A five-year study of more than 3,400 primary school children in London and Luton has found that youngsters living within London's Ultra Low Emission Zone (Ulez) showed rapid lung recovery after the scheme cut vehicle pollution from 2019 onward. Researchers say children whose lung growth had previously been stunted by dirty air caught up with less-polluted peers within a few years, offering what they describe as the clearest evidence yet that local clean air policies can undo some of the physical harm pollution inflicts on developing lungs.
The research, known as the CHILL study, tracked children aged six to nine attending 85 schools in central London and Luton, testing their lung function and capacity every year from before Ulez launched through four years afterward, according to Queen Mary University of London's project page. The Luton children served as a comparison group because the town has a similar mix of air pollutants to London but lower overall pollution levels, and it also introduced some smaller-scale clean air measures during the same period.
What did the study find?
At the outset, children in London had measurably smaller lung capacity than their counterparts in Luton, but by the end of the five-year tracking period the two groups had reached nearly identical lung capacity levels. Researchers say this amounts to an accelerated "catch-up" in lung growth among the London children that would not be expected under normal development.
"While we would expect children's lungs to grow year on year, our results indicate that the London children's lung growth had accelerated during the four years to 'catch up' with the control group in Luton to reach similar levels of lung capacity," the researchers say.
The proportion of London children whose lung capacity was classed as "clinically impaired" — a marker associated with coughing or breathlessness — fell from 14% to 9% over the study period, a figure also highlighted in separate reporting on the study. In Luton, where more limited pollution measures were in place, the equivalent figure moved from 9% to 7%.
Why are researchers so surprised by the results?
The scale and speed of the improvement caught even the study's own team off guard, according to lead researcher Professor Chris Griffiths of Queen Mary University of London.
"I was absolutely stunned when I first saw the results," Prof Chris Griffiths, a senior author on the study, told a national broadcaster.
"The speed of catch up in lung capacity in the London group was surprising and impressive."
"This shows an ambitious clean air zone can drive pollution levels down, rapidly restoring children's stunted lung growth."
The core measurement used in the study assessed how much air a child could forcibly exhale in one second following a deep breath. In everyday terms, Griffiths explained, the improvements recorded could translate into London children being able to run as fast as their Luton counterparts without becoming breathless, or matching them in a task as simple as blowing out the same number of candles. A second lung capacity test also showed meaningful gains, though smaller ones, suggesting further improvement may still be possible.
How does this connect to earlier pollution research?
The findings build directly on a 2018 Queen Mary University study, which found London children exposed to diesel-heavy pollution had lung capacity roughly 5% smaller than expected, according to the university's earlier research summary. That work, part of the EXHALE project, was published in The Lancet and had already established a link between traffic pollution and stunted childhood lung growth, according to the CHILL project's news archive. CHILL was subsequently designed as a natural experiment specifically to test whether Ulez would reverse that damage, according to the Health Research Authority's study summary.
Air pollution's toll on children has drawn wider attention in the UK beyond this study. In 2020, Ella Adoo-Kissi-Debrah, a nine-year-old from London who died following an asthma attack in 2013, became the first person in the country to have air pollution formally listed as a cause of death. Separately, research has also examined pollution's effects on reproductive health, with a study of over 2,000 men finding that exposure to pollutants including ozone and nitrogen dioxide produced epigenetic changes in sperm DNA that could affect male fertility.
Why is London's Ulez considered significant globally?
Researchers describe London's scheme as the largest and most ambitious clean air zone in the world, according to coverage of the published findings. Introduced in 2019 by Mayor Sadiq Khan to curb nitrogen dioxide emissions from motor vehicles, the scheme required older, more polluting vehicles to pay a daily charge to enter central London, with the zone later expanded to cover the entire capital. The policy attracted both strong support and vocal political opposition, including public protests.
"There are more than a billion kids living in cities around the world, most of them in polluted environments, getting a really bad start in life. That's why these are important findings with global relevance," Griffiths said.
What caveats do independent experts raise?
Professor Anna Hansell of the University of Leicester, who was not part of the research team, said the improved lung function documented in the study is likely to bring lasting health benefits for the children involved. She described the work as carefully conducted by well-respected researchers and noted that comparing London directly against Luton was valuable, since air pollution has been falling generally over time regardless of local policy.
"This proved particularly useful, given that the study period included the Covid-19 pandemic – and demonstrates the findings in London are not due to changes related to the pandemic," she said.
She also pointed out that children in the London group may have become more likely to walk or cycle to school after Ulez was introduced, which could itself have influenced the results. Kevin McConway, emeritus professor at the Open University, said the pandemic's full impact still needs careful accounting for, and suggested that examining other cities could help determine whether unmeasured differences between London and Luton — rather than Ulez itself — explain some of the gap.
What does the wider evidence base show?
Dr Helen Wood of Queen Mary University of London, the study's lead author, said the results are encouraging but should not lead to complacency, since pollution levels in London, Luton and other UK cities still exceed World Health Organization guideline limits.
"We know clean air zones are a complex area and the impact on businesses and individuals must be considered. What we are doing is adding new evidence to inform the debate," Griffiths added.
The full study, titled "Impact of the Ultra Low Emission Zone on lung function growth in children in London, UK: a prospective parallel cohort study," has now been published in The Lancet Public Health, carrying the DOI 10.1016/S2468-2667(26)00163-5, according to medical research reporting on the paper. The project involved researchers from the universities of Bedfordshire, Oxford, Cambridge, Edinburgh and Southern California alongside Queen Mary University of London.
What happens next?
Documentation from the CHILL research team indicates that final data analysis from the study programme was still ongoing as of a 2024 project poster, suggesting further detailed findings may yet emerge from the same cohort, according to the CHILL study's published materials. Separately, other efforts to shield children from urban pollution and traffic have continued elsewhere in England, including temporary closures of residential streets in Leeds that campaigners hope could feed into a national play strategy giving children safer outdoor space.
- The study tracked more than 3,400 primary school children aged six to nine across 85 schools in London and Luton over five years.
- London children's lung capacity, initially smaller than Luton's, reached near-identical levels by the study's end.
- The share of London children with clinically impaired lung function fell from 14% to 9%, compared with 9% to 7% in Luton.
- The findings build on a 2018 Queen Mary study that found London children's lung capacity was around 5% smaller due to diesel pollution.
- The research is published in The Lancet Public Health and involved five UK and US universities.
