High pollution levels in London have led to air quality alerts being declared at bus stops, Tube stations and roadsides.

Air quality alerts are being displayed at 2,500 bus countdown signs and river pier signs across the capital.

Alerts are also being shown on 140 roadside message signs on the busiest main roads into London, with instructions to switch engines off when stationary to cut emissions.

And there are updates in the entrances of all 270 London Underground stations.

Image: St Paul's Cathedral on Thursday morning

The alerts were issued at around 4.30pm under orders from the capital's mayor, Sadiq Khan.


He 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."

People who suffer discomfort such as sore eyes, a cough or sore throat have been advised to reduce their activity outdoors.

Image: Air pollution alerts at bus stops in London

In recent months Mr Khan announced plans to issue such alerts at times of high pollution.

A spokeswoman for the mayor said the warning is expected to remain in place until later on Friday, although this will be kept "under close review".

Weather forecaster Dr Chris England said: "The problem at the moment is we're under high pressure with very little wind, so any pollutants really haven't anywhere to go and hence build up.

"There will be changes over the next few days, with the high drifting off.

"So air quality should improve later on Friday and over the weekend as the breeze picks up."

The Department for Environment, Food and Rural Affairs measures air pollution on a scale of 1-10, from lowest to highest.

Image: The HMS Belfast on the River Thames can barely be seen through Thursday morning's fog

Anything between seven and nine is recorded as "high". Greater London is currently at seven, while the South West is at nine. Ten is "very high".

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.

:: How different areas of London are affected

Particulate matter can worsen lung and heart conditions when there is high air pollution and with long-term exposure can raise the risk of dying early.