Humans have an upper environmental limit for thermal survivability, one that we’ve covered extensively in this series (see “Human Thermal Limits” if you need a refresher). As long-time readers will know, that survivability limit can be quite a bit lower than you’d expect, depending on many factors: what you are doing, wearing, your heat acclimatization, and how long you’re exposed to the heat.
Knowing the upper limit for survivability in the heat is, of course, critically important. Temperatures approaching survivability limits affect work and health are becoming increasingly common. In some parts of the world, outdoor work is even banned during the hottest periods of the year.
So, it’s understandable that the most extreme temperatures generate the most interest. Just not today.
In this article, we’ll focus on conditions that don’t approach life-threatening but are still uncomfortably hot. The 100oF+ day might make the headlines, but 92oF days aren’t ideal either. We don’t do our best work in such conditions, with performance suffering even as we push through. You don’t have to live in the desert to know hot days; days above 90oF are summer conditions nearly all of us face, at least occasionally (some places more than other; West Texas logged 108 straight days of 90oF+ heat this year). Even if the heat isn’t deadly, it still affects us.
A Natural Experiment: Marathon Performance
There is perhaps no cleaner example of data showing heat’s ability to degrade performance than what we find from marathons. If your immediate reaction is “doesn’t apply to me” (I don’t have plans to run 26.2 miles either), consider this: marathon races are a near-perfect natural experiment to analyze temperature’s effect on human performance.
Marathons are standardized, results measured to the second, the age of contestants is known, and athletes are motivated to apply maximum effort. The weather on race day is the same for every participant but changes between races on different years. Any worthwhile scientific test keeps as many variables as possible the same while changing only the variable being considered (in this case, temperature). While the study population (i.e., athletes) isn’t perfectly homogenous, marathon racers are highly similar in fitness and motivation, especially in high-profile races and in the upper tier of competitors. As a final bonus, races like the Boston Marathon have records going back over 125 years, providing plenty of data points to draw statistically robust conclusions.
Given such a clean natural experiment, it’s unsurprising that many researchers have considered how temperature affects race times. Spoiler: athletes are slower in hot conditions. One study, which considered Boston Marathon race times between 1897 and 2018 (all told, over 244,000 male finishers), put hard values to this performance degradation: for every 1.8oF of temperature increased, race times, on average, were nearly two minutes slower.
The hotter it got, the more performance degradation added up. Over the years, more than 217,000 competitors finished the Boston Marathon in temperatures between 46-59oF, with an average time of about 3 hours, 33 minutes. More than 50,000 racers finished during years when the race temperature was 60-73.4oF. Races on years with this hotter temperature averaged finish times just over four hours, or about thirty minutes slower. The conclusion: despite the same population of highly fit and motivated runners, increasing temperature around 15oF dropped performance substantially.
This performance degradation shows up even in top-tier athletes. Among the top ten finishers, races on years with temperatures between 60-73.4oF saw finishes just over 14 minutes slower when compared to years when the temperature was only 46-59oF.
While the Boston Marathon might be the most famous example, similar results show up in studies using other races. Using nearly thirty years of data from the Stockholm Marathon, a researcher at the Finnish Meteorological Institute found not only did run times slow during hotter years, the percentage of race dropouts also spiked. For any competitor who’s put months of training behind them, “DNF” – Did Not Finish – is the worst possible outcome on race day. In 2007, the hottest race in Stockholm Marathon history, nearly 15% of competitors did not finish, a huge increase from the typical 5% or less during cooler years.
There is one more insight we can learn from marathon performance. The largest dataset ever compiled (at least to my knowledge), with over 1.7 million race records, found optimal marathon temperature is less than 50oF. More importantly, it found performance doesn’t decline steadily with rising temperature. Instead, it drops sharply (along a “quadratic curve”, or inverted U shape, when plotted). A few degrees higher than 50oF has a very small impact, but competitors slow more and more as temperatures climb. In the researcher’s own words, “the more the temperature increases, the larger the decreases in running speeds”.
Figure 1: Stylized graph of performance degradation as thermal stress increases. Performance decrease isn’t steady; the hotter it is, the more quickly performance suffers.
Beyond The Marathon: Performance on the Field
While marathons provide an easy way to assess temperature’s impact on human performance, they remain an unusual activity. Few among us travel 26.2 miles on foot in a day, let alone race that distance in one continuous effort!
The soccer field offers a potentially more practical understanding of heat’s impact on performance. Professional soccer is athletically demanding but is a better analogy to daily work or most athletic events. Players alternate periods of intense work, like sprinting for the ball or into scoring position, with lower intensity periods of walking. The difference in a player’s metabolic heat production between these two extremes can be more than seven-fold (1750 watts while the ball is in play compared to about 240 watts when walking).
Between June and July 2025, FIFA held the Club World Cup in the United States. (Not to be confused with the 2026 World Cup, also held in the U.S. this summer. Peer-reviewed heat impact articles for those games haven’t published yet). Catering to fans overseas, many matches were scheduled in the early afternoon, U.S. time, to allow European viewers the opportunity to watch an evening or early night game in their local time zone.
Early afternoon, of course, is often the hottest time of the day, and coincided with a June heatwave across the United States. Thirteen of the 57 matches were played in conditions when wet bulb globe temperature (WBGT)[1] exceeded 86oF, and two with WBGT above 89.6oF. For some perspective, moderate- and high-intensity U.S. military training is often modified to mitigate heat illness during such conditions.
