On 12 July 1924, cross-country runners competing in the Paris Olympic Games struggled to cross the finish line of the 10,650m (6.62 mile) course due to extreme heat. Twenty-three out of 38 long-distance runners dropped out of the last-ever Olympic cross-country race as temperatures in the shade in the city soared to 33C (91F). Eight of the athletes who did finish were reputedly taken away on stretchers.
An unofficial thermometer placed near the start line (probably in the sun) measured an eye-watering 45C (113F), temperatures one newspaper described as “Senegalese”. The heat created a “suffocating, putrid atmosphere, given off by the fermentation of the garbage, making it an asphyxiating and unhealthy receptacle for pestilential odours”.

The day of the cross-country race was exceptionally hot – the average daily maximum temperature during the Olympic period in 1924 was 24C (75F). Now, 100 years later, Paris is again host to the Olympic Games, and the Paralympic Games shortly after. And once again, heat is a challenge that athletes and spectators alike may have to contend with.

The difference now is that the climate has changed dramatically since Paris last hosted the Olympics. Average temperatures in the city are significantly higher today than in 1924. Air pollution, on the other hand, has fallen as the city has moved away from fossil fuels, especially coal, and clamped down on car use.

During the Paralympics, which take place in late August and early September, average temperatures have warmed by 1C (1.8F) over the past 100 years, the scientists conclude. Overall, annual temperatures have warmed in the French capital by 1.8C (3.2F) since 1924.

“Heat could be a killer if you aren’t able to get your nutrition and hydration right,” she says. Mohan has previously suffered from diarrhoea, mental fogginess and dehydration after competing in extreme heat. “I have competed in temperatures 40C (104F) or higher before,” she says. “After the competition you really start feeling the effects – your body goes into shutdown mode.”

Sarah Safieddine, a climate scientist at the French National Centre for Scientific Research, says it is still too early to predict whether a heatwave will hit Paris during the Olympics.

But the national weather service, Météo France, has said that this summer is likely to be warmer than average. The recent El Niño, which ended in April, is still influencing global temperatures, says Safieddine. “So we have global temperatures that are higher on average than previous years,” she says.

Summer temperatures in Paris, like many major cities, are intensified by the “urban heat island effect”, when concrete roads and buildings absorb more heat than in rural areas. But there is some relief. “We are lucky to have big parks in the middle of the city which act as urban cool islands,” says Safieddine. (Read more about how green spaces, white roads and water features can help keep cities cool during deadly heatwaves).

The French capital has experienced deadly heatwaves in the past. In August 2003, extreme heat killed 15,000 people in France, including 735 people in Paris. A 2016 study concluded that the majority of the 2003 heatwave deaths in Paris were a result of human influence on the climate.

Extreme heat has become much more common in Paris since the city last hosted the Olympics. In 1924, there were 29 “hot” days, with maximum temperatures of 25C (77F) or higher, and two “scorching” days, when temperatures reached 30C (86F) or higher. Last year, there were 86 hot days and 26 scorching days in Paris, according to the Rings of Fire report.

Athletes have expressed concern about competing in the heat, following the Tokyo Olympics in 2021, which were “the hottest in history” and saw competitors fainting and struggling to breathe due to the oppressive temperatures.

The rower says that he struggles with thermoregulation due to his disability, a spinal cord injury that means he is paralysed from the waist down. “I have done a lot of work on cooling methods and appropriate warm-up techniques to ensure that the heat doesn’t affect me too poorly,” he says.

Extreme heat has a detrimental impact on athletic performance, says Mike Tipton, co-author of the Rings of Fire report and professor of physiology at the University of Portsmouth’s Extreme Environments Laboratory in the UK.

“As the temperature goes up, you get a reduction in performance – marathon performance declines by about one minute for every 1C (1.8F) increase in air temperature above 15C (59F),” says Tipton.

The ideal temperature for continuous endurance, such as cycling or running, is 11C (51.8F), says Tipton.

Heat compromises blood flow to the gut and leads to excessive sweating, which can cause dehydration, cramping, exhaustion and fainting. Muscle glycogen, the body’s stored form of glucose, is used up more quickly when it is hot, impairing endurance performance, says Tipton.

It’s not just athletes’ physical performance that is diminished by heat, he adds. “Heat changes your cognitive function in terms of complex decision making, makes you more irritable and more agitated, reduces motivation, and decreases mood.”

This means that scorching temperatures can also impact more “passive sporting events”, such as archery or shooting, where the “margins of error are very small”, he says. (Read more about the sinister ways heatwaves warp the mind.) Acclimatising to high temperatures is now a critical part of training for athletes, says Tipton. “If you get your heat preparation wrong, it’s a risk to life and limb.”

Mohan is preparing for high temperatures at the Paris Olympics by spending up to an hour in saunas several times a week and turning off the fans while doing her indoor cycling training.

