Running in heat and humidity is tough. Here’s how (and why) it affects your performance, how you can compensate and what to do about heat-related illnesses.

Nothing’s worse for a marathoner than waking up on the morning of the big race, only to be met with extremely hot weather. Running in heat and humidity can be particularly dangerous. In fact, last year’s London Marathon saw record high temperatures and fresh health concerns. In the UK, we’re not exactly great at dealing with the heat. Knowing how to prepare is essential. In this blog post, we’re going to look at how your body reacts to heat and how it can affect your athletic performance.

Regulating body temperature when running in heat and humidity

Healthy body temperature falls between 35°C and 42°C. The average person has a baseline of between 37°C and 37.8°C. One of the biggest physiological problems facing runners when they exercise is how to lose excess body heat.



Blood flow to the muscles increases as we exercise. Not only does the heart pump more blood, but blood is diverted from nonessential organs towards the working muscles and skin. The blood is heated as it passes through the muscles. This distributes more heat throughout the body, particularly to the skin. Heat is then conducted to the skin’s surface.

Man running in heat and humidity

When sweat in the skin evaporates, surface heat is lost. It’s important to remember that sweating doesn’t cause heat loss. It’s the evaporation of the sweat into the atmosphere. Complete evaporation of 1 litre of sweat leads to a heat loss of 580 calories. Little heat is lost when sweat drips off the body.

Factors that affect heat response

A number of factors can affect heat response when running in the heat and humidity. As described in Fitness and Health, 7th Editon these include body fat, sweat glands, fitness level, sex and clothing.

Body Fat

People with higher levels of body fat might have better insulation from the cold. However, this doesn’t mean they’re not less prone to losing excess heat to the environment. This is because the body learns to route blood around the fat for cooling purposes. Excess fat is a hindrance as the body needs extra energy just to carry it around.

Sweat Glands

All of us have a certain number and pattern of sweat glands. As evaporative heat loss is the most important protection against heat stress, a good supply of active sweat glands is important. Like almost everything else, sweat glands respond to use. If you use them a lot, they become more efficient.

Physical Fitness

Fitness seems to improve the body’s ability to regulate temperature. It does this by lowering the temperature at which sweating begins. Therefore, fitter people can workout with lower heart rates and core temperatures than those who are unfit. Cordes and Sharkey, 1995 found that a highly fit subject had a heart rate of 118, whereas less-fit people had rates of 170bpm while running on a treadmill in heat. Acclimatisation also helps to lower the point at which sweating begins. This means, physically fit and heat-acclimated individuals are better prepared for running in the heat (Nadel, 1977).

Sex

Men produce more sweat than women do for a given increase in body temperature, perhaps too much. Women’s sweat production is more suited to the heat load, so they don’t waste water. When we compare men and women doing the same task, men seem more able to work in the heat. However, this difference could be down to fitness, not gender. When fitness levels are the same or when the workload is equal in terms of a given percentage of maximal oxygen intake, women are able to work well in heat. In several recent marathons, women seemed to tolerate the heat as well as, or better than many men did.

Clothing

In Lore of Running, 4th Edition, author Tim Noakes mentions clothing as a factor that affects heat balance. The usual reason for wearing specific clothing is to trap a thin layer of air next to the body. As air is a poor conductor of heat, this thin layer quickly heats to body temperature and acts as an insulator, preventing heat loss. When running in heat and humidity, clothing needs to have the opposite effect.

Marathon runners have learned that light, porous garments, such as mesh singlets and shorts made of breathable fabric, are best for achieving this. Recent examples including Nike creating cooling vests for its sponsored athletes to wear during the 2008 Beijing Olympics. They also created a cooling hood for decathlon world-record holder Ashton Eaton to wear at the 2015 World Championships.

Decathlon world-record holder Ashton Easton wearing a cool hood designed by Nike.

Performance effects of running in heat and humidity

Anyone that’s been for a run in the heat knows it’s it’s much more difficult than running in cooler conditions. Ultimately, there’s no denying that running in heat and humidity affects performance, whether you’re racing or just training.

