With the hot summer months upon us, it is time to review the single most serious threat to the runners' life--heat. It has a higher fatality rate than sudden death (heart attacks), or being killed by automobiles while running.

Even elite athletes have been known to have heat problems, but it is 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.

We can generally tolerate temperatures as high as 80 to 90 degrees F. because we are 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 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 is not particularly efficient in this respect--seventy-five percent of our expended energy is turned into heat. Thus, the faster and longer we run, the higher the heat load placed on our 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.

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 is 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 it returns 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 twenty-five percent of heart output to function) and the working muscles, which need more blood but are getting less and less. It is here that 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 are by now becoming 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 over 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. Possible death.

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

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 is how to treat these symptoms.

Treatment of Heat Exhaustion

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

Treatment of Heat Stroke

1. Same as 1-5 above.
2. Treat for shock, but do not cover victim with blankets.
3. Take immediate measures to cool body quickly as for steps 7, 8 and 9 above.
4. Or apply cold ice packs or ice continuously.
5. Or place victim 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 (cotton) 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 are not, don't compete. Be wary of races organized by local charities--they may be directed by amateurs who have no idea of the dangers of heat stroke, and may have inadequate precautions for these eventualities, such as not enough water at drink stations, no medical coverage, etc.

Drink lots of cold water before, during, and after your training and racing efforts. Make sure you eliminate excess water from the bladder a half hour before you run, then drink 200 to 500 ml 15 to 20 minutes before show time. Try to drink at least one cup of water every 20 minutes during the run. Don't wait until you are thirsty--it will be too late. Drink during your training runs to get used to it. If you live in the Midwest, South or East coast this is necessary for your survival anyway.

Keep your body wet. The temporary relief is well worth it. Putting ice under the cap is an old trick used by seasoned marathoners. 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 will 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 will 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. Are salt tablets necessary? Some research has shown the chloride in table salt can inhibit your body's ability to deal with heat stress, so high levels of salt intake may not be necessary, or may even be counterproductive. You'll get adequate salt intake from your meals, even without salting them.

Alcohol should be avoided because of its diuretic effect, causing you to dehydrate more quickly. Wearing rubber or nylon sweat suits on hot days is an extremely dangerous practice. Presumably people do this to "sweat off" pounds, but this weight loss is fluid loss and is replaced as soon as you drink water.

Electrolyte replacement drinks are highly advanced these days, and lots of research has gone into their efficacy. However, watch out for the imposters that are simply loaded with sugar and no better for you than soft drinks. One problem that runners may encounter with electrolyte drinks is that they are too concentrated, making them feel nauseous. If this is the case, dilute the drinks one hundred percent or more to make them palatable.

Lastly, there is nothing macho or intelligent about shunning water 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 of dealing with heat injury. Knowing them could save your life, or a fellow runner's life. It is a good idea to be familiar with these guidelines to prevent unnecessary hyperthermia. Use these precautions to avoid 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. He set a New Zealand younger then 20 record when he ran a 2:42:28 marathon at age 19 and competed in New Zealand championships on the track, road, and cross-country. He has coached several hundred runners from around the Puget Sound.