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The Eiffel Tower was an ideal structure for taking these measurements and scientists have used these records to infer smoke pollution levels in the French capital in the 1890s.

But these records are not directly comparable to those made today. The closest equivalent is data on levels of particulates: microscopic particles released by burning fossil fuels and from traffic exhausts.
Exposure to particulate pollution is extremely harmful to health and 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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Security forces in Venezuela have fired tear gas and rubber bullets at people protesting Sunday’s disputed election result.
Thousands of people descended on central Caracas on Monday evening, some walking for miles from slums on the mountains surrounding the city, towards the presidential palace.
Protests erupted in the Venezuelan capital the day after President Nicolás Maduro claimed victory.
The opposition has disputed Mr Maduro's declaration as fraudulent, saying that after reviewing 73.2% of the voting tallies, it was clear that its candidate, Edmundo González, had won convincingly.

Opinion polls ahead of the election suggested a clear victory for the challenger.
Opposition parties had united behind Mr González in an attempt to unseat President Maduro after 11 years in power, amid widespread discontent over the country's economic crisis.
Choreographed celebrations but opposition demands proof
A heavy military and police presence was on the streets of Caracas with the aim of trying to disperse protesters and prevent them from approaching the presidential palace.
Crowds of people chanted “freedom, freedom!” and called for the government to fall.
Footage showed tyres burning on highways and large numbers of people on the streets, with police on motorbikes firing tear gas.

In some areas, posters of President Maduro were ripped down and burned while tyres, cars and rubbish were also set alight.
Armed police, military and left-wing paramilitaries who are sympathetic to the government clashed with protesters and blocked off many roads around the city centre.
Venezuela's government also announced a temporary suspension of commercial air flights to and from Venezuela with Panama and the Dominican Republic starting from 20:00 local time on Wednesday.
In a speech on Venezuelan state television, Mr Maduro said it is his "obligation to tell you the truth".
"We are all under the obligation to listen the truth, to gear up with patience, calmness and strength because we are familiar with this movie and we know how to face these situations and how to defeat the violent.”

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