In 2008, London created its Low Emission Zone (LEZ) to encourage the most polluting heavy diesel vehicles driving in the city to become cleaner.

FORBES: Air Quality In China

In January, 2012 the LEZ emissions standards became more stringent, and tens of thousands of trucks and buses were retrofitted with filters and other systems to enable them to comply.

FORBES: Air Quality In China

He said the introduction of the Congestion Charge in 2003 and the Low Emission Zone (LEZ) in 2008 had only made a "tiny difference" to air quality in the capital.

BBC: News, London

The LEZ covers most of Greater London, and to drive within the LEZ without paying a daily charge, vehicles must meet certain emissions standards that limit the amount of particulate matter coming from their exhausts.

FORBES: Air Quality In China