When London introduced the ultra low emission zone (Ulez) in 2019 , the idea was straightforward: cut the dirtiest vehicle emissions and improve the air people breathe. But the big question at the time was whether this measure would actually translate into measurable health benefits.
Authors
- Ian Mudway
Senior Lecturer, School of Public Health, Imperial College London
- James Scales
Senior Research Fellow, Wolfson Institute of Population Health, Queen Mary University of London
- Rosamund Dove
Research Associate, Wolfson Institute of Population Health, Queen Mary University of London
Our new study, published in The Lancet Public Health, suggests that it has had benefits for children's lung development .
Between July 2018 and April 2019, we recruited 3,414 children aged six to nine, and followed their development over five years. A total of 1,664 of these children lived in central London where the Ulez was introduced. A further 1,750 children lived in Luton - a comparable urban area without a clean air zone.
At the start of the study, before the Ulez was introduced, children in London had both higher exposure to air pollution and lower lung function than children in Luton. We assessed lung function using non-invasive breathing tests, including FEV₁, which measures the amount of air a person can forcibly breathe out in one second after taking a deep breath.
After accounting for other factors (such as sex, ethnicity, deprivation and whether they had asthma), average FEV₁ was about 38mL lower among London children. Their modelled residential exposure to nitrogen dioxide (NO₂), a pollutant closely linked to vehicle exhaust, was also almost 19μg/m³ (micrograms per cubic metre) higher.
Between 2019 and 2023, after the Ulez was introduced, the picture of air quality changed. According to a model of the two residential areas, nitrogen dioxide fell much faster in central London than in Luton. It dropped by 3.77μg/m³ in London compared with just 1.77μg/m³ per year in Luton.
Over the same period, children's lung function improved faster in London. Annual growth in FEV₁ was around 10mL per year greater than in Luton. That may sound small, but lung growth happens gradually. Modest differences accumulated across childhood can matter for long-term health.
By the end of follow-up, the earlier gap had closed almost completely. The average FEV₁ was essentially the same for children living in both places. The proportion of children with clinically low FEV₁ (which is below the predicted range for their age) also fell more in London, from 14% to 8.8% - compared with 8.9% to 6.9% in Luton.
We also looked at forced vital capacity, or FVC, which reflects lung volume. This is the total volume of air a person can quickly exhale from their lungs after taking a deep breath. London children started with about 55ml lower FVC measurements. Their FVC increased faster than Luton children's over follow-up, but the gap did not disappear. By the end of the study the London children still had approximately 25mL lower average FVC than the children in Luton.
In other words, one measure of lung function caught up fully, while another showed substantial but still incomplete recovery. Given FVC more closely reflects structural lung volume and growth than FEV₁ does, this may explain why it has taken longer to change.
These findings are important because impaired lung growth in childhood is linked to worse respiratory health later in life .
Our findings also point to a plausible mechanism. Across the whole cohort, higher long-term exposure to nitrogen dioxide and particulate matter pollution was associated with poorer lung function and growth overall.
While all pollutants were linked to lower lung development, the clearest association was with nitrogen dioxide. Given nitrogen dioxide is strongly linked to road traffic exhaust emissions, it is therefore particularly relevant to an intervention such as the Ulez.
Pollution and lung function
As this was a natural experiment rather than a randomised trial, it cannot prove causation definitively. It does, however, provide strong real-world evidence that cleaner air was associated with healthier lung growth.
This also does not mean that Ulez explains everything. London and Luton are not identical - and weren't even at the start of the study. Our pollution estimates were based on modelled residential exposure, which cannot capture non-residential exposures - including school hours, travel and indoor environments.
The COVID pandemic also disrupted school-based follow-up - although we conducted analyses that tested whether this changed the interpretation. Overall, the main findings held up.
We are cautious about saying the Ulez "caused" these improvements in the way a clinical trial might establish cause-and-effect. But the timing, the faster fall in traffic-related pollution, the comparator town and the pattern of lung growth together make a causal interpretation plausible.
The broader message is that transport policy is also health policy. Clean air zones are often debated in terms of charges, fairness and inconvenience. Those concerns are real, and policies need to be designed fairly with support for people on low incomes and for those who rely on vehicles. But there is another side to the ledger: children's health.
Many of the children most exposed to traffic pollution are not from the communities most responsible for generating it .
In central London, where car ownership is relatively low , many children still live, learn and play near busy roads. That means children can bear the health burden of traffic pollution even when their own households contribute relatively little to it. That makes air pollution an equity issue as well as an environmental one.
Back in 2019, one of us (Ian Mudway) argued that Ulez should be evaluated as a public health intervention and evaluated carefully. Our study suggests that it was indeed a medicine worth testing, and that it may already be delivering benefits.
Cleaner air will not solve every urban health problem. But our findings suggest that reducing traffic-related pollution can help children's lungs grow better. That is not a trivial gain. It is an investment in healthier lives.
Ian Mudway receives funding from the European Union - Horizon programme, NERC/MRC UKRI, NIHR and Barts Charity. He is affiliated with Gresham College, where he is presently the Visiting Professor for Environmental Health. He is also a Trustee of the educational charity, Off The Curriculum.
James Scales and Rosamund Dove do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
