As somebody who groped his way to school through winter smogs in Sheffield in the 1950s and 1960s, I have always been sceptical about the claims of environmental campaigners that air pollution in British cities is now reaching critical levels of toxicity. I recall playing football on pitches where neither goal was visible from the halfway line. No doubt any therapeutic benefits of exercise were outweighed by the damage to our youthful lungs.

Yet recent headlines proclaim that our children are being exposed to illegal levels of toxic air, and London mayor Sadiq Khan has declared a public health emergency in the capital. The mayor quotes epidemiological studies claiming that 9,000 Londoners are dying prematurely every year because of poor air quality. Estimates of national fatalities have increased from 40,000 to 60,000 per year.