Running in The Heat

by Patti and Warren Finke, Team Oregon
"It was 85 degrees and 90% relative humidity. I remember trying to stay with Bill Rodgers but being unable...feeling funny...then I woke up in a tub of ice. I lost 20 or 30 minutes. My body temperature was over 108." - 1994 Comrades Marathon Champion Alberto Salazar discussing his life threatening heat stroke episode at the 1980 Falmouth Road Race.

Performance may be influenced tremendously by temperature. As air temperature rises, the combination of environmental heat and increased body heat from exercise may result in adverse effects ranging from decreased performance to death. The best defense the runner has is to prepare himself for warm weather conditions and understand how to recognize and deal with the effects of heat. For runners who may compete in hot weather it is critical for performance and safety to develop and maintain heat acclimatization.

The human body is able to maintain a fairly constant temperature under varying environmental conditions. To do this, it must be able to gain or lose heat. The core temperature is regulated to remain relatively constant, but the temperature of the shell, the skin and the tissues directly beneath it, varies directly with environmental conditions. The hypothalamus in the brain controls the body temperature and calls into play either heat loss or heat production mechanisms. Regulation comes in response to changes in the skin or blood temperature.

Normal metabolism in the body produces heat. Increased heat production can come from higher metabolic rates, disease, shivering or exercise. During exercise, the increased metabolic rate and energy production both generate heat. Most of the heat gain is due to the lack of efficiency of the body. It converts only 20-30% of energy produced into work; the rest is dissipated as heat. Heat gain or loss is governed by the following physical means:

  • Conduction - transfer of heat to or from the body by direct physical contact.
  • Convection - transfer of heat by movement of air or water over the body.
  • Radiation - radiation of heat from the body into space and absorption of radiation, (sunlight) on the bodies surface.
  • Evaporation - loss of heat by the body when converting sweat to vapor.

    In a cold or cool environment, conduction and convection, along with some evaporation of sweat, can maintain the heat balance. As the temperature rises, evaporation of sweat becomes the main way of controlling the rise in core temperature. Evaporation can keep the body's exercising temperature in the normal range of 102-105 F under normal environmental circumstances.


    Several studies have shown that the optimum temperature for long distance running performance seems to be around 50-55 degrees Fahrenheit. Above and below this range performances degrade as much as 2% for every 5 degrees. Three additional environmental factors can interact to alter performance further. They are relative humidity, air movement, and radiation.

    High humidity, because it inhibits evaporation, has the same effect as increasing the ambient temperature. This effect is worse for higher temperatures where it can increase the effective ambient temperature by as much as 10 degrees.

    Air movement over the body enhances the ability to lose heat by convection and evaporation. Movement is generated both by the runners speed and by any prevailing wind. These can combine to lower the effective temperature by as much as 8 or 9 degrees while increasing evaporation and fluid loss. Running downwind cancels out this cooling effect.

    Direct sunlight adds heat to the body by radiation. The effective temperature increase can be as much as 8 or 9 degrees.

    It is easy to see that by combining 80+ degree temperatures with direct sun exposure and high humidity serious performance degradation will occur in long distance races.


    Special caution should be advised when the temperature is above 80 F or when the relative humidity exceeds 50-60%.

    Running unwisely under environmental heat stress may lead to a variety of heat illnesses which can be life threatening. These illnesses are caused by three factors: increased core temperature, loss of body fluids, and loss of electrolytes. While running in the heat, monitor your condition for signs of weakness, dizziness, nausea, disorientation, cessation of sweating and piloerection, (the standing up of body hairs). If these signs occur, stop running and start the appropriate treatment. They could be symptoms of any of the major heat illnesses described below.

    Heat Cramps

    Salts can be lost in the sweat while running in the heat. If they are not replenished properly, muscle pain and cramps can occur. The body temperature does not become elevated. Prevention can come from heat acclimatization, ingestion of large amounts of water and by increasing the daily salt intake several days before the heat stress. Treatment is rest in a cool environment and replacement of lost salts.

    Heat Exhaustion

    Poor circulatory response to heat and reduction of blood volume due to increased sweating can lead to symptoms of general weakness, dizziness and nausea. The skin is usually cool and pale, but the person is probably still sweating. Body temperature is not elevated to dangerous levels (under 106F). Exercise must be stopped. Treat by rest in a cool environment, ingestion of cool liquids and cooling the body externally with water or ice.

    Heat Stroke

    When the body's temperature regulating system fails, excessively high body temperature and heat stroke can result. This is a serious condition which, if untreated, may well lead to death. It requires IMMEDIATE medical attention. The symptoms are dry, warm and red skin, a reduction or loss of sweat and a body core temperature over 106F. Treatment is to immediately stop exercise, seek medical attention and start cooling the body with ice packs and cold water. The person may or may not be conscious. Cool liquids may be consumed if the person is conscious.


    There are ways to reduce hazards when running in the heat and/or humidity, most are common sense:

    Check the conditions before exercising and adjust your plan if needed. Slow the pace or decrease the duration of activity if training when hot or humid. If racing when hot and humid, realize that performance will less than expected. If the event is not a key one, relax and save the bigger effort for a cooler day.

    Run in the early morning or late evening to avoid the heat of the day. In many climates, late afternoon is the hottest time of the day and running then should be avoided.