There have been some well-documented and somewhat legendary examples of athletes that have struggled when running in heat and humidity. All of these races were run in unusually warm conditions:

Jim Peters in the 1954 Empire Games Marathon in Vancouver

Alberto Salazar in the 1980 Falmouth 12km Road Race

Gabrielle Andersen-Schiess in the 1984 Los Angeles Olympic Marathon.

Jim Peters collapsed during the 1954 Empire Games Marathon in Vancouver due to high temperatures and a lack of hydration.

As discussed in Running Anatomy, 2nd Edition, acknowledging the fact that heat and humidity can affect performance is important. You can use it to make pace adjustments, allowing you to normalise your effort and get the most out of you run.

The best way to do this when running in heat and humidity is to compare your overall pacing from previous runs. For example, a marathon finish time of 4:40:00 could be adjusted by 2% to account for a temperature and heat index sum of more than 130 (65°F and 65 dew point). This adjustment would result in a finish time of 4:45:42. Although, five minutes slower than goal pace, should compare well with the rank that a 4:40:00 marathoner usually achieves in normal conditions. These same percentages apply to training . Whether it’s just for a workout or an easy run, pace will be affected.

Heat-related illnesses

Prolonged exposure to hot temperatures when exercising can cause a number of heat-related illnesses. These include heat cramps, heat exhaustion and heat stroke (also known as sunstroke).

Heat cramps occur when electrolytes are lost in the sweat. Take a sports drink and use stretching and massage to relieve the cramp .

occur when electrolytes are lost in the sweat. Take a sports drink and use stretching and massage to relieve the . Heat exhaustion occurs when heat stress exceeds the capacity of the temperature regulating mechanism. A person with cold and pale skin, a body temperature of 39°C to 41°C, a weak pulse and dizziness should be given fluids and allowed to rest somewhere cool.

occurs when heat stress the capacity of the temperature regulating mechanism. A person with cold and pale skin, a temperature of 39°C to 41°C, a weak pulse and dizziness should be given fluids and allowed to rest somewhere cool. Heat stroke means the temperature regulating mechanism has failed. The skin is flushed, hot and dry. Sweating stops and the body temperature might rise above 41°C. Heat stroke can lead to permanent damage of the temperature regulating centre of the brain. Or in extremely serious cases, even death. The victim needs to be cooled rapidly and fluids administered, if possible, whilst waiting for an ambulance.

Gabrielle Andersen-Schiess suffered effects of heat exhaustion and dehydration during the 1984 Los Angeles Olympic Marathon.

More information on heat-related illness can be found in Common medical conditions in athletes, their effects, treatment and recovery.

Summary

Taking information from various sources, we’ve looked at how best to prepare for running in heat and humidity. Solutions range from improving your fitness and acclimatisation to dressing properly and maintaining hydration. Fitness, although especially important in the heat, improves performance in all environments. Fluid replacement is also critical in the heat. Maintaining energy levels is essential to prolonged performance. Finally, proper clothing is also required to cope with the demands of the environment.

Time needed: 1 minute. How to run in the heat and humidity (8 steps) Wear the right clothes Think breathable, wicking and light-coloured. Don’t forget a visor and shades. Stay hydrated Especially important for longer runs and in remote areas. There are many options available to buy for transporting water with you. Go off-road Asphalt absorbs heat. Trees provide shade. Look for routes that favour the prevailing weather conditions. Listen to your body Heat-related illnesses can be serious. Learn to recognise the symptoms and don’t ignore them. Don’t go it alone Run with a partner, carry a mobile phone or consider telling someone where you are going. Seems overly cautious until it doesn’t. Run early or late The sun is more powerful at midday, avoid it if you can. Run during the ‘golden hour’ of morning or evening instead. Don’t be a hero Let personal records stand for another day. Accept the conditions and adjust your expectations accordingly. Wear sunscreen If we could offer you only one tip for running in heat and humidity,

sunscreen would be it.

Further Reading

Our running books