The warning has reignited calls from organisations for a "clean air act" with targets to reduce pollution levels in the UK.
Friends of the Earth say an act will be needed to protect the public from air pollution once the UK leaves the EU to "help safeguard existing legal protections".
The charity is also calling for:
"Clean air zones" in all areas failing limits for NO2, for all vehicle types, in place by 2018
A government commitment to phasing out diesel by 2025
Vehicle tax changes so that diesel vehicles are discouraged
A "carefully-designed" vehicle scrappage scheme for clean vehicles and alternatives to driving
Jenny Bates, Friends of the Earth air pollution campaigner, says road traffic is the biggest problem and diesel vehicles are the "worst of all".