Like top marathon runners, Club World Cup soccer players are world class athletes. Unlike marathoners, soccer players don’t spend an entire game at maximum effort. Short bursts of speed are required to make plays, interspersed with periods of lower-effort passing and walking. One group of researchers decided to find if the hotter, more humid early afternoon games affected player speed and effort during Club World Cup matches.
The results are what you’d expect. During games with high temperature, high humidity, and high WBGT values, players reduced the overall distance covered during a game. Simply put, they moved around the field less and at a slower pace.
Assisted by FIFA technical reports including 1,070 observations, researchers quantified this speed and distance reduction, finding for every 1.8oF (1oC) increase in WBGT, players decreased distance covered during high-speed sprints by 0.182 meters per minute. During moderate and low speed running where players spend the majority of a soccer match, they slowed even more.
A separate study found, in hot conditions, teams may even change their style of play. A possession-oriented game focuses on passing and maintaining control of the ball, usually at a slower tempo. Alternatively, a transition-based style of play has more rapid, direct attacks immediately after gaining control of the ball. In games under hot conditions, more passes were successful, and there were fewer challenges to ball possession, compared to games played during cooler conditions.
From Athletes to the Assembly Line
World class athletes have thermal regulation advantages over us regular folk. At a peak level of fitness, they start with a more robust cardiovascular system, making them likely to shed heat more effectively. At the elite level with access to dedicated training staff, they may conduct a deliberate heat acclimatization program. They start the race or game in peak condition and properly hydrated. Their uniform and gear are as light and breathable as possible, designed by researchers at athletic companies like Nike and Under Armour.
Compare this with your average worker, who mostly arrives at the job site slightly dehydrated and may spend the day wearing thermally restrictive PPE.
Despite these advantages, fundamental human physiology is the same for the athlete, the worker, and you and me. Athletes running slower in the heat don’t consciously feel they are giving less effort. The opposite is actually true; in hot environments, perceptual effort feels greater when in fact performance is suffering. This is as true for the average worker as it is for world class athletes. As Dr. Park, a Wharton School professor puts it, “all these little drips of productivity loss and subtle burns here and there really add up”.
The reduction in worker performance due to hot temperatures is covered in a 2022 review article of occupational heat stress literature. Across 38 field studies involving more than 2,400 outdoor workers performing 41 jobs in 21 countries, occupational heat was found to account for an amazing two-thirds of the variation in how hard people worked.
That is worth restating: the thermal environment, not supervisor, task, or deadline, accounted for more than two-thirds of the variation in worker physical effort.
The study’s researchers came to this conclusion by assessing workers’ metabolic rates across different thermal environments. Your metabolic rate directly correlates to how hard you are working; metabolic rate is literally the amount of energy (i.e., calories) burned during a given time. Work harder, burn more calories, and your metabolic rate goes up. And, since humans are highly inefficient at turning calories to physical work, higher metabolic rates increase the amount of waste heat we must dissipate to prevent heat illness like heat stroke. In cold conditions, dissipating lots of waste heat might be no problem, but it becomes challenging in hot conditions; above about 95oF, sweat evaporation is the only avenue to dissipate waste heat generated through work.
Like athletes, workers in hot conditions don’t necessarily feel they are working less. The opposite is probably true, with the work feeling harder. However, consciously or not, they self-pace, reducing their metabolic rate to lower the amount of metabolic waste heat they need to dissipate.
The link between athlete and worker performance under hot conditions doesn’t stop there. According to the first nationwide analysis of heat and workplace injuries in the U.S., workplace injuries follow a pattern surprisingly similar to marathon performance.
This study used a heat index[2] baseline of 80oF, similar to how our previously mentioned marathon study used <50oF as an optimal race temperature baseline. After all, most jobs aren’t as physically demanding as running a marathon and can be comfortably performed in 80oF weather. However, at 90oF, the risk of occupational injury increases by 3%, rising to 10% at 100oF and 20% at 110oF. Just like marathon run times decrease faster as temperatures rise, occupational injuries increase faster and faster as temperatures climb.
Qore Performance: Shifting the Thermal Threshold
Heat burden reduces performance. For both athletes and workers, performance begins to degrade as temperatures rise. Faced with an uncomfortable thermal burden but still needing to get the job done, our natural inclination is to slow down.
Active cooling can push that thermal threshold higher before performance begins to suffer. Qore Performance's ICEAGE ECOSYSTEM products act directly on the dissipation side of the heat balance equation through conductive cooling, removing metabolic heat that otherwise must dissipate more slowly to the environment. Doing so allows us to stay productive past the point where we’d otherwise slow down.
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About the author: Dr. Erik Patton holds a PhD from Duke University where he conducted research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120oF) to Arctic conditions in central Alaska (-42oF).
[1] WBGT is a thermal index that combines temperature, humidity, radiation exposure (i.e., the sun), and windspeed – all factors that influence our ability to thermal regulate - into a single variable.
[2] There are many thermal indices used in heat research. While we typically use ambient (i.e., air) temperature and the more occupationally focused WBGT, the heat index is also common. Heat index incorporates ambient temperature and humidity to provide the “feels like” temperature we commonly see in weather forecasts.
