Air pollution in London passed levels in Beijing this week, figures have shown, with popular wood burning stoves blamed for exacerbating the problem.
On Monday London mayor Sadiq Khan issued the highest air pollution alert in London for the first time, and said on Tuesday that the capital's 'filthy air' is now a 'health crisis.'
Readings at 3pm on Monday showed that air at locations in the capital were worse than in notoriously smoggy Beijing, hitting a peak 197 micrograms per cubic metre for particulate matter on the Air Quality Index. Pollution in the Chinese city only reached 190, which is still deemed 'unhealthy.'
Although nitrogen dioxide levels in London rose higher than China in 2014, it is believed to be the first time particulate readings have exceeded those in the far east.
Experts at King's College London said the recent spell of unhealthy pollution was the worst since April 2011 in the capital and was being caused by cold, calm and settled conditions combined with 'traffic pollution and air pollution from wood burning.'
Temperatures have fallen below zero overnight over the last few days, meaning householders are burning more fuel to keep warm.
"This was the largest contribution from wood burning measured during the winter so far," said a spokesman for King's College.
More than a million homes in Britain now have a wood burning stove with 175,000 new ones installed every year.
Demand for the stoves, which cost between £400 (NZ$700) and £7,000 (NZ$12,000), has tripled in the last five years - partly down to the savings they can make to energy bills.
Last year experts at the University of Southampton warned that wood burners 'liberate significant amounts of particulate pollution into the outdoor air' and said they risked undoing the good work of the Clean Air Act which was brought in following the Great Smog of 1952, which is estimated to have killed 12,000 people.
Over the past few days, many parts of the capital have recorded double the legal limits of emissions. Some schools banned children from playing outdoors, and Public Health England warned people not to exercise outside.
The mayor said the situation was becoming so toxic to children that hundreds of schools will now be audited to see whether gates and play areas can be moved away from busy roads.
'No idling' zones are likely to be implemented to prevent drivers leaving their engines running while waiting for children on the school run, while the most polluting vehicles may be banned entirely from driving up to entrances.
Schools will also be encouraged to plant hedges and bushes around their sites to provide barriers to block out fumes and children will be encouraged to walk and cycle to cut down on lifts.
Mr Khan said: "Every child deserves the right to breathe clean air in London and it is a shameful fact that more than 360 of our primary schools are in areas breaching legal pollution limits.
"London's filthy air is a health crisis and our children are particularly vulnerable to the toxic effects of air pollution.
He said the new air quality audits would be "a strong step towards helping some of the most polluted schools in London identify effective solutions to protect pupils from toxic fumes".
Air pollution from sources including factories and vehicles, particularly diesel engines, is linked to the early deaths of around 40,000 people a year in the UK - and causes problems such as heart and lung diseases and asthma.
In children it can lead to coughs, bronchitis and asthma, and harm the development of their lungs and brains.
A letter from 100 London schools, coordinated by Greenpeace, is calling on the mayor to tackle diesel vehicles on the roads, make walking and cycling to school safer, boost public transport and speed up the switch to clean vehicles.
The British Lung Foundation also called for more to be done to clean up London's air and said the government must consider banning diesel cars.
"Children living and attending school in highly polluted areas are more likely to have damaged lungs when they grow up," said Dr Penny Woods, Chief Executive of the British Lung Foundation.
"Air pollution contributes to 9,500 early deaths in London every year. It worsens existing lung conditions and increases the risk of getting lung cancer.
"It's a complete no-brainer: investing in making cycling and walking safer and more accessible in our cities - and moving towards ditching diesel will not only help clear up our roads, but will clean up the air we're all breathing too."
Rebecca Abrahams, head teacher at St Luke's Church of England school in Tower Hamlets, said: "We have a duty to protect the children in our care, but sadly, even while they play outside at lunch, they are being harmed by invisible air pollution from traffic.
"Given what we know about the life-long consequences of exposure to air pollution as a child, it's imperative we clean up London's air without delay."
Friends of the Earth London campaigner, Sophie Neuburg said: "We strongly welcome any action to protect children's growing lungs, and no-idling zones around schools are an important way to reduce children's exposure to pollution.
"But schools shouldn't have to take dramatic steps such as moving playgrounds and entrances to keep their pupils safe - London's children deserve better."