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.
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Photographer Jeffrey Milstein flew over the capital and took some pretty eye-catching images