The evidence is clear: low-emission zones like London’s Ulez work

Many cities across the UK have not met legal limits for air pollution that should have been reached by 2010. Photograph: LeoPatrizi/Getty

Clean-air or low-emission zones are being rolled out in more UK cities, and the London mayor is proposing to extend the ultra-low emission zone to the suburbs.

These zones are designed to speed up the attainment of legal limits for air pollution and to lessen the health burden from breathing poor air. While not new, such zones can be controversial, especially when they are introduced or expanded.