FER FGBF Paris may well have been about 100 microgram RGRG

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.