Search below to find additional information or return to the sports medicine main page to browse the index.


We comply with the HONcode standard for trustworthy health
verify here.

Running In The Heat & Heat Related Injury

by Stephen M. Pribut, D.P.M.


Summers are getting warmer. Records are being set while the polar icecaps melt. But that doesn't seem to make us any cooler. Running in the heat requires wisdom and caution. We'll try to provide information to keep you safe this summer. The table and javascript heat index illustrate what it Image: Blazing Sunfeels like at 82 degrees with 60 percent humidity which is probably not very good. The best defense against most heat injury while running short to moderate distances is hydration. So drink up. In general you should drink about 6 - 8 glasses of water per day under normal circumstances. Always drink before you run and drink about 4-6 oz. every 20 minutes while you run. Water is just fine for runs of up to an hour, but you may find that sports drinks will help both your recovery and your performance for runs over one hour. Most popular sport drinks have a low level of electrolytes and also contain carbohydrates (both simple and more complex polymers) to help speed up glycogen replacement. Caffeine may improve performance, and may not be as strong a diuretic as once though so you may not want to skip it, but make sure you get adequate fluids and don't use beer to fuel your run.


Try a protein-carb mix to more rapidly replenish your glycogen stores after a long run. But don't forget your overall fluid replacement needs should be met with a sports drink or water.

On long runs and competitive events such as marathons and ultra events, hyponatremia is a significant and serious danger. You must be careful not to over hydrate. Weighing yourself along a course will keep you from weighing more when you finish and being at significant danger of hyperthermia (see more below).

Immediately following exercise the muscles are able to replenish glycogen stores at a much higher level than a few hours later, so a sports drink or a fluid source such as water with another source of carbohydrates for glycogen replenishment would be fine. Don't forget that if you are using Sport Gels or Bars for carbohydrates drink 4 oz to 8 oz of water with them to aid your digestion and assist in their absorption. There is evidence that a protein-carbohydrate drink may assist in glycogen replacement better than carbohydrates alone. This also assists in preventing the body from eating or catabolizing its muscle for energy and will help your muscles to repair themselves by having amino acids available for protein synthesis. One way to get a protein-carb mix would be to mix whey protein (which weight lifters love) and fruit juice (or orange juice). Some whey protein comes in flavors such as praline, vanilla, chocolate, but the taste would be called an acquired taste. However, if you can down those yucky gels, you can probably get this stuff down also and it could help build lean and strong muscles and is definitely good for a recovery from your long run. You can also mix it with low fat milk and a chocolate milk mix for added flavor and carbs. The choclate milk mix I use has less than 5% fat and no cholesterol. It does help it go down and after a workout or long run actually tastes good. But don't forget that water and sports replacement drinks are the best way to replenish your fluid deficit. This protein-carb mix is to help replenish your glycogen loss and speed your muscular recovery from a hard or long workout.

Read "Dr. Pribut's Top Ten Summertime Running Tips"

To help you stay hydrated during this long, hot summer of running you might consider stopping often for water. Don't forget to give yourself a few weeks to acclimate to the heat. This is best done by running 3 to 4 miles in the heat, gradually building up your mileage before you try for speed. Avoid the mid-day hours if at all possible by running early in the morning or late at night. You can consider using a treadmill on the worst days. I have on occasion mixed a warm up and interval repeats on the treadmill with an outside longer and slower run. This way you can get a workout and a few more miles in a cooler environment.

Try combining treadmill running and outside running to get more distance on the really hot days.


And now on to the facts.

Heat Production:

During exercise, heat is produced in direct relationship to the amount of energy expended. In running, this results in speed being the primary determinant of body heat production. Elite runners would run at an energy level expenditure of 1500 kcal/hr while a novice runner would be likely to run at an energy level expenditure rate of 500 kcal/hr. Body metabolism as a means of energy production is estimated to be only 25 percent efficient. That leaves 75 percent of the energy used going solely to the production of heat. That is great for cold weather, but not of much help to hot weather runners.

The radiant energy of the sun may be responsible for up to 150 kcal/hr on a clear and cloudless day. Heat production alone would thereby give an average runner a heat burden of approximately 1000 to 1200 kcal/hr. If there were no means of heat dissipation the core temperature would rise by approximately 23 degrees. In other words, if there were no functioning thermoregulation, the body temperature would rise approximately 1 degree centigrade for every 5 to 8 minutes of running.

