Running in the Heat: Safe Summer Running
by Roy Stevenson
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With the summer months looming up on us, it’s time to review the single most serious threat to the runners’ life—heat. Heat contributes to more fatalities than sudden death (heart attacks), or being killed by automobiles while running.

Elite athletes have even been known to have heat problems, but it’s the semi-conditioned rank and file runner who is most susceptible to heat injury. Even dehydration can lead to unconsciousness and death if allowed to escalate into heat exhaustion or heat stroke.

Let’s examine the factors that combine to cause heat problems: air temperature, air movement, humidity, exposure to the sun, acclimatization, and the intensity or duration of your run.

The Body’s Reactions to Heat
We can generally tolerate temperatures as high as 80 or 90 degrees F because we’re able to sweat as much as two liters per hour. Given dry air, most of this sweat evaporates, cooling the body as it does so. However, as humidity increases the already saturated air absorbs less sweat, and body heat begins to build up.

The greater the air movement around us, the greater the cooling effect as air currents enhance evaporation. A headwind helps evaporation, but note that a tailwind actually reduces the airflow over the body, hindering evaporation.

Sunlight acts as an insulating blanket by warming the skin. Direct sunlight causes a rapid rise in body heat by elevating skin temperature—which should always be at least two degrees cooler than your core temperature to allow for a cooling heat gradient.

Your workout intensity and the length of your workout contribute to stress produced by heat. We generate heat during exercise and the human body isn’t particularly efficient in this respect—75% of our expended energy is turned into heat. Thus the faster and longer we run, the higher the heat load placed on the body.

A runner’s previous exposure to heat is also a major factor in determining our susceptibility or resistance to heat illness. This is called acclimatization. Through training we can partially, but never completely, adapt our thermoregulatory mechanism. In addition, people respond differently to heat, so adjustments to exercising in heat should be made on an individual basis.
Thus heat will always be a limiting factor to our endurance performance. Unfortunately, each summer thousands of runners discover this the hard way and for some, it’s a fatal experience.

Even fit runners can only tolerate a narrow range of internal core temperatures. The good news is that a fit person can tolerate a higher core temperature than an inactive person, so heat problems usually arise when runners are inadequately conditioned for a race or pushing beyond their limits.

A combination of two or more of the above factors can increase your risk of heat injury. The most formidable combination is simultaneously elevated heat and humidity. An air temperature of 60 degrees plus 95% humidity is more dangerous than a “dry” 85 degrees. This combination places an extra burden on the cardiovascular and thermoregulatory systems. It’s no coincidence that runners with undetected cardiac problems “choose” hot races to collapse in.

The blood to the skin carries heat from the body core, where evaporating sweat cools the blood before its return to the body’s core. But when your skin absorbs heat faster than evaporation can cool it, you run into problems. The hypothalamus—the body’s thermostat—detects this discrepancy and responds by dilating the blood vessels in the skin to be cooled. It also makes the heart pump faster to shunt more blood to the surface, causing your sweat glands to produce more sweat.

Soon a vicious competition for blood ensues between the brain (which needs 25% of heart output to function) and the working muscles, which need more blood but are getting less and less. It’s here the inexperienced or foolhardy runner makes a mistake. Instead of slowing down, they keep pushing themselves. By continuing to push, the runner worsens his or her current state of dehydration. And with this increased sweat loss, the plasma becomes thicker and more viscous, causing the heart to pump harder.

Continuing sweating without taking in adequate fluids amplifies these demands on the circulatory system, which soon become intolerable. At this stage you are a prime candidate for heat exhaustion, and if you ignore the signs, heat stroke. Usually running performance declines by this stage, pressuring the overly competitive runner to pick up his pace. Then the competition for blood becomes unbearable and the circulatory system and hypothalamus shut down. Blood pressure drops. Unconsciousness, and possibly death, results.

Now that you understand the mechanisms involved in hyperthermia, you’ll need to recognize it in yourself and others. Warning signs of heat exhaustion include dizziness, profuse sweating, weakness, dehydration, parched throat, and hot red skin.

The warning signs of heat stroke are headaches, dizziness, disorientation, nausea, pale dry skin, decrease in sweating, fatigue, blurred vision, pounding head, fainting, and a tingling sensation (goose bumps) on the trunk. Here’s how to treat these symptoms.

Treatment of Heat Exhaustion

• Do not try to run through these symptoms. Stop!
• Find shade and pour water on the person.
• Seek medical help.
• Raise your legs to get blood to the brain. (Raise heels 8-12 inches)
• Keep person lying down.
• Give person cold water to drink for 1 hour.
• Loosen or remove clothing.
• Sponge bare skin with cold water or rubbing alcohol.
• Use fan or air conditioner to create draft over person’s body.

Treatment of Heat Stroke

• Same as 1-5 above.
• Treat for shock, but do not cover person with blankets.
• Take immediate measures to cool body quickly as in steps 7, 8, and 9 above.
• Or apply cold ice packs or ice continuously.
• Or place person in tub of cold water.

Prevention of Heat Injury
Generally males handle heat less efficiently then females, as do larger people who have less cooling surface per pound of body weight than slim people. Food digestion interferes with the blood flow to the working muscles, so avoid large meals before a long run or race. Wear light colored clothing that breathes well and repels the sun’s rays.

On hot or humid days don’t start too fast for your current level of fitness, and don’t push beyond your limits under these conditions. Know your current state of fitness and be adequately conditioned for your race—if you aren’t, don’t compete.

Drink lots of cold water before, during, and after your training and racing efforts. A well-hydrated runner will have pale yellow, but not completely clear, urine. Make sure you eliminate excess water from the bladder a half hour before you run, then drink 200-500 mls 15 to 20 minutes before show time. Try to drink at least one cup of water every 20 minutes during the run.

If your training runs or races will be lasting over 45 minutes, it’s important to choose a sports drink over plain water. The carbohydrate, potassium, and other electrolytes will replenish lost nutrients and stave off the depletion of muscle glycogen, and the sodium will help keep you from becoming hyponatremic. Hyponatremia is a dangerous condition in which blood sodium levels become dangerously low. This can occur with excessive sweating in hot conditions, or by overdrinking—the latter causes you to dilute your blood sodium levels. This can occur in colder conditions when fluid loss is not as great as fluid intake. These factors make hyponatremia somewhat complex; for example, it is possible to be both dehydrated and hyponatremic at the same time. It’s a good idea to weigh yourself before and after runs; if you gain weight after a workout, you’re drinking too much.

On hot, muggy days don’t try to stick to your planned distance. Be prepared to cut back if conditions are dangerous. Try to run in cooler shaded areas on hot days. Another precaution is to run with a partner and keep an eye on each other.

For good acclimatization, run at least three days a week in conditions similar to those you’ll race in. If you can’t do this, avoid races held in the heat of the day. Early morning and evening runs will not fully prepare you for the midday heat. You’ll need to allow 10 to 14 days of slowly progressive running to adjust to the heat. The benefits of acclimatization are less sweating at a given workload, and less elimination of electrolytes in your sweat.

Lastly, there is nothing macho or intelligent about shunning fluids on your racing or training efforts. This practice is detrimental to performance and can lead to heat injury. Recognition of the signs, symptoms and treatments of heat exhaustion and heat stroke is half the battle to dealing with heat injury. Knowing them could save your life, or your training partner’s life. Use these precautions and guidelines to prevent running into problems when exercising outside in hot and humid weather.

Roy Stevenson is an exercise physiologist with a master’s degree in exercise science and teaches physical education and exercise science at community colleges in Seattle. Jeff Venables contributed to this article

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