Running in conditions characterised by high temperature and humidity places significant physiological stress on the body, affecting not only comfort but also performance and safety. A thorough understanding of how heat and humidity influence thermoregulation and endurance capacity allows runners to develop effective strategies for training, competition, and recovery. By integrating evidence-based insights into preparation and pacing, athletes can optimise performance even in challenging conditions.

Heat production in running

During physical exertion, the body is remarkably inefficient at converting energy into mechanical work. Only approximately 20-30 % of the energy produced by muscular activity contributes directly to movement, whereas the remaining 70-80 % is dissipated as heat (1). This heat must be effectively managed to maintain performance and prevent overheating.

For runners, heat originates from two primary sources. The first is internal, generated by the repeated contractions of skeletal muscles during exercise. Each muscle contraction produces metabolic heat that must be transported to the skin and released into the environment. The second source is external, arising from ambient environmental conditions, including air temperature, solar radiation, and surrounding surfaces. Both internal and external heat stressors interact to challenge the body’s thermoregulatory capacity, making it essential for athletes to employ strategies that mitigate heat accumulation during training and competition.

Understanding how the body manages heat is essential for optimising running performance in hot conditions. Human thermoregulation relies on four main mechanisms. Radiation transfers heat through infrared rays, both directly from the sun and indirectly via reflective surfaces. For example, during large city marathons, sunlight reflecting off glass windows in narrow streets can increase localised heat, while tall buildings can block airflow, reducing convection and raising perceived temperature. Conduction occurs when heat moves through direct contact with objects, such as the foot on hot pavement. Convection transfers heat via air or water movement across the skin, like a cooling breeze or rainfall. Evaporation is the key cooling method during exercise, as the conversion of sweat into vapour removes substantial heat from the body.

Dew point and its impact on running

Although air temperature is often the first factor runners pay attention to, the dew point provides a more comprehensive picture of the environmental strain, as it combines the effects of heat and humidity into a practically meaningful measure. The dew point refers to the temperature at which air reaches saturation and can no longer hold additional water vapour. The higher the dew point, the greater the amount of moisture in the air.

A high dew point impairs the body’s ability to regulate temperature because sweat does not evaporate efficiently in humid conditions. Since the body cools itself primarily through the evaporation of sweat, a high dew point increases thermal strain and cardiovascular load. For this reason, running in a high dew point often feels significantly more strenuous, even if the air temperature itself is not exceptionally high.

Sweating, particularly the evaporation of sweat from the skin, is the body’s primary method for dissipating heat during exercise. When the air is dry, meaning low humidity percent, sweat evaporates efficiently, maximising cooling. Conversely, when humidity is high, sweat is unable to evaporate, pooling on the skin and removing minimal heat. This reduction in evaporative cooling can significantly impair performance.

For elite runners, ideal marathon conditions are characterised by a dew point between 2 and 13°C. Humidity begins to noticeably affect performance once air temperatures rise above about 18°C. For a more precise evaluation of environmental heat stress, measures such as the Wet Bulb Globe Temperature (WBGT) would give more performance information, as it incorporates temperature, humidity, wind speed, solar radiation, and cloud cover.

Strategies to stay cool

Effective thermoregulation in running relies on both pre-cooling and mid-cooling strategies to mitigate heat stress. Pre-cooling is designed to enhance the body’s heat-storage capacity before exercise, allowing athletes to tolerate higher internal temperatures for longer periods (1). Common methods include cold water immersion, application of ice packs to areas of high flow such as the armpits, groin, and neck, wearing ice vests or specialised cooling garments, exposure to cold air, and ice-slurry ingestion, which combines crushed ice with water (2). While these strategies can provide measurable performance benefits, their effect is typically short-term, making them more suitable for shorter distances.

Mid-cooling, aka cooling during activity, is crucial for maintaining performance in hot conditions. Effective strategies include using water, ice packing the torso with running vests or sports bras, icing the neck, head and peripheral arteries using bandanas, collars, or sleeves, and wearing light-coloured, loose-fitting clothing to enhance heat dissipation. Notably, ice-slurry ingestion during exercise can sometimes overstimulate stomach receptors, reducing sweat production and creating a physiological mismatch in which the brain perceives the body as cooler than it is, potentially impairing evaporative cooling (3).

Heat Acclimation

Heat acclimation is a critical strategy for runners preparing to perform in hot environments. By gradually exposing the body to elevated temperatures over a period of one to two weeks, athletes can significantly enhance their thermoregulatory and cardiovascular efficiency. Physiological adaptations include improved heat dissipation, increased plasma volume, which reduces cardiovascular strain, more efficient sweating, and greater thermal comfort, all of which help athletes maintain performance under heat stress. Additionally, heat acclimation supports psychological resilience, allowing runners to tolerate better the perceived discomfort of hot conditions during training or competition (4).

When travel constraints make pre-race acclimation challenging—such as living in cooler climates and racing in hot environments—alternative methods can help simulate heat exposure. These include sauna sessions, hot water immersion, or wearing heated clothing during light exercise. Such interventions can accelerate heat adaptation and partially mitigate the negative effects of unfamiliar heat during competition. While no short-term approach fully replaces the benefits of living and training in the target environment, these strategies can still improve performance and safety when fully acclimating before the race is impossible.

Conclusion

Running in hot and humid conditions demands a clear understanding of the body’s physiology, environmental factors, and effective cooling strategies. By combining pre-cooling, mid-cooling, and heat acclimation, runners can optimise performance, maintain comfort, and reduce the risk of heat-related issues. It is also essential to adjust pace according to conditions, as running speed naturally declines when the dew point rises and humidity limits effective heat dissipation. Careful planning and personalised experimentation with these strategies can be the difference between struggling through a race and achieving a strong, controlled finish.

REsources

1. Bongers, C. C., Hopman, M. T., & Eijsvogels, T. M. (2017). Cooling interventions for athletes: An overview of effectiveness, physiological mechanisms, and practical considerations. Temperature,4(1), 60-78. doi:10.1080/23328940.2016.1277003
2. Tyler, C. J., Sunderland, C., & Cheung, S. S. (2013). The effect of cooling prior to and during exercise on exercise performance and capacity in the heat: A meta-analysis. British Journal of Sports Medicine,49(1), 7-13. doi:10.1136/bjsports-2012-091739
3. Osakabe, J., Matsumoto, T., & Umemura, Y. (2019). Ice slurry ingestion as a cooling strategy in the heat. The Journal of Physical Fitness and Sports Medicine,8(2), 73-78. doi:10.7600/jpfsm.8.73
4. Coudevylle, G. R., Sinnapah, S., Robin, N., Collado, A., & Hue, O. (2019). Conventional and Alternative Strategies to Cope with the Subtropical Climate of Tokyo 2020: Impacts on Psychological Factors of Performance. Frontiers in Psychology,10. doi:10.3389/fpsyg.2019.01279