The Green Party has published this picture, claiming that is shows "why we need a very low emission zone for central London ASAP, not in 2020".
It is a map of nitrogen dioxide (NO2) pollution in the congestion charging zone, put together by Assembly Member Jenny Jones.
It is based on NO2 emissions modelling carried out by Kings College London, not on actual readings.
More top news
Georgina Ferguson was on a carriage behind the one that was bombed. This is her very personal story of 7/7.
David Cameron unexpectedly appeared during a cookery segment.
Robert Quick, who was head of special operations for Scotland Yard questioned whether the Government should lay on charter flights.