The mayors of Mexico City, Paris, Madrid, and Athens say they will ban diesel vehicles from their cities starting in 2025. The proposed ban is intended to improve air quality by reducing nitrogen-oxide (NOx) and particulate emissions from diesel engines.

NOx emissions from diesel engines made headlines around the world last year when it was revealed Volkswagen had developed software that enabled some of its diesel engines to pass U.S. emissions tests but allowed up to 40 times the legal limit of NOx to be emitted during normal on-road driving. NOx is instrumental in the formation of ground-level ozone, which can cause breathing difficulties, and has been linked to cancer.

But diesel engines have also been coming under increased scrutiny because they produce fine soot particles during combustion that cannot be trapped by particulate filters. These fine soot particles not only form visible pollution, but can penetrate the lungs and have been implicated in the development of cardiovascular illness.

The announcement, made at a Mexico City summit of mayors and leaders from 40 of the world's largest cities, is another hammer blow to the already battered reputation of the diesel engine. While only four cities have committed to banning diesels by 2025, few observers doubt more will follow.

Paris has been an early leader in the move to get diesels out of cities, having already banned diesel vehicles built before 1997 from entering the city, with restrictions increasing through 2020. In London, commuters at rail and bus stops are now shown air quality warnings, and there are growing calls for bans on diesel vehicles so the city can meet its legally mandated air quality targets. Chinese government policy favors the development of electric vehicles over diesels.