A high air pollution warning has been issued for London for the first time since August 2020, City Hall has said.
The Mayor of London warned that the capital is "importing pollution from the continent" alongside a build-up of local emissions.
An Imperial College London forecast said the capital would see a “further sustained import of particles” on Wednesday.
Adults and children with lung or heart problems and older people have been advised to avoid exercising outdoors.
Londoners have been advised to walk, cycle, or take public transport and avoid unnecessary car journeys.
"Toxic air is extremely dangerous for Londoners – and I’m doing everything in my power to tackle it,” said Mr Khan.
“Today we have seen the capital experiencing high levels of air pollution and are expecting this to continue for two days.
“For this reason, I am issuing a ‘high’ air pollution alert from today until Thursday.
“I’m urging Londoners to look after each other by choosing to walk, cycle or take public transport, avoiding unnecessary car journeys, stopping engine idling and not burning wood or garden waste, all of which contributes to high levels of pollution.
“This is particularly important in order to protect those who are more vulnerable to high pollution.
“While this alert is in place people with heart and lung problems should avoid physical exertion.”