Running in the Heat

Running in the heat is far more stressful and can be dangerous.

Running in the heat is much harder than running in cooler temperatures, reducing performance and creating serious health risks. The simple guidelines are to slow up and avoid pushing the pace, drink to thirst, and to dress for the conditions.

1 Guidelines for Running in the Heat

• Don't die. Running is generally a pretty safe sport, but there are some serious hazards to running in hot conditions.
  • Heat stroke. If your core temperature gets too high, you can suffer from heat stroke. Runners who get heat stroke typically ignore the warning signs and push on until they collapse. Risk is higher with shorter distances and faster paces, such as 5K where the rise in core temperature can be extremely rapid but can happen at the marathon distance as well. The key to avoiding heat stroke is to take it easy and don't push hard (see below on "go slower"), especially if you're new to running. You should watch out for the warning signs of overheated listed below. Note that some antidepressant medications have been linked to heatstroke.
  • Dehydration. Contrary to expectations, it's tough to die from dehydration unless you're running in a remote location. A runner died from dehydration in 2004, but they were running in the Grand Canyon in July where the temperatures can hit 120f in the sun. If you have access to drinks, dehydration is self-limiting. Contrary to popular belief, which has been promoted by the sports drink industry, the evidence is that mild dehydration (<=4%) doesn't impact performance. See The Science Of Hydration for details.
  • Hyponatremia. Unlike dehydration, drinking too much is far more likely to be fatal. This condition, called Hyponatremia, has killed several runners over the years. Drink to thirst and keep your salt intake high. Adding 1/4 teaspoon of salt to each quart/liter of drink helps offset the electrolyte loss (most sports drinks don't contain enough salt to be noticeable.) Be careful about drinking cold drinks to lower your body temperature, as this can lead to overdrinking. See Practical Hydration for details.
  • Sunburn. Not only does sunburn cause long-term health issues, it can also cause more immediate problems with your body's temperature control. Burned skin doesn't sweat so well in the heat, and in colder conditions burned skin can't contract the capillaries to reduce heat loss.
• Adapt. There are various changes you can make to your running that will make summer running easier.
  • Take your temperature. The most accurate way of measuring your core temperature is with an in-ear infrared sensor. I use a Braun ThermoScan and I've been pleased with it.
  • Go slower. With increased heat stress, you'll be slower. In fact, the optimum temperature for performance is generally about 40f/5c, and temperatures above this are likely to slow you down. With mild temperatures (40-70f/5-20c) you're more likely to feel tired rather than hot as your mind shuts down muscle recruitment to prevent overheating. At higher temperatures, the impact becomes steadily greater. If your core temperature goes above 38.5c/101.3f you should ease off or stop exercising until you cool off. If your temperature goes above 39.7c/103.4f you should terminate your activity and cool off as soon as possible. See Impact of Heat on Marathon Performance for some guidelines on how much heat changes pace.
  • Acclimate. With acclimation, you will do better in the heat, though you'll never do as well as you will in cool conditions. Heat acclimation produces many benefits, including increased blood volume, more rapid onset of sweating, reduced electrolyte levels at moderate sweat rates, and more. See Heat Acclimation Training for more details
  • Take breaks. Taking Walking Breaks can provide time to cool off, allowing you to maintain your training distance. Running a short route repeatedly can give you access to drinks and ice, as well as providing a natural break. You can check for blisters which can be more of a problem in hot conditions. If you're looking to do speedwork, Interval Training is far better than tempo runs as the rest period between intervals allows you to cool off. However, be cautious with faster paced intervals like Tabata HIIT, as the faster pace can cause your core temperature to spike dangerously high.
  • Avoid the heat. Run in the cooler parts of the day such as early morning and later in the evening, avoiding late afternoon. Also aim to avoid the sun both by running shady routes and choosing the right time of day. (Remember sunburn not only has long term health risks, it also reduces your ability to control your core temperature.)
• Dress for the conditions. What you wear can make a big difference to your performance, comfort, and even safety. However, you must dress for the conditions, as the humidity and sun change things. Running in high and low humidity conditions is rather different. Evaporation will help keep you cool in low humidity, but it's quite ineffective when the humidity is higher. (In the notes below, low heat is roughly <85f/30c, moderate is 85-100f/30c-40c, extreme is >100f/40c, though details will depend on your height and weight, as noted in Running Heat Model.)
  • Moderate heat with low humidity. When the humidity is low, evaporation will help keep you cool. Thin, form fitting clothing like UA's heat gear can spread out sweat over a greater area and keep you cooler than bare skin.
  • Extreme heat with low humidity. When the heat becomes more extreme your sweat rate may not be enough to keep you cool. Under these conditions it becomes worthwhile to wear thicker clothing that will hold more water and spray yourself off. At Badwater I wore a loose long-sleeved shirt that I kept soaked, which made a huge difference. When the heat is above core body temperature, air movement over dry skin will heat rather than cool your body, so it's only when you're wet that wind will help.
  • Low heat with high humidity. These conditions can be deceiving, as the overall heat stress is much higher than you might expect. When the humidity is close to 100%, even small changes of temperature have a big impact on the fastest pace you can maintain without overheating. Cooling in these conditions comes predominantly from air movement across your skin (convection), not evaporation. Your best option is to wear as little as possible in order to expose your skin to the air, and clothing can easily act as an insulator. If you need to cover up because of the sunlight, then wearing very loose-fitting clothes that don't interfere with air movement may be an option. Ideally this loose clothing may act as a radiator for the heat, though in practice this is hard to achieve. Consider thin, form fitting clothes as another approach, but it needs to be as thin as possible. Consider applying ice to provide direct cooling (see below). Wind will help cool you off.
  • Moderate heat with moderate humidity. The combination of the heat and humidity can be both brutal and intractable. The humidity limits the cooling from evaporation and the warm air limits the cooling from convection. In these conditions, applying ice is one of the few ways of cooling off, and without it running can become impractical. Clothing should be selected to spread out the meltwater from the ice, so thin, form fitting clothing is ideal.
  • Extreme heat with high humidity. These conditions are relatively rare, but there comes a point where running becomes impractical and ill advised.
  • Daytime. As you probably know, white clothing reflects heat where black absorbs heat. If you're in direct sunlight, then wearing white is critical, and even in partially shady conditions white clothing is an advantage. A white hat should be used and consider a 'legionaries cap' to help protect your neck and face. See Running Hats for more details.
  • Nighttime. After dark it may be better to wear dark clothing to help radiate more heat.

