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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.