Sear factor: How heat impacts an athlete

A body under superhuman strain, heating up and dehydrating faster than usual, is a dangerous cocktail. In addition, athletes don’t always know when to stop.

Just how hot is too hot, for an athlete?

‘I was shivering. Nothing made sense. It felt like my body was shutting down... I’m now trying to become more aware of my body’s signals and recognise when to stop,’ says Indian triathlete Pragnya Mohan.

The two core factors are heat and humidity (in high humidity, even if temperatures aren’t hazardous, perspiration cannot evaporate, and so the body cannot cool itself).

The measure used to calculate these factors is, of course, the wet bulb temperature. This is the temperature recorded by a thermometer placed outdoors, covered with a wet cloth, thus accounting for heat, humidity, the potential cooling effects of perspiration, and the natural movement of air.

For a reasonably well-hydrated human at rest, a wet bulb temperature of 35 degrees Celsius is said to be dangerous. It puts someone at risk of heat stroke.

With endurance-based sports — anything that includes a fair bit of running — things become dangerous at a much lower temperature. By some estimates, this mark can be as low as 18 degrees Celsius.

If wet-bulb temperatures breach 28 degrees Celsius, the event should be shut down or the athlete should withdraw, states an advisory by the American College of Sports Medicine.

“The ideal temperature for endurance-based sports is 11 degrees Celsius (because of how much heat the body itself generates during such activity),” says Mike Tipton, a professor of human and applied physiology at the Extreme Environments Laboratory at University of Portsmouth.

Last year, ahead of the Games, Tipton co-authored a report titled Rings of Fire: Heat Risks at the 2024 Paris Olympics. (It was commissioned by the British Association for Sustainable Sport and FrontRunners, which work to promote sustainability and address the impacts of climate change in the realm of sport.)

Tipton worked on the report with Jo Corbett, a professor of human applied and environmental physiology at University of Portland, and climate scientists and communicators from the non-profit organisation Climate Central.

Together, they interviewed athletes from 15 sports, 11 of them Olympians, and recorded their experiences of performing amid rising temperatures around the world.

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“I was shivering. Nothing made sense. It felt like my body was shutting down,” says Indian triathlete Pragnya Mohan, describing runs in Ahmedabad and Chennai, in ambient temperatures of over 40 degrees Celsius.

It feels “scary”, “nasty”, “like your body is fighting you every step of the way”, said others such as British rower and two-time World Champion Imogen Grant, Japanese race-walking world champion Yusuke Suzuki, and Olympic pole-vault medallist Eliza McCartney and Olympic bronze-winning tennis player Marcus Daniell, both from New Zealand.

“Asking athletes to perform in ambient temperatures above 30 degrees Celsius, in sports that themselves raise body temperature sharply, can be unsafe,” Tipton warns.

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What the athletes are describing is called exertional heat stroke (EHS).

Core temperatures rise from 36.5 to 37.5 degrees Celsius to over 40 degrees Celsius, affecting the central nervous system. Immediate symptoms include confusion, loss of balance, apathy. Also, thirst, profuse sweating, fatigue and muscle cramps.

EHS can quickly slip into a red zone of chills, dizziness, an increased heart rate and some version of a shutdown event (risks include fainting, heart incidents and stroke).

In the initial stages, an athlete who doesn’t recognise the symptoms may push harder, because this is what they are trained and are able to do. “They may also push harder because they’re simply very suddenly disoriented due to cognitive impairment caused by the heat,” Tipton says.

If this seems strange, it is perhaps worth mentioning that elite athletes perform at rates that so far exceed the average human experience that, where the average man can run about 10 km per hour, Usain Bolt sprints at rates of over 44 kmph.

When a body so finely tuned unexpectedly collapses, there is long-term fallout too: anxiety. Ever since her experiences in 2016 and 2023, Mohan has had a nagging fear of the heat, she says. “Especially when I must adjust race strategies and pacing on the fly, I cannot tell if I am making the right decision.”

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Mitigation strategies need to be suited to the sport, says Walker J Ross, a lecturer in sport management at University of Edinburgh, and director of The Sport Ecology Group, a collective of academics dedicated to the intersection of sport and ecology.

Rescheduling or cancelling when required is likely to become the norm, Ross adds. For intermittent activities such as hockey, football or tennis, additional and extended breaks will become crucial. Heat training during workouts will become vital too.

“Athletes have historically cared largelyabout their skills and diet. That will change,” says Tipton.

A key challenge is also a fairly tricky one, he adds: athletes don’t yet know when to stop pushing and pause instead.

Mohan, for her part, says she is trying to learn from those who already seem to do this well.

“One quality that stands out in Olympian javelin thrower Neeraj Chopra is that he knows his body extremely well and stops before risking injury. This level of self-awareness is rare, even among Olympic gold medallists,” she says. “While I’m not at that level yet, I’m trying to become more aware of my body’s signals. I’m pushing myself to recognise when to stop.”