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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Concern is growing for the welfare of a 28-year-old woman who has not been seen for more than two weeks.
A police officer in today's London Pride parade surprised his partner by proposing in the middle of the parade.
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