A London-wide air quality warning has been issued as the cold, clear conditions cause pollution in the capital.
Mayor Sadiq Khan has triggered the air quality alert, which will be displayed at bus stops, tube stations and roadsides to give Londoners information about the situation.
Air which has brought pollutants from industrialised and urban areas of Germany and the other side of the Channel, is combining with local emissions that have built up in the still conditions in London on Thursday.
The London Air Quality Network run by King's College London said there is expected to be moderate levels of pollutants known as particulates, with high levels of PM2.5 particulates at busy roadsides.
Moderate levels of nitrogen dioxide are expected close to busy roads as a result of poor dispersal of the pollutant.
Under such conditions, adults and children with lung problems, and adults with heart problems, who experience symptoms, are advised to consider reducing strenuous physical activity, particularly outdoors.
The mayor's air quality alert system was first triggered in December, after it was announced in August.
Mr 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."
He said London was delivering on an ambitious programme to tackle air pollution, including bringing in ultra-low emissions zone and cleaning up the bus fleet, but called on the Government to play its part.
"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."