London air pollution
Air quality alerts have been issued in some parts of LondonREUTERS/Suzanne Plunkett

Mayor of London Sadiq Khan has issued air quality alerts in some parts of the capital as pollution reached levels high enough to affect the health of vulnerable people. Messages have been displayed at 2,500 bus countdown signs, at the entrance of all 270 tube stations and on 140 roadside signs, where motorists have been warned to switch off engines when stationary to cut emissions.

People with asthma, heart or lung problems have been warned not to over-exert themselves, as is anyone with sore eyes, a cough, or sore throat. Cyclists, people playing outdoor sports and anyone eating outside are also warned to be careful and parents are warned to avoid taking small children and babies near busy roads.

The warning was issued at 4.30 pm on Thursday (1 December) and remains in place until Friday at the earliest. The problem is exacerbated by high air pressure and minimal wind, which means the pollutants are not dispersed and instead sit over the capital. The wind is expected to pick up over the weekend which should disperse the worst of the pollution.

Mayor 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."

Environmental group Friends of the Earth (FOE) said it was unacceptable to ask pedestrians to avoid going out when the pollution was being caused by vehicles. FOE said the mayor should be given new powers to restrict car usage when pollution reaches high levels.

"The solutions proposed for dealing with the latest smog have things backwards: the first step should be restricting traffic not people," said a spokeswoman, reports The Guardian. "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 the air they breathe."

Diesel vehicles are responsible for much of the pollution across the capital, and there have been moves to charge people who own diesel-powered cars more to use the road network. According to a report published by the Royal College of Physicians and the Royal College of Paediatrics and Child Health, air pollution kills 40,000 people prematurely in the UK each year, 9,500 of them in London.