In 2017, the worst location for nitrogen dioxide pollution was Kensington and Chelsea, followed by Leeds and Doncaster.
Nitrogen dioxide (NO2) is one of the most harmful pollutant gases, irritating the lungs and potentially causing breathing difficulties. Road traffic is a leading source.
On Tuesday the mayor of London issued a high pollution alert for the capital as people across the UK basked in the unseasonably warm February weather. It was the first alert since last July, towards the end of a long heatwave and dry spell.