    Find a shady road or trail to run on.

    Dress accordingly, wear as few clothes as you decently can. Try loose fitting white shorts and a white mesh top to reflect the heat and to permit evaporation. Protect your head from intense sun with a lightweight hat that can breathe. The back of the neck can be protected by the hat or a cotton kerchief. Ice can be wrapped in the kerchief and carried under the hat.

    Drink fluids while running. Carry a water bottle or pick a route with water fountains. Drink 6-8 oz. of water for every 15-20 minutes of running. Also pour water over your head and chest. Dehydration has been shown to adversely effect performance after as little as 45 minutes of activity.

    Weigh yourself after workouts and replenish lost water at the rate of 1 pint per pound of weight loss. Body weight should be back to normal before the next workout.

    Try hyperhydration by drinking 2-4 cups of water 30 minutes before running.

    Be aware of lost electrolytes if you've sweated excessively. Put an appropriate amount salt on foods and eat bananas and citrus fruit.

    Avoid excess protein intake. Protein metabolism produces extra heat.

    Know the signs and symptoms of heat illness and their treatments. If you have any of the symptoms, stop running, get to a cool place and consume cold fluids.

    If you are going to compete in an event in hot conditions, acclimatize first.


    With proper acclimatization the body can perform as if it were in temperatures 10 to 15 degrees cooler. Acclimatization is the process of adapting your body to be able to run more efficiently under hot environmental conditions. When it is hot the blood goes to the skin for cooling the body as well as to the working muscles. This increases the workload of the heart and the exercising heart rate. Intensity of exercise will need to be reduced when running in the heat and when acclimatizing for proper adaptation.

    The body makes several adjustments during the heat acclimatization process. The circulatory adaptations to acclimatization provide better transport of heat from the core to the skin. There is better distribution of the blood to regulate temperature. This frees a greater portion of the heart output for the working muscles. Sweating mechanisms undergo complementary changes. Sweating starts at a lower body temperature and the capacity for sweating nearly doubles. The sweat becomes more dilute, contains less salt, and is more evenly distributed over the skin. Major changes occur during the first week of heat exposure and are mostly complete after 10 days.

    Heat acclimatization can also be lost in 10 days. This is why it is important to wear extra clothing during unusually cool summer weather, ( like the summer of 1993 in the northwest ). You should try to maintain acclimatization for typical hot weather conditions which could occur on short notice at your next race.

    Ways to Acclimatize

    Begin early in the season when the temperature is moderate and wear one more layer of clothing than usual on 3 runs per week. If you would normally wear a T-shirt wear a long sleeved one or a jacket. If you would normally wear shorts, wear cross training shorts or tights. This early constant acclimatization works well in climates such as in Oregon where the weather is often unpredictable and occasional hot days are experienced relatively early in the year.

    To develop and maintain acclimatization in weather that is unseasonably cool or in preparing for a race in a warmer climate assume that each layer of dry clothing or degree of coverage, (i.e. going from short to long sleeved shirt or from shorts to tights), is equivalent to 15 or 20 degrees in temperature.

    Adding a waterproof jacket such as Tyvec provides a hot, humid microatmosphere and prevents evaporation which would normally cool you once your clothes became wet.

    If the weather suddenly turns hot, reduce the training load; run slower and less distance. Slowly build back up to usual mileage and intensity. Work on heat acclimatization every other day and make certain to replace lost fluids. Run in the cooler part of the day on the nonacclimatization days. Do not overdo and get heat symptoms.

    If you plan to race under hot conditions, remember that acclimatization takes about 10 days. Plan to be acclimatized a week in advance. During the week before the event, avoid extra heat stress which may dehydrate and fatigue you for the race.


    Optimal performance depends on proper hydration. Dehydration or excessive loss of body water reduces the amount of time you can exercise as well as necessitating slowing down. Changes that take place at the cellular level adversely effect muscle contraction. Water losses of 2% or more of body weight impair circulatory function, create heat imbalance and degrade performance. You should drink 6-8 ounces of fluid every 15-20 minutes during exercise. You can also hyperhydrate by drinking 2-4 cups of cold fluid 15-30 minutes before exercise.

    Sweat is comprised mainly of water and sodium and chloride ions. These ions are known as electrolytes. Other electrolytes are also present in small amounts. Studies of electrolyte balance during and after exercise have shown increases in the electrolytes in the blood, but these changes are probably due to water loss and muscle use.

    There is some evidence that glucose electrolyte solutions (sports drinks) help replenish body water better than plain water. While the electrolytes are probably not necessary for replacement in runs of marathon length or shorter, they may improve fluid absorption by the body and encourage further drinking by stimulating the bodies thirst mechanism. In runs longer than 90 minutes, the carbohydrate in sports drinks helps spare liver glycogen depletion. For optimum absorption drinks should contain 5 to 10% carbohydrate (glucose or sucrose) and should be cool (40-50 degrees F).

    If running in the heat for several consecutive days, try to replace fluids and eat a balanced diet. Add salt to foods and select foods high in potassium such as bananas and citrus fruits. Salt tablets are unnecessary and may be harmful when not taken with adequate water.

    Team Oregon Running Tips are Copyrighted by wY'east Consulting and Team Oregon which reserve all rights to republication.