The mayor has issued air quality alerts across the capital, with moderate pollution expected until the weekend across 17 boroughs.
Some boroughs in particular are expected to have the worst of it today: Westminster, City of London, Hammersmith, Kensington and Chelsea, Kingston and Hounslow and Wandsworth. There is also a chance of moderate nitrogen dioxide at roadside locations during tonight's rush hour, according to King's College London's Air Quality Network.
Alerts have gone live across the bus countdowns, at Tube stations, roadside signs and on Transport for London's (TfL) website. People have been advised to spend less time outdoors during the alert.
Sadiq Khan said: "The extent of our air quality crisis means that I have today triggered a London-wide air quality alert, ensuring Londoners have all the information possible. It's crucial that Londoners who are vulnerable, such as asthma sufferers, are able to take appropriate measures to protect themselves."
In December, the mayor pledged to invest £875m to tackle air quality over the next five years and has outlined a range of programmes aimed at tackling the issue, including plans to launch an ultra low emission zone from 2019.
Khan also unveiled the capital's first double-decker hydrogen bus as part of a commitment to phase out diesel buses, which will be trialled on London roads this year.
He has pressed for more action on air quality from the government.
Read more: The mayor is doubling funding on cleaning up London's air
"I am again appealing to the government to urgently play their part too," he said. "By funding a national diesel scrappage scheme, reforming vehicle excise duty and bringing in a new Clean Air Act the government could finally prove that they match our ambition. This requires all levels of government to work together as an urgent priority to finally tackle the problem once and for all."
London breached its legal limits for toxic air for the entire year within the first five days of 2017.