Grosvenor Place, which runs alongside the Queen's London residence, had the highest level of the toxic gas nitrogen dioxide in 2012.
Researchers found 152 milligrams of the gas per cubic metre of air, with a main contributor being diesel engine fumes. The EU legal limit is 40.
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The Hammers moved to the Olympic Stadium in May leaving their old site to be developed for housing.
The first officer shouted "look!" as the drone passed next to the right wing at around 11,000ft above south-east London.
They found a sheepish looking ewe with a traffic cone on her head in the middle of a field.