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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Arriva Rail North, Greater Anglia and South Western Railway are all affected.
The Duchess of Cambridge was beaming on the steps of the Lindo Wing hours after giving birth. Mums are also revealing how they looked.
Everyone has their heads down and their eyes averted but I know they can sense my presence.