Published in The Lancet Public Health, the Children's Health in London and Luton (CHILL) study found that cleaner air following the introduction of the ULEZ was associated with improved lung growth in primary school children. The authors say this is the first study to demonstrate that improvements in air quality following the introduction of a clean air zone are associated with improved lung growth in children.
Led by researchers at Queen Mary University of London and the Nuffield Department of Primary Care Health Sciences, the study involved collaborators from Imperial College London, and the Universities of Bedfordshire, Cambridge, Edinburgh and Southern California.
The team followed more than 3,400 children aged 6–9 years from 84 primary schools between 2018 and 2022, studying children in central London and in Luton. Luton does not have a clean air zone and so it provided a reference group against which the London findings could be assessed. All the children were recruited before the ULEZ was introduced, allowing researchers to track changes in lung growth over time. Because researchers cannot assign children to breathe cleaner or dirtier air, the study was a natural experiment: it compared what happened in a city that introduced a clean air zone with what happened in one that did not.
Before the ULEZ was introduced in 2019, children living in central London had significantly smaller lung capacity than children in Luton. Over the following four years, nitrogen dioxide (NO₂) pollution – a key marker of traffic exhaust emissions – fell more than twice as fast in London as in Luton. At the same time, children's lung growth accelerated, so that by the end of the study average lung function in London had caught up with that of children in Luton. Forced vital capacity – a measure of total lung volume – also improved faster in London, but had not fully caught up by the end of the study, suggesting further gains are possible.
The findings also showed a meaningful reduction in the number of children with clinically impaired lung function. In London, this fell from 14% of children at the start of the study (around one in seven) to 9% (around one in 11) after four years. In Luton, the proportion fell from 9% to 7% following other measures introduced by the local council to improve air quality.
Researchers also found important differences between ethnic groups. Across the whole cohort, long-term exposure to air pollution was associated with reduced lung growth, with Asian and Black children experiencing significantly larger pollution-related deficits than White children. The findings highlight that the health impacts of air pollution are not experienced equally, reinforcing the importance of clean air policies in reducing health inequalities.
The study period overlapped with COVID-19 travel restrictions, which reduced traffic at both sites; the researchers note that the findings rest on individual children's growth trajectories measured repeatedly over five years rather than on year-to-year pollution comparisons.
Failure to achieve full lung growth during childhood and adolescence is associated with a greater risk of chronic respiratory disease, cardiovascular disease and premature death later in life. While the study found substantial improvements in children's lung growth, air pollution levels in both London and Luton remained above World Health Organization guideline levels, indicating that further progress is needed.
Professor Chris Griffiths, Professor of Primary Care at the Nuffield Department of Primary Care Health Sciences, University of Oxford, and Queen Mary University of London, and lead investigator said:
"The findings of our study provide strong evidence that ambitious clean air policies can improve children's long-term health and development. Cities around the world facing traffic-related air pollution should consider clean air zones as an important public health intervention."
Dr Helen Wood, Research Fellow at Queen Mary and lead author, said:
“We already knew that the ULEZ reduced air pollution but now we know that children’s lung health has improved at the same time, which is a really important finding for children and parents living in London. However, we must not be complacent – air pollution in both London and Luton – as well as other cities across the UK – remains above WHO guideline levels, so there is still work to be done.”
Professor Sir Aziz Sheikh OBE, Head of the Nuffield Department of Primary Care Health Sciences and an author of the study, said:
"This study demonstrates the value of investing in prevention. Cleaner air doesn't just benefit the environment, it improves children's health during the years when their lungs are still developing and particularly vulnerable, with benefits that will potentially be lifelong."
Professor Monica Fletcher OBE, Professor of Practice in Translational Primary Care at the Nuffield Department of Primary Care Health Sciences and an author of the study, said:
"Children's lungs continue to develop throughout childhood, so creating cleaner environments during these years is crucial. It's particularly important that we recognise the unequal impact of air pollution, with larger pollution-related effects seen in Asian and Black children. Reducing air pollution has the potential not only to improve health overall but also to help reduce health inequalities."
Karin Smyth, Minister of State for Health, said:
"This study is a powerful reminder of the impact that air quality has on children's lung health, and will help to improve our understanding of how reductions in air pollution can restore normal lung growth among children, reduce the risk of respiratory illnesses in adulthood, and support our goal to give every child the best start in life.”
Professor Lucy Chappell, Chief Scientific Adviser at the Department of Health and Social Care (DHSC) and Chief Executive Officer of the NIHR, said:
“This research, funded by NIHR, will provide crucial evidence on real-life interventions that deliver life-changing health benefits for children. I would like to thank the 3,400 children and their families who took part. Research can only take place with the input and support of participants, and this study is a great example of how the UK's vibrant research sector is working with communities to transform lives.”