2 Heat and Fatigue

It is generally accepted that exercise at a constant effort will be voluntarily terminated once the core body temperature reaches about 40c/104f. However, if subjects are allowed to vary their pace freely, they will preemptively reduce their exercise intensity to prevent their core temperature form reaching the critical point. One mechanism for this reduction is that fewer muscle fibers are recruited in hot conditions, even before the core body temperature starts to rise. Elevated core temperature reduces the voluntary maximum force generated, though it is unimpaired when electrical stimulation is used, indicating the central nervous system is responsible . When exercising at a constant Rating of Perceived Exertion, subjects steadily reduced their power output at 59f/15c, 77f/25c or 95f/35c even though their perception of their temperature comfort remained constant.

2.1 Mechanisms

Exercise in the heat burns more carbohydrate than in cool conditions . However, it is not clear to me if this is because of a direct change in the use of carbohydrate, or if the hot conditions reduce V̇O₂max and therefore the exercise intensity is effectively increased. Exercise in the heat also burns more muscle Protein, and the waste products from this Protein metabolism could result in mental fatigue due to a rise in GABA.

3 Heat Stroke

Running in the heat can kill, as the body's protective mechanisms that normally cause us to slow down can be ineffective. There are a number of risk factors for heat stroke.

1. Being overweight, as fat can act as an insulator.
2. Being untrained, as fitness gives some heat adaptation over the unfit.
3. Lack of heat acclimatization, as this acclimatization gives some protection from the heat.
4. Thinking you can run faster than you can in the conditions.
5. Having suffered from heat illness before.
6. Ignoring warning signs (see below).

Being overweight, unfit and attempting to run hard can be a catastrophic combination. Intense exercise can spike your core temperature and this rise can continue even after collapse as the heat from the muscles warms the blood while the fat reduces the cooling effect.

4 The Warning Signs of Overheating

The best advice seems to be to take things cautiously if you are not used to running in the heat. Pushing yourself harder than normal in familiar heat, or attempting to run normally in heat you are not used to is dangerous. Traveling to a warmer area for a race is especially risky. Look out for the following warning signs, and if you have any doubts, slow down or stop and cool off.

• Nausea or vomiting. These symptoms can occur before true heatstroke, as running makes digestion harder.
• Weakness. An unusual muscular weakness could be due to low blood sugar, but elevated core temperature also creates weakness.
• Headache. This can also be caused by dehydration, or low blood sugar. Having had headaches from each of the three causes, I have found the type of headache is different. My limited experience is that a headache caused by heat is particularly painful and intense.
• Dizziness or confusion. This is a serious symptom that suggests either extremely low blood sugar or heatstroke.
• Flushed/Hot Skin. I've found that an early warning sign is the feeling that my skin is burning, especially my face.
• Panting. Another symptom I've found of overheating is that my Breathing becomes labored beyond what is reasonable for the exercise intensity.
• Core Temperature. The only sure test is to check your core temperature using something like an in-ear thermometer. Using a mouth thermometer may not be accurate if you've been Breathing hard.

If you have any doubts, stop and check your temperature. Avoid High Intensity Interval Training in the heat; the intense work can spike your core temperature too high too quickly for you to recover. It's possible that a runner that suffers heat stroke may exhibit none, or only one of these warning signs.

5 Using Ice

Ice can be a huge help in hot, humid conditions. I'd recommend trying a Jimbo Bandana while running (as well as for Precooling.) You can also put ice under a hat, though this can be quite painful when the cold hits your scalp. Even holding some ice in your hand, or holding a cold water bottle can help. Pouring water over your head, neck and body is an alternative, but watch out for the shock of the temperature differential. It's important to never use gel packs, only frozen water! Gel packs start off too cold, causing frostbite and skin damage, then warm up too quickly. If you use ice, it will start off just below freezing and then maintain that temperature until it's melted.

6 The effect of clothing color

A study comparing clothing color in hot conditions (38C) and strong sun showed that black clothes result in 2.5x the gain heat from the sun compared with white clothes. Tan clothing of a military uniform gained 1.7x more than white clothes and just shorts (semi-nude) gained 2.2x more. Therefore, it's important to wear white clothes in hot sunny conditions.

7 Tight or lose clothing?

The study of clothing color used black and white versions of the traditional Bedouin clothing, which has two layers of material and allows air to flow freely between them, creating a chimney like effect. This clothing mitigated most of the extra heat absorbed from the black clothing, as the hotter air was able to escape. This suggests that lose clothing may be an advantage, but only if the air can freely circulate. Also, the study used stationary people, so the benefit of loose clothing may not transfer to exercising athletes.

8 See Also

• Running calculators
  • Running Heat Model
  • Perceived Temperature For Runners
  • Heat limited running pace
  • Optimum Running Temperature
• Heat Acclimation Training
• Impact of Heat on Marathon Performance