MONTREAL — AS scandals go, it was a juicy one. Volkswagen, one of the world’s highest grossing automakers, persuaded consumers it had created a new generation of so-called clean diesel cars — until investigators discovered that defeat devices, which activated emissions controls only when the cars were tested, were concealing the fact that its vehicles emitted up to 40 times the permitted levels of pollutants during regular use.

Volkswagen has been punished with consumer opprobrium, a costly recall and plummeting sales. Yet the public outrage over the fraud obscures a much larger issue.

Volkswagen played a leading role in convincing people to accept a technology that in many countries is causing a precipitous decline in air quality for millions of city-dwellers: the diesel engine. Monitoring sites in European cities like London, Stuttgart, Munich, Paris, Milan and Rome have reported high levels of the nitrogen oxides and particulate matter, or soot, that help to create menacing smogs.

Recently, officials in Milan temporarily barred cars from the city; in Rome, too, persistent smog has forced the authorities to limit the use of private vehicles. Back in March, Paris was enveloped in a gray cloud of choking fumes that obscured the Eiffel Tower for hours and briefly earned the French capital the title of the world’s most polluted city, beating out even Beijing. An air quality expert in Britain reported that much of it was “stale diesel” from traffic emissions generated in European cities.