Mayor Sadiq Khan issues air quality alerts across London
Air quality alerts were issued across London ahead of the evening rush hour.
Mayor of London Sadiq Khan issued the warnings for the first time at bus stops, Tube stations and roadsides at around 4.30pm on Thursday because of what he described as "high pollution levels".
Anyone experiencing discomfort such as sore eyes, a cough or sore throat was advised to consider reducing their activity, particularly outdoors.
Mr Khan said: " Londoners need to know when the city is suffering from high pollution levels so they can take any necessary appropriate measures to protect themselves from poor air quality.
"This is particularly crucial for Londoners who are vulnerable, such as asthma sufferers."
The alerts were being displayed at 2,500 bus stops and river piers, the entrances of all 270 Tube stations and on 140 signs next to the busiest main roads into London with instructions to switch engines off when stationary to reduce emissions.
A spokeswoman for the mayor said the warning is expected to remain in place throughout Thursday night and come to an end tomorrow, although this will be kept "under close review".
King's College London described air pollution in London as "high" on Wednesday due to a n area of high pressure over the UK resulting in calm, settled and cold conditions and poor dispersal of local pollutants.
The Environment Department's (Defra) air quality monitoring system UK-Air has notifications for high air pollution in two regions.
In London, several sites have high air pollution levels for tiny particles known as PM10s or PM2.5s, which come from sources such as burning fuel in vehicle engines.
There is also a high pollution notification for the South West due to high PM2.5 and PM10 levels in Bristol.
Particulate matter can worsen lung and heart conditions in pollution episodes and with long term exposure raise the risk of dying early.
Official advice warns that adults and children with lung problems and adults with heart problems should reduce strenuous physical activity when air pollution levels are high.
People with asthma may need to use their reliever inhaler more often and older people should reduce physical exertion.
UK-Air said no areas of the UK were suffering from very high air pollution and all other regions had low or moderate levels of pollution.
Jenny Bates, Friends of the Earth air pollution campaigner said solutions for tackling high pollution incidents should focus on restricting traffic, not people.
"It's outrageous that those with vulnerable lungs, including children and the elderly, are told to stay at home when the air is bad.
"Everyone should be able to go about their business, without being afraid of their air they breathe.
"When pollution episodes are high, the mayor should introduce emergency traffic restrictions to bring pollution levels down quickly," she urged.