Heat Loss:

“An individual who is heat acclimated may perspire almost twice as much as an unacclimated individual.”
As we have already mentioned, perspiration and evaporation of perspiration, is the primary means for the body to cool during exercise. Skin blood flow increases significantly during exercise. Blood flowing near the surface results in cooling by both conduction and convection. There appears to be a 25 to 40 percent lower increase in Skin Blood Flow, however, in the older athlete. Sweat glands become active as body core temperature rises. One liter of sweat is generated during the expenditure of about 500 kcal. Studies have shown that training increases both sweating and skin blood flow.

An individual who is heat acclimated may perspire almost twice as much as an unacclimatized individual. One of the effects of acclimatization is to allow an individual to begin perspiration earlier in the course of exercise, this allows for a quick, effective and efficient beginning to heat dissipation and alleviation of early heat buildup. Each gram of perspiration that evaporates cools the body by 0.6 kcal. Acclimatized individuals may produce up to 30 g/minute of perspiration. This would allow for considerable cooling.

The highest concentration of sweat glands are in the palms of our hands and the soles of our feet. There are between 2 and 4 million sweat glands over our entire body. Each sweat gland is small and weighs little, but all together they weigh as much as one kidney, about 100 grams. An individual can perspire up to several liters per hour and up to 10 liters per day.

The sole actually has the most sweat glands with 620/cm2. The back has the least with64/cm2. The soles of the feet alone have between 250,000 to 300,000 sweat glands in total.

As we noted, the environment has a major impact on heat loss. Humidity, playing the largest role, but temperature, of course, also a major factor. These both impact upon heat loss via conduction and convection besides the impact on evaporation of perspiration. The worst possible conditions would be a hot, humid, windless day, following a cool spell.

Heat Induced Illness

Heat Exhaustion

Heat exhaustion is caused by dehydration. The symptoms include chills, lightheadedness, dizziness, headache, and nausea. The body temperature is usually between 100 - 102 degrees and profuse sweating is evident.


Treatment should be to get the individual to a cool shaded area and administer fluids either by mouth, if conscious or IV if the individual is unconscious. Seek medical advise.

Heat Stroke (actually a form of Hyperthermia)

Heat stroke is a serious life threatening medical emergency. It is caused by a sudden failure of the thermoregulatory system of the body. Heat stroke may be fatal. Some consider it to lie on a continuum with heat exhaustion. It initially appears similar to heat exhaustion, but may rapidly progress to manifest more serious neurological symptoms: disorientation, loss of consciousness and seizures (status epilecticus). The body temperature may be higher than 105 degrees Fahrenheit. A genetic susceptability to hyperthermia increases the risk of this disorder. Sweating is often absent, but the skin may be quite moist from earlier perspiration. The core temperature must be brought down immediately. In fun runs, perspiration is usually noted, and you do not need the absence of perspiration or an rectal temperature to make the diagnosis. There is usually a sinus tachycardia present, with the pulse noted to be faster than 160. The blood pressure may be low. Kidney damage (acute nephropathy) occurs in about 35 per cent of cases. Rhabdomyolysis (muscle breakdown from) occurs and the myoglobulinuria (excretion of muscle breakdown products) contributes to the kidney injury. Liver damage is also evident when liver enzymes are measured following heatstroke. Disseminated intravascular coagulation also occurs in exertional heat stroke.

The individual will often be confused and ataxic. Seizures can occur as well as coma.

Ecchymoses (bruised appearance to skin), hematuria (blood in the urine) and epistaxis (bloody nose) may also be seen and indicate coagulopathy. Current treatment includes rapid cooling to get the temperature below 102F. Evaporative assistance may help. Spray water, increase air flow and remove restrictive clothing if present. Ice baths or cooling packs to the arm pits, groin and neck may also be used.

These are the individuals who get "packed in ice". Rapid medical attention is vital.


