of the blood volume is lost, sweating stops and the body temperature rises, leading to heat-stress illness. Heat stress illness is a serious problem that can be avoided by following these guidelines designed to preserve the body’s fluid level:
Estimating Water Loss
- Weigh yourself nude before and after exercise.
- Towel off sweat completely after exercise and then weigh yourself.
- Each pound of weight loss represents about 1 pint of fluid loss. Be sure to drink that and more after exercise.
- Modify the exercise program by (1) working out during cooler times of day, (2) choosing shady routes where water is available, (3) slowing the pace or shortening the duration of exercise on particularly oppressive days, and (4) wearing light, loose, porous clothing to facilitate the evaporation of sweat.
- Never take salt tablets. They are stomach irritants, they attract fluid to the gut, they sometimes pass through the digestive system undissolved, and they may perforate the stomach lining.
- Exercise must be prolonged, produce profuse sweating, and occur over a number of consecutive days to reduce potassium stores. For the average bout of exercise, you do not need to worry about depleting potassium or make a special effort to replace it. The daily consumption of fresh fruits and vegetables, as suggested by the food pyramid, is all that is needed.
- Remember to use a sunscreen lotion when the weather is sunny or hazy. Be sure that the sunscreen you select has a sun protection factor (SPF) of at least 15, and apply it liberally over exposed skin.
Guidelines for Exercise in the Cold
Problems related to exercise in cold weather include frostbite and hypothermia (abnormally low body temperature). Frostbite can lead to permanent damage or loss of a body part from gangrene. This can be prevented by adequately protecting exposed areas, such as fingers, nose, ears, facial skin, and toes. Gloves, preferably mittens or thick socks, should be worn to protect the fingers, hands, and wrists. Blood vessels in the scalp do not constrict effectively, so a significant amount of heat is lost if a head covering is not worn. A stocking type hat is the best head covering because it can be pulled down to protect the ears. In very cold or windy weather, use surgical or ski masks and scarves to keep facial skin warm and to moisten and warm inhaled air. All exposed or poorly protected flesh is vulnerable to frostbite when the temperature is low and the windchill high. Air temperature plus wind speed equals the windchill index. This value will help you know how to dress appropriately for outdoor exercise.
People often experience a hacking cough for a minute or two after physical exertion in cold weather. This is a normal response and should not cause alarm. Very cold, dry air may not be fully moistened when it is inhaled rapidly and in large volumes during exercise, so the lining of the throat dries out. When exercise is discontinued, the respiratory rate slows and the volume of inhaled air decreases, allowing enough time for the body to fully moisturize it. Coughing stops within a couple of minutes as the linings are remoistened.
Hypothermia is the most severe of the problems associated with outdoor activity in cold weather. Hypothermia occurs when body heat is lost faster than it can be produced. This can be a life-threatening situation. The adjustments made by the body to avoid excessive heat loss include shivering, nonshivering thermogenesis, and peripheral vasoconstriction. Shivering is the involuntary contraction of muscles. These contractions increase the body’s heat production by four to five times that produced under normal resting conditions. Nonshivering thermogenesis raises body temperature through neural stimulation that increases metabolic rate. Peripheral vasoconstriction occurs from the neurally mediated contraction of smooth muscles located subcutaneously (beneath the skin). The contractions of these muscles constrict the small arteries beneath the skin, leading to decreased blood flow to the skin. This adjustment prevents unnecessary heat loss. However, hypothermia may still occur because these adjustments can be overcome by excessive exposure to cold.
Exercise in cold weather requires insulating layers of clothing to preserve normal body heat. Without this protection, body heat is quickly lost because of the large temperature gradient between the skin and environment. A layer or two of insulating clothing can be discarded if you get too hot.
Hypothermia can occur even if the air temperature is above freezing. The rate of heat loss for any temperature is influenced by wind velocity. Wind velocity increases the amount of cold air molecules that come in contact with the skin. The more cold molecules, the more effective the heat loss. The speed of walking, jogging, or cycling into the wind must be added to the speed of the wind to properly evaluate the impact of windchill.
You should wear enough clothing to stay warm, but not so much as to induce profuse sweating. Knowing how much clothing to wear comes from experience exercising in various environmental conditions. Clothing that becomes wet with sweat loses its insulating qualities. It becomes a conductor of heat, moving heat from the body quickly and potentially endangering the exerciser.
If you exercise or work outdoors in cold weather, you may want to wear polypropylene undergarments. Polypropylene is designed to whisk perspiration away from the skin, so that evaporative cooling does not rob heat from the body. You should wear a warm outer garment, preferably made of wool, over this material. If it is windy, wear a breathable windbreaker as the third, outer layer.
If you follow the guidelines for activity in hot and cold weather, you can usually participate comfortably all year long. Other hazards associated with outdoor exercise are discussed in Real-World Wellness: Exercising Safely in an Urban Environment.
Fluid Consumption Before, During, and After Exercise
The American College of Sports Medicine has issued the following recommendations about fluid consumption:
- Make a special effort to drink plenty of fluid every day, so that you will be fully hydrated prior to exercise.
- The daily water needs of most moderately fit active people range between 3 and 5 liters (a little more than 3 to 5 quarts).
- It is a common occurrence that fluid losses exceed fluid replacement during intense physical exercise for a variety of reasons.
- Drink fluids during exercise that contain carbohydrates (sugars) and sodium because these will enhance performance and delay fatigue more effectively than an equal amount of plain water for exercises lasting 45 to 50 minutes or during high-intensity intermittent exercises.
- It is imperative to consume carbohydrate/sodium fluids during prolonged physical activity. Replacing with plain water, combined with sweat loss, reduces blood levels of sodium. If the sodium deficit becomes excessive, it will lead to exertional hyponatremia. The symptoms of hyponatremia include progressively worsening headache, confusion, disorientation, nausea, vomiting, aphasia (impairment of speech or understanding speech), muscle cramps, and muscle weakness. It sometimes results in death.
6 . Women are at greater risk of incurring exertional hyponatremia because their fluid intake is more likely to exceed their sweat rate and because they have less body water than males and a smaller body mass that is more readily affected by overdrinking.
- The general rule after exercise is to drink until thirst is satisfied and then drink a bit more in order to satisfy your tissue needs.
- Rehydration after exercise requires fluid replacement of 125 to 150% of the loss of body mass (weight) during exercise. Each pound lost during exercise represents the loss of one pint of fluid. The loss of 3 pounds of body mass would require the consumption of 3.75 pints (125%) to 4.5 pints (150%) of fluid. The replacement fluid should be tasty and contain sugar, sodium, and possibly potassium and magnesium in amounts that can be found, for example, in sports drinks.
- Caffeine, alcohol, and protein can modestly increase urine water loss and should not be consumed immediately after exercise. This is counterproductive because rapid and complete hydration is desirable at this point.