London's ULEZ linked to faster lung growth in children, five-year study finds

The Children's Health in London and Luton study — CHILL — followed more than 3,400 children aged six to nine across 84 primary schools, comparing London, where the Ultra Low Emission Zone launched in 2019, with Luton, a town with a similar mix of traffic pollution but no clean-air zone. Every year from 2018 to 2022, the children blew into spirometers to measure how their lungs were growing.

The results, published in The Lancet Public Health, surprised the researchers themselves. They had expected cleaner air might at best halt the damage traffic pollution does to developing lungs. Instead, the data suggests recovery.

At a glance

• The CHILL study followed more than 3,400 children aged six to nine across 84 primary schools in London and Luton
• Children had lung capacity tests every year from 2018 to 2022
• London children's NO2 exposure fell twice as fast, and more than twice as much, as children's in Luton
• London children's FEV1 lung capacity grew 10ml per year more than their Luton counterparts
• Clinically significant lung impairment in the London group fell from 14% to 9% over the study

What the numbers show

Over the five years after the ULEZ was introduced, London children's exposure to nitrogen dioxide fell twice as fast — and by more than twice as much — as that of the children in Luton. As exposure dropped, lung function followed: the London group's FEV1, the volume of air a child can force out in one second, increased by 10ml per year more than their Luton counterparts.

The proportion of London children with clinically significant lung impairment fell from 14 per cent to 9 per cent across the study period, against a fall from 9 per cent to 7 per cent in Luton. By the end of the five years, lung function in the two groups was comparable — the London children had, in effect, caught up.

Why the comparison matters

The study's design is what gives the finding its weight. Luton served as a control: a town whose children breathed similarly polluted air at the outset but saw no equivalent policy intervention. Divergence between the two groups is therefore hard to attribute to anything other than the difference in air quality trajectories.

The research was led by Queen Mary University of London with Imperial College London and the Universities of Bedfordshire, Oxford, Cambridge and Edinburgh, along with the University of Southern California, and funded by the National Institute for Health and Care Research.

From pause to reversal

Professor Chris Griffiths of Queen Mary, one of the study's leads, said traffic pollution in cities damages children's health and argued that ambitious clean air zones "should be considered a priority for cities globally".

His colleague Professor Ian Mudway made the broader case: air pollution is a global problem, but the CHILL data shows clean air zones can work as a genuine public health intervention — not simply a traffic measure with incidental benefits. Children's lungs grow fastest in exactly the years the study covered, which is why damage done then tends to persist into adulthood, and why recovery at that age matters so much.

The caveats

The findings are an association, not a controlled experiment, and the researchers are careful on that point. And there is no declaration of victory on air quality itself: as the team noted, pollution in both London and Luton remains above World Health Organization guideline levels.

The ULEZ also remains politically contested, having drawn sustained opposition over its costs to drivers as it expanded across Greater London. The CHILL results will not settle that argument, but they shift its terms — the health side of the ledger now has hard longitudinal data behind it.

What happens next

The research team's message to policymakers is explicit: clean air zones should be treated as public health infrastructure, and cities beyond London now have a measurable benchmark for what one can achieve. With pollution still above WHO guidelines even inside the zone, the study reads less like a conclusion than a proof of concept — evidence that children's lungs can recover when the air improves, and a reason to keep improving it.