A section of Oxford Street near Marble Arch registered an annual average NO2 reading of 150 micrograms per cubic metre of air, the second-highest in the country.
Four sections of Marylebone Road, including near Madame Tussauds, were among the 50 locations in the UK - all of them in London - with the highest NO2 readings, along with Regent Street outside Hamleys, the toy shop.
"The thought that hundreds of thousands or millions of tourists and Londoners in a year may be exposed to air pollution this high is deeply troubling," said Simon Birkett, director of Clean Air in London.
A spokesperson for the mayor of London, Boris Johnson, claimed that the number of people living in areas where NO2 levels exceeded legal limits had halved since his election in 2008.
