Schoolchildren living in London’s ultra-low emission zone (Ulez) were nearly four times as likely to switch to walking and cycling after its introduction compared with those outside the zone, new research suggests.

The study, published on Wednesday, followed the travel habits of almost 2,000 children over two years in London and Luton.

The Ulez was introduced in an effort to reduce traffic-related air pollution, which is known to affect neurodevelopment, cognitive ability, and lung function in children.

The authors said that further monitoring is needed, but it is evidence for other cities of the potential impact of clean air zones.

“This study is important because it addresses multiple, interconnected health issues that children face today,” said Dr Christina Xiao, the lead author of the study, and a population health researcher at the University of Cambridge.

“We know that [car and van] use negatively impacts children’s health by reducing opportunities for physical activity and increasing exposure to air pollution, which can contribute to conditions like childhood asthma,” she said.

Dr Xiao added that until now there has been limited evidence of the direct impact on children’s health of these policies.

The researchers surveyed 1,000 children across 44 schools in central London about how they travelled to school in 2018-19, prior to the Ulez’s introduction, and then again in 2019-2020 after it was introduced.

The same survey was repeated for a similar number of children outside the Ulez zone in Luton, a town chosen because it has a similar demographic structure and household income levels to central London.

The results showed that four in 10 children in central London, who previously travelled to school by car, then switched to walking, cycling, or public transport following the Ulez’s introduction.

However, in Luton only two in 10 children changed their school journeys over the same period.

London’s walking and cycling commissioner, Will Norman, said: “The Mayor and I are delighted that this study shows our decisive action is having an impact.

“The decision to introduce the Ultra Low Emission Zone was a major step forward in our work to tackle London’s toxic air. It was a difficult decision, but necessary to save lives.”

London’s former Conservative Mayor, Boris Johnson, proposed in 2015 that the Ulez should be introduced in central London, and now following later expansions, it is the largest clean air zone (CAZ) in the world – covering the most citizens.

In 2016, before it started, London had the highest concentration of nitrogen dioxide – a key air pollutant from road traffic – of any city in the UK. The capital breached UK legal limits for the gas for 4,130 hours that year, and nearly 10,000 people died prematurely from air pollution.

“[The Ulez] is what scientists call a natural experiment, if we show big health effects then that has a big implication for how cities around the world manage their air quality,” said Dr Chris Griffiths, professor of primary care at Queen Mary University and author of the study.

London is now one of 14 major cities across England and Scotland, including Birmingham, Portsmouth and Glasgow, with a CAZ.

In 2023, the decision by Labour Mayor Sadiq Khan, to expand the Ulez to cover all London boroughs, was met with significant public and political opposition.

Five Conservative-run councils even challenged the decision in the High Court – but eventually lost.

And in July last year, Sir Keir Starmer said there was “no doubt” the issue was the reason his party failed to win the Uxbridge and South Ruislip by-election.

Neil Garratt, member of the London Assembly and leader of the city’s Conservative Group, said: “This study confirms what we have been saying all along, which is the benefits of the Ulez in central London, which the study is looking at, are large and the costs are quite small.”

However, he said that the recent expansion to the outer areas has diminished these benefits because “the alternatives to your car are almost non-existent”.

Dr Xiao said: “The Ulez is one effective policy measure, among other complementary initiatives, including providing more accessible public transport and providing safer walking and cycling infrastructure.

“So decisions to introduce similar policies should consider local context, such as existing air quality initiatives and transportation needs.”

The paper was based on the travel patterns of children for just one year after the Ulez was introduced. But the broader study, called Chill, has been surveying children and the way they travel to school for the last five years.

Dr Griffiths said the team now plans to evaluate the most recent data to see if the Ulez’s initial impact of the has been maintained.