Air quality alerts have been issued by Mayor of London Sadiq Khan, with moderate pollution expected until the weekend across 17 London boroughs.
The problem is expected to peak on Thursday in Westminster, City of London, Hammersmith, Kensington and Chelsea, Kingston and Wandsworth.
Pollution warnings will be displayed at bus stops, Tube stations and roadsides.
Mr Khan first announced the introduction of the alerts in August 2016.
Mr Khan has described London’s dirty air as a “public health emergency” and has said his office will continue to closely monitor pollution levels over the coming days.
The elderly and those with heart or lung problems should reduce strenuous activity, particularly outdoors, according to the official advice.
London breached its legal limits for toxic air for the entire year in the first five days of 2017.
The environmental law group Client Earth, which has successfully sued the UK government twice over poor air quality, said it was a “shameful reminder of the severity of London’s air pollution” and “shows why the mayor has rightly made tackling it a top priority”.