Avoiding Heat Stress Injury


In longer events, in which competitors are running for much longer than a 10 mile distance and staying out and hydrating themselves over a longer period of time hyponatremia is a large concern. Of particular concern are events which take longer than 4 hours. This problem was brought to the fore by Dr. Tim Noakes who had vast exerience with ultra events, particularly the Comrades Marathon. Hyponatremia is a more dangerous and insidious problem than dehydration. Hyponatremia, by definition is low blood sodium (below 130 mm/L). In athletes, it most often occurs primarily because of overhydration. Over the past 20 years much attention has been paid to proper hydration, in some cases, going over the line into over hydration. The drink till you slosh theory, is a bad one for marathoners. Many years ago salt pills were in vogue for athletes, then they were called ridiculous. For runners who are out for 4+ hours, it may be time to rethink this and strongly consider bringing back a bit of salt again. On the other hand, a simple weight check can help keep your hydration under control. Weigh stations have been proposed. Check your weight at the start, and make sure you aren't gaining weight as you continue along the race course.

The symptoms of hyponatremia include lightheadedness, a blurring of consciousness, nausea, vomiting, and dizziness. With increasing severity, the athlete may become unconsious, have seizures or stop breathing. The symptoms of hyponatremia and heat injury are similar and are differentitated by a blood test, and aided by keeping in mind the possibility of hyponatremia in long distance, long time, events.

While gatorade and other sports drinks include more electrolytes than water, they do not include enough to treat hyponatremia.


Additonal Risk Factors:

First time marathoner
Lack of acclimitization
Overhydration - drinking when not thirsty




Weigh In Test:

Why weigh in? Weighing in before and after running will give you a general idea of your fluid balance. If you are several pounds lighter after a run you have likely not replaced adequate fluid and are in danger of dehydration. Likewise, if you weigh more after your run, you may have over hydrated and could be at risk for developing hyponatremia.

Heat Index:

Derived from temperature and relative humidity


Heat Index

Enter temperature and relative humidity below to calculate heat index

Temperature: Calculate Heat Index
Relative Humidity: spacer
Heat Index: spacer


The Heat Index Table


The Heat Index
Environmental Temperature Fº
  70º 75º 80º 85º 90º 95º 100º 105º 110º 115º 120º
Relative Humidity
Apparent Temperature Fº
0% 64º 69º 73º 78º 83º 87º 91º 95º 99º 103º 107º
10% 65º 70º 75º 80º 85º 90º 95º 100º 105º 111º 116º
20% 66º 72º 77º 82º 87º 93º 99º 105º 112º 120º 130º
30% 67º 73º 78º 84º 90º 96º 104º 113º 123º 135º 148º
40% 68º 74º 79º 86º 93º 101º 110º 123º 137º 151º  
50% 69º 75º 81º 88º 96º 107º 120º 135º 150º    
60% 70º 76º 82º 90º 100º 114º 132º 149º      
70% 70º 77º 85º 93º 106º 124º 144º        
80% 71º 78º 86º 97º 113º 136º          
90% 71º 79º 88º 102º 122º            
100% 72º 80º 91º 108º              
Apparent Temperature Heat-stress risk
90º-104º Heat cramps or Heat Exhaustion possible
105º-130º Heat cramps or Heat Exhaustion likely. 
Heat Stroke possible.
130º and up Heat Stroke highly likely.

Note: Exposure to full sunshine can increase the impact of temp and humidity.

In case nothing jumps out at you in this jumble of numbers, let's look at a typical example for Washington, D.C. Let's say you get up a bit late on a Saturday morning, after a Friday night of "renting" beer. A touch dehydrated, but unaware of this, you decide to go out for a run around 11:30 A.M. You head for Rock Creek Park, in the shade. It's hot but what the heck, you planned to run about 7 to 8 miles. Off you go. It's humid, sticky, hot, one to two miles later you feel as if you'd run 5 miles already. You wise up, turn around and have completed between 3 - 4 miles, feel as if you'd done 10 without training. Later you find out the temperature was 102 degrees.

“Eggs could be cooking on the sidewalk.....”

With the humidity at greater than 70%, 100 degrees would feel like 144 degrees. When the humidity is 90%, 90 degrees will feel like 113 degrees. Hmmmmm, maybe it is not such a good idea to run then!

