Other highly polluted areas include Oxford Street near Marble Arch, which registered at 150 micrograms, and Cockspur Street near Trafalgar Square on 138 micrograms.
The fourth and fifth locations were Park Lane and Knightsbridge - which measured 135 and 134 micrograms respectively.
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Police say he may have attacked more children and urged them to come forward.
The suspect walked up to a Tesco employee and shoulder barged him to the ground before punching him repeatedly in the face.
Up to 200 noisy motorists regularly meet in Imperial Way, Croydon attracting crowds of up to 400 people.