And now the mayor has pledged to invest £875m to tackle air quality over the next five years - more than doubling spending on it, after his predecessor Boris Johnson committed £425m.
Earlier this year, Mercer said London ranked 39th in the world for quality of living due to smog and traffic in a ranking of 450 cities.
Ahead of the publication of the Transport for London's (TfL) draft business plan later this week, Khan announced the investment, with many parts of London in breach of legal limits for nitrogen dioxide (NO2).
He said: "With nearly 10,000 Londoners dying early every year due to air pollution, tackling poor air quality is a public health emergency that requires bold action."
He also threw down the gauntlet to the government to follow suit.
London’s air quality continues to breach legal levels, which is why I have taken this unprecedented action. The onus is now on the government to prove that they share my ambition.
By funding a national diesel scrappage scheme and reforming vehicle excise duty the government could also prove that they understand the enormity of our air quality emergency.
The mayor has outlined a range of programmes to tackle the issues arising from poor air, including looking to launch an ultra low emission zone (ULEZ) from 2019 (which'll see £610m of investment overall), consulting on the emissions surcharge which will remove older polluting vehicles from central London and introducing five low emission neighbourhoods across eight boroughs.
Plans have already been laid out for transforming London's buses by phasing out diesel-only vehicles and bringing in hybrid of zero-emission double-deckers from 2018.
Khan showed off the capital's first hydrogen bus last month - it will be trialled on London's roads next year.