WBGT (Wet Bulb Globe Temperature) Index & Thermal Stress

The above table is a simplification of the usually accepted means of determining the risk of environmental heat. As can be seen from the table, humidity plays a great role in the risks associated with heat and exercise. Clearly, humidity will decrease the evaporation of perspiration, and thereby inhibit one of the primary means by which the body can cool itself. This is what led the development of the WBGT Index method of determining heat illness risk. This is also known as the wet bulb globe temperature. This method depends upon temperature measured with 1) A dry thermometer to measure ambient temperature, 2) Wet Bulb Temperature, and 3) Black globe temperature to determine radiant heat. The wet bulb temperature is measured by placing a wet wick over a thermometer bulb. This reading will be affected significantly by both humidity and wind velocity. The relative humidity is the ratio of the wet bulb to dry bulb temperatures. If the humidity is high, evaporation and consequent cooling will not take place and the reading will be close to that of the ambient temperature. Likewise, higher wind velocity will increase evaporation and lower the wet bulb reading. The wet bulb reading can never exceed the ambient temperature.

Calculation of WBGT by Formula

WBGT = 0.7 x Twb + 0.2 x Tg + 0.1 xTdb

Twb = Wet bulb temperature
Tg = Black globe temperature
Tdb = Dry globe temperature

From this formula, it is evident that the humidity is valued as the major determinant of heat stress, as it is weighted at 70% of the value. Below a WBGT value of 64 degrees (18 C) the risk of heat injury is small. Above a WBGT value of 82 degrees (28 C) races should not be undertaken.

Heat Regulation

Body core temperature is the result of the balance between those processes that create body heat and those that dissipate heat. Winslow, Herrington and Gagge in 1937 used the following formula to approximate how the body's thermoregulatory system worked:

S= M + R + CD + Cv - E

S = stored heat
M = metabolic heat production
R = amount heat gained or lost by radiation
Cd = conductive heat lost or gained
Cv = convective heat lost or gained
E = evaporative heat loss

From this formula it is evident that heat balance in the body depends upon the amount of heat produced by muscle activity and metabolism and the amount of heat that is gained or lost either by the effect of the environment and evaporation of perspiration. Note that perspiration that drips from your body does not appreciably contribute to evaporative heat loss. Air flow either through the effect of wind or your own body motion does aid the cooling of the body. The sudden loss of this air flow is one of the reasons why you will start sweating more when you stop running.

The heat index formula gives good results for temperatures above 70 degrees Fahrenheit. The formula is as follows: t = temperature in degrees Fahrenheit, r = %relative humidity (-42.379+2.04901523*t+10.14333127*r-.22475541*t*r-
(6.83783e-3)*t^2- (5.481717e-2)*r^2+(1.22874e-

This formula was posted to rec.running by Phil Margolies and he reports it is more accurate than the Javascript above.

For the heatedly obssessed Phil Margolies also posted the following Excel formula:
You can paste the following forumula on an Excel spreadsheet. If you put the temperature in cell A1 and the relative humidity in cell B1 and paste this formula in cell C1: =-42.379+2.04901523*(A1)+10.14333127*(B1)-

This Excel formula, however does not appear to be consistently accurate below 80 degrees and should not be relied on. Spreadsheets have been known to have rounding errors with fractions, and we are dealing with many fractions in this equation.

Additional information: The Physics of Temperature and the History of Thermometers


The Killing Fields

Football practices during the months of August and September are potential times of danger. In 1970 eighteen players died from heatstroke. In 2005 3 players died. In a two week period in August, 2006, two high schoolers aged 15 and 16 have already died (one in Atlanta, GA and the other at Stafford High School in Virgina. At Shippensburgh University a 17 year old died from heatstroke during the second week of August, 2006. These heatstroke injuries are now properly recognized as hyperthermia events. The press reports that as of August 12 there have been 5 hyperthermia deaths of high school athletes.

Good sources of information on high school sports and on football injuries may be found at: National Federation of State High School Associations and at the Annual Survey of Catastrophic Football Injuries maintained at University of North Carolina.

Many teams now offer water on the field and even have tubs of ice available for emergency injury treatment. Players are warned to monitor how they feel, and the color of their urine with clear being preferred and a dark color being an "alert" sign. If you are a player please pay attention to how you feel and pay attention to what your peers are doing. One player who died this year reportedly passed out twice and pushed himself to continue. This should have been stopped by a buddy stopping the player from continuing or reporting to a coach what had occurred.


