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A cohort published in the Lancet Public Health looks at London’s Ultra Low Emission Zone (ULEZ) and lung function in children.
Prof Anna Hansell, Professor in Environmental Epidemiology, University of Leicester, said:
“This is a carefully conducted follow-up study collecting data over a five-year period, with a large team (there are 41 authors) led by well-respected researchers in air pollution and health. As well as conducting a before-after analysis, the researchers conducted the same study in a comparison area, Luton. The geographical comparison is important as there have been general reductions in air pollution levels over time. It proved particularly useful, given that the study period included the COVID-19 pandemic – and demonstrate that the findings in children in London are not due to changes related to the pandemic.
“The good news is that air pollution fell faster in London than in Luton as a result of ULEZ, and the reduction in air pollution was associated with catch-up growth in lung function in the children in London. Lung function levels were initially lower in children in London than in Luton, but reached similar levels by the end of the study.
“The bad news is that air pollution levels were still above World Health Organization guidelines by the end of the study (2022) and that higher air pollution levels were associated with lower measured lung function in both London and Luton. Further, the impact of air pollution on lung function was greater in Asian and Black children compared with White children.
“Why does lung function matter? Lung function is an important marker of general health. Reaching maximum growth potential in childhood provides a protective reserve. Lung function in adult life is closely linked, not just with risk of chronic respiratory disease, but also cardiovascular disease and mortality risk. The falls in air pollution related to ULEZ were associated with improved lung function in children and this is likely to have lifelong benefits for their health. The study is an evaluation of an intervention, not an observational study. This provides support for the association being causal.
“There was large loss to follow-up over time of more than half the children, raising the possibility that children more badly affected by air pollution moved out from central London, which could have affected the results. However, this explanation is unlikely to explain the findings, as the drop-out rates were similar in both London and Luton. The authors also conducted additional statistical analyses, which confirmed that the drop-out rates did not change the interpretation of the results.
“The most notable fall in air pollution in London was in nitrogen dioxide. However, as the authors note, the ULEZ intervention led to a fall in diesel exhaust fumes (which were not directly measured) and to other changes including more active travel to school. These may also have contributed to the observed effects.
“The study findings are consistent with observational studies in the USA and Sweden looking at lung function in relation to falls in air pollution (which can be considered a natural experiment). They are also consistent with research1 from my own group in children born in the Bristol area in the early 1990s, with lung function measures at ages 8, 15 and 24 years. Air pollution fell markedly over this time period. We commented on the possibility that “the negative effects of early-life exposure to higher, more toxic air pollution were offset by increased lung growth …in the absence of continued cumulative high exposure”. Our follow-on study2 suggested the impact of air pollution on lung function development was greatest between ages 8-15 years, which covers the ages of children in the ULEZ study.”
1 Cai et al, 2020. https://academic.oup.com/ajrccm/article/202/1/112/8494593
2 https://doi.org/10.1016/j.envres.2026.124278; see Guardian article on our study: https://www.theguardian.com/environment/2026/may/29/air-pollution-slows-lung-growth-childhood-uk-study
Prof Nick Hopkinson, Professor of Respiratory Medicine, National Heart and Lung Institute, Imperial College London, said:
“There is abundant evidence that air pollution is harmful to growing lungs.
“This study, which measured lung function in primary school children annually for five years, provides strong evidence that reducing exposure to air pollution can allow lung growth to catch up, reversing this harm if done in time.
“A strength of the study is that rather than measuring lung function in children at different ages it follows the same children over time. The rate of healthy lung growth increased after the London ULEZ was introduced, which reduced children’s exposure to particulates and nitrogen dioxide. In a control group of children living in an area where a clean air zone was not introduced there was no change in the rate of lung growth.
“Failure of lung growth in early life is not just an issue for child health – it is strongly associated with ill health in adulthood as well and makes people more vulnerable to asthma and COPD, and is also associated with cardiovascular disease.”
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“This is certainly a major piece of work. Overall I don’t completely object to an interpretation that the greater increases in children’s lung function in the original ULEZ area in London, compared to what was observed in Luton, may have been caused by the introduction of the ULEZ zone, at least in part.
“But this feeling comes from a balance of the different arguments. This paper does not establish that this is the only explanation, and the researchers indicate this by talking about the ULEZ introduction being “associated with”, that is, correlated with, faster growth in lung function, rather than definitely being a cause of that difference. I do feel that the research paper spends too much time discussing reasons why the ULEZ introduction is likely to be the cause of the difference, and perhaps not enough in discussing possible alternative explanations.
“The fact that there might be plausible other explanations stems from the way the study was designed. Some of the details are pretty complicated, but fundamentally the researchers compared the growth of children’s lung function in just two areas – the more central parts of London, and Luton.
“It’s true, of course, that one of these (London) introduced a clean air zone, the ULEZ, and the other one didn’t. The researchers followed up changes in children’s lung function over the period after the ULEZ was introduced, and found greater improvements over time in London than in Luton.
