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Despite significant improvements in recent years, London's air pollution is still a concern. The Low Emission Zone (LEZ), which covers much of Greater London, was introduced in 2008 to encourage the most polluting heavy diesel vehicles to become cleaner.

To drive within it without paying a daily charge these vehicles must meet certain emissions standards that limit the amount of particulate matter (a type of pollution) coming from their exhausts. On 3 January 2012, the LEZ emissions standards became more stringent. More vehicles are affected, and those that were already affected need to meet tighter emissions standards (Link Transport for London).