LONDON — When I moved back to London in 2017 after more than 10 years living abroad, the first thing that hit me was the smell of diesel and the dusty haze hovering over the city.

Within months I was short of breath and coughing regularly. I decided to download an app that monitors the city’s air-quality levels. The data was startling, to say the least.

Toxic air in London has been at illegal levels since 2010, and for a brief period last year, air pollution here was worse than Beijing’s, according to statistics published by the London mayor’s office.

More than 9,000 Londoners die prematurely each year because of the toxic air in Britain’s capital, where approximately 50 percent of the pollution comes from vehicles on the road, with 40 percent coming from vehicles that use diesel.