“But the two areas, and the children living in them, differ in very many ways other than in whether they introduced a clean air zone of not. Luton is not a clone of central London.
“The researchers carried out a pretty comprehensive set of statistical adjustments to allow for differences between individual children, in terms of some physical properties including ethnicity, a measure of deprivation of where they lived, and whether they had asthma. That’s good practice.
“But that wouldn’t account for differences in other policies and practices to reduce air pollution that were going on at the same time. The paper reports on some of these. The fact that air pollution measures reduced over time in Luton as well as in the London ULEZ indicates that changes in air pollution weren’t just due to the ULEZ. Indeed air pollution by small particles (measured by PM10 and PM2.5) reduced more in Luton than in London, though nitrogen dioxide pollution reduced more in London.
“A standard way to allow, statistically, for the fact that the individual people or places or things differ from one another, possibly in ways that researchers don’t even know about, is to include more people or places or things in the study. Here, though the measures of changes in lung function were made on individual children, I’d argue that the key comparison is between the geographical areas, and there were only two of them, London and Luton.
“I think that the results might have had a clearer interpretation if the researchers could have included data from a few more places where clean air zones were introduced, and a few more places where they weren’t. Otherwise, to use an exaggerated metaphor, it’s a bit like taking one man and one woman (not chosen at random), measuring their foot size, and if the woman’s feet just happened exceptionally to be bigger, concluding that generally women have bigger feet than men.
“I’m not saying the new study is anywhere near as crude as that, but the fact that only two areas were used does make it difficult to be sure that the difference in lung function improvement was due to just one of the differences between the two areas, the fact that only one of them introduced a clean air zone.
“The researchers recognise this difficulty in the section of the paper where they discuss limitations of their study. They write, “Including additional sites with or without clean air zones would have been helpful, but we were unable to extend our data collection, and no existing datasets with comparable lung function data were available.”
“In other words, they didn’t design their study to include more sites, and nobody else had collected data that they could have used alongside their own. I recognise that it would probably have been difficult to extend their own study to other areas, but in my view it’s a major limitation that they didn’t do so. This is just part of the reason why they go on to say, “There might be unmeasured reasons why children in London could have more rapid lung function growth than those in Luton.”
“They mention other limitations too. For example, their study unfortunately ran over the period when travel and other aspects of life, including school closures, were heavily affected by COVID-19. There’s nothing the researchers could have anticipated about that. But I think it’s going too far to claim, as they do, that children in both areas would have had similar experiences because the COVID restrictions were national. There were differences in implementation and enforcement between places – maybe that didn’t matter to these findings, but maybe it did. Again, it’s hard to see how the researchers could have taken account of that, but it’s another reason for doubt about the extent to which the introduction of the ULEZ was the crucial factor.”
Prof Stephen Burgess, Professor of Biostatistics, University of Cambridge:
“I must admit that I approached this study with a high level of scepticism. My expectation was that the ULEZ regulations would have little measurable impact on child health. However, the findings are striking – children in Central London had poorer lung function than children in Luton at the beginning of the study in 2018, but this difference narrowed to the point of disappearance by 2023.
“Three points by way of caveat:
“First, it isn’t clear how much of this change is attributable to reductions in air pollution. Other changes in behaviour around the ULEZ transition include an increase in active travel to school. It may be that the improvement in lung function is attributable to greater levels of physical activity rather than reductions in air pollution.
“Second, it isn’t clear how much of this difference is truly attributable to the ULEZ regulations. The years investigated overlap the COVID-19 pandemic, a time that was characterized by lower pollution levels worldwide and drastic changes in human behaviour. While this affected both Central London and Luton, its effect was not necessarily equivalent in both places. The pandemic could have led to greater similarity in the environment and experience for children in both Central London and Luton.
“Finally, it is not clear how generalizable the results are. If similar policies were introduced in other cities, would we see the same improvement in children’s lung development?
“This study shows a striking change in lung function over time – the lung function of children in Central London has improved substantially relative to those in Luton. Whether this improvement can be attributed to the introduction of ULEZ regulations is more questionable. Nevertheless, the finding is important because lung function in childhood is an important measure of health, and the study provides encouraging evidence that changes in the urban environment may benefit children’s lung development.”
‘Impact of the Ultra Low Emission Zone on lung function growth in children in London, UK: a prospective parallel cohort study’ by Helen E Wood et al. was published in the Lancet Public Health at 23:30 UK time on Tuesday 18 August 2026.
DOI: 10.1016/ S2468-2667(26)00163-5
Declared interests
Prof Anna Hansell: “I do not have conflicts of interest to report. I am Chair of the government Scientific Advisory Committee, the Committee on the Medical Effects of Air Pollution (COMEAP), but comments here are in a personal capacity as Professor of Environmental Epidemiology at the University of Leicester.”
Prof Nick Hopkinson: “None.”
Prof Kevin McConway: “I have no conflicts of interest to declare.”
Prof Stephen Burgess: “I have no relevant conflict of interest to declare.”
This Roundup was accompanied by an SMC Briefing.