The Ultra Low Emission Zone (ULEZ) is an area in London, England where a fee is charged for driving the most polluting vehicles. Plans were laid out under Mayor Boris Johnson and introduced by Sadiq Khan in April 2019 in Central London, covering the same area as the congestion charge. In the four months following its introduction there was a 20% reduction in emissions in Central London and the number of the worst polluting vehicles dropped from 35,600 to 23,000. In August 2023 it will extend to cover all of Greater London.