“Being an outdoor sport, whatever conditions you can think of, we have trained in them,” says Pritchard.

Organisers say they are taking a number of precautions to protect athletes from extreme heat. Outdoor endurance events such as the marathon and triathlon will start early, at 8am, when it is cooler, and athletes will have access to water stations and shaded areas at regular intervals, says Lambis Konstantinidis, executive director of planning and coordination at the Paris Olympics. The triathlon will be postponed if the temperature rises above 32.2C (90F), Konstantinidis says.

The AC question
In a bid to halve the carbon footprint of the Paris Olympics compared with previous games, air conditioning units have not been installed in the Olympic Village.

Instead, the village will be cooled by a geothermal system which pumps water from deep below the ground to keep indoor temperatures 6C (10.8F) lower than outside, according to the organising committee. More than 9,000 trees have also been planted in the Olympic and Paralympic villages to provide natural cooling. The “natural freshness” of the River Seine will also help cool the city centre, says Konstantinidis.

Some delegations, including the US, have said that they plan to bring their own air-conditioning units, for athletes’ rooms.

“In our conversations with athletes, this was a very high priority and something that the athletes felt was a critical component in their performance capability,” Sarah Hirshland, chief executive of the Olympic and Paralympic Committee leadership committee, said during a press briefing in June.

Analysis included in the Rings of Fire report shows that in the past 100 years there has been a significant increase in the number of “tropical” nights, when temperatures do not drop below 20C (68F). Between 1924 and 1933, only four tropical nights were observed in Paris. In 2014-2023, the city experienced a total of 84 tropical nights – 21 times more.

Mohan says she has “mixed feelings” about the lack of air conditioning in the village. She says she understands organisers’ commitment to sustainability but is worried about the impact heat will have on her health and recovery following the triathlon. “Recovery is very important for athletes,” she says. “I want to be safe. I don’t want to come back injured and I don’t want to not have my best performance on the day.”

The organisers of the games have said that Paralympic delegations will be provided with one free air-conditioning unit for every three athletes for medical reasons, as some athletes experience thermoregulation problems.

Particulate levels in 1920s Paris may well have been about 100 micrograms per cubic metre, estimates Giles Harrison, professor of atmospheric physics at the University of Reading in the UK. This is about five times higher than pollution levels today, BBC analysis reveals.

The main sources of Parisian pollution in 1924 were coal-powered factories and domestic heating as well as motor vehicles, says Harrison.

The 1924 Stade de Colombes Olympic stadium, located in north-west Paris, was surrounded by factories, says Catherine Radtka, a historian of science at the French National Conservatory of Arts and Crafts.

“There were tyre factories, sugar and alcohol refineries and gas plants in the area, emitting fumes, dust and noxious gases,” says Radtka. “There were already complaints from farmers at the beginning of the 20th Century…they said the fumes were destroying their crops.” Physicians had also started sounding the alarm about the health risks of air pollution, she says.

Iconic landmarks, such as the Notre Dame, were caked in thick, black dust. “Everything was black in Paris because of the smog,” says Radtka.

In 2023, by contrast, monthly levels of large particulates (PM10) in the city averaged 19 micrograms per cubic metre of air – a five-fold reduction compared with 1924.

Much of this improvement has occurred relatively recently. In 2003, monthly PM10 levels still averaged 67 micrograms per cubic metre, but air quality improved in the decades since as the city has prioritised cyclists and pedestrians over cars. Levels of the smallest, most dangerous particles, measuring less than 2.5 microns across (known as PM2.5), have fallen by similar proportions since 2003.

“Cars will be very restricted in the centre of Paris during the Olympics,” says Safieddine. “I don’t think pollution will be a problem unless we have high temperatures. Heatwaves are often associated with pollution events.”

Mohan says her bigger concern is polluted water in the River Seine. The river failed water quality tests a month before the start of the games due to high levels of E. coli. But organisers say that the Seine is safe to swim in, with Paris mayor Anne Hidalgo and other members of the Olympic committee going in for a dip in mid-July to prove that the water is clean. (Read about the efforts to tackle the Seine’s historic pollution.)
Mohan hopes to swim in the Seine at the Paris Games. “Growing up, I never got to swim in my own river in Ahmedabad because it was so polluted,” she says. “It’s scary that in future the triathlon might not be an outdoor sport due to climate change.”
Extreme heat is already changing her sport. Mohan says she can no longer train outdoors in India due to the scorching temperatures. This reality has forced her to move away from her home and train in London instead. The conditions faced by the cross-country runners in the 1924 Olympics are becoming increasingly common for athletes. Mohan has competed in temperatures exceeding 40C (104F) and 50-80% humidity.
“If I had a choice, I wouldn’t have done those races,” she says. “But as athletes we are wired to perform our best and to keep pushing ourselves.”

_________________________________________
The graphs in this story were created by Diana Mota.

Writer: Isabelle Gerretsen and Miriam Quick