Other Summer Outdoor Risks


Solar Energy: Sunburn Risk

Exercising in the heat of summer requires a certain amount of respect for the environmental conditions of this season. Heat, humidity and sun can all have deleterious effects on your performance and may also result in long term problems.

Sun exposure is now thought to be one of the more dangerous long term risk factors for the development of skin cancer and premature aging of the skin. For light skinned individuals the pasty white look is in, rather than that healthy tanned look. Although even individuals with more melanin pigmentation should also be careful of excessive UV exposure. Daily dosing with sun screen of SPF 15 or greater and wearing funky hats contributes to protection from the risks of sun exposure. Sun burns are considered to do the most harm. The majority of a individual's lifetime sun exposure occurs before they are out of their 20's. The CDC reports that 80% of an individual's sun exposure occurs before the age of 18.

When applying sunscreen make sure to do so 15 to 30 minutes before exposure. Use about a tablespoon per limb and a few tablespoons for your torso. The protection studies are all based on using quite a bit of sunscreen. Don't forget the tops of your feet and front of your ankles. Clothes provide even more protection than sunscreen - so hats and shirts can be of great help in reducing your exposure. If you sit under an umbrella at the beach, remember that sand and water will reflect up to 85% of UV light.

Make sure you reapply sunscreen every 2 hours and after swimming. The SPF measurements noted above pertain to the UVB rays. Tanning booths use UVA and used indiscriminately can contribute to a significant increase in the risk of developing melanoma and other skin cancers. Make certain your sunscreen covers both UVA and UVB.

Besides the well known and highly lethal melanoma, which is diagnosed in more than 40,000 Americans each year, there are other types of skin cancer caused by sun exposure. Basal Cell Carcinoma strikes over 900,000 people each year. This is sometimes seen as ulcers or sores on the face that do not heal, although other body parts are also often affected. Squamous cell skin cancer strikes about 100,000 people per year. A pre-malignant skin injury called actinic keratoses is also found in many people each year.

Protective agents found in some of the best sunscreens include Parsol 1789, avobenzone and butyl methoxydibenzozlmehtane.

Cosmetics and medications can cause increased susceptibility to solar injury. Certain cosmetics contain alpha-hydroxy acids which are particularly apt to increase solar injury. These may be labeled with one of several different ingredients.

Protect your skin. Protect your feet. Protect your eyes.



Alphahydroxy Acids

  • glycolic acid
  • lactic acid
  • malic acid
  • citric acid
  • glycolic acid + ammonium glycolate
  • alpha-hydroxyethanoic acid + ammonium alpha-hydroxyethanoate
  • alpha-hydroxyoctanoic acid
  • alpha-hydroxycaprylic acid
  • hydroxycaprylic acid
  • mixed fruit acid
  • tri-alpha hydroxy fruit acids
  • triple fruit acid
  • sugar cane extract
  • alpha hydroxy and botanical complex
  • L-alpha hydroxy acid
  • gloomier in cross linked fatty acids alpha nutrium (three AHAs)


Glasses are an important summertime item. Use glasses that eliminate 98% or more of the UV light. This will reduce your chances of getting cataracts. Wrap-around glasses will help keep the peripheral UV light from getting in your eyes. Many stores have machines available (photometers) which may measure the actual amount of UV protection.


Beach - Dreaming of Barefoot Running




Following are some helpful summertime links:




About Dr. Pribut: Dr. Pribut is a member of the Advisory Board of Runner's World magazine. He has run in races from 200 meters to the marathon. He is a past president of the American Academy of Podiatric Sports Medicine (AAPSM). He served as chair of the AAPSM Athletic Shoe Committee for 5 years and has served on the Education Committee, the Research Committee, the Public Relations Committee and the Annual Meeting Committee. He is co-Editor of the AAPSM Student's Manual. Dr. Pribut is a past president of the District of Columbia Podiatric Medical Association, serving in that post for 4 years. Dr. Pribut served on the American Podiatric Medical Association's Clinical Practice Advisory Committee and Internet Committee. Dr. Pribut is a Clinical Assistant Professor of Surgery at the George Washington University Medical Center.

Privacy Policy - Advertising Policy 

Content on this site is not meant to replace or substitute for a visit to a physician. It is only to be used as a supplement to your visit.