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responsible for the vast majority of air pollution-related deaths.

Heat could be a killer if you aren't able to get your nutrition and hydration right – Pragnya Mohan

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."   

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The graphs in this story were created by Diana Mota.

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