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know exactly how polluted Parisian air was in 1924, when air quality monitoring was in its infancy. There are some records dating back to the early 20th Century, such as measurements of suspended dust from industrial processes or measurements of atmospheric electricity. 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.

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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)