The lung capacity of thousands of children stunted by traffic fumes on London’s streets recovered fully in the five years after the city imposed limits on its most polluting vehicles, a landmark study has found.

The research, published Tuesday in The Lancet Public Health, tracked more than 3,400 children in London and Luton, a commuter town north of the capital, from before the Ultra Low Emission Zone (ULEZ) took effect in 2019, providing what its authors describe as the most definitive evidence to date that a clean air zone can improve lung growth in children.

No previous study across a literature review spanning back to the year after the dust settled from World War II in 1946 had addressed the question, researchers said, calling their results “the first evidence that air quality improvements following introduction of a clean air zone are associated with improved lung growth trajectories in children” across a literature search spanning back to 1946.

“Traffic pollution in cities damages children’s health and development,” said Chris Griffiths, professor of primary care at the University of Oxford and Queen Mary University of London and joint senior author. “We provide the strongest evidence yet on how these harms can be prevented. Ambitious clean air zones should be considered a priority for cities globally with traffic-related air pollution.”

The ULEZ, which charges drivers of older, more polluting vehicles £12.50 per day to circulate inside its perimeter, was introduced in central London in April 2019 and expanded to cover the entire city by August 2023, making it the world’s largest clean air zone.

When London mayor Sadiq Khan, who was diagnosed with adult-onset asthma at 43 after training for the 2014 London Marathon on the city’s polluted roads, launched the zone’s first phase, he was attacked by politicians and voters across the aisle, cast as having “declared war on drivers in blind pursuit of a policy that may not even work,” he wrote in the Guardian in an op-ed accompanying the study’s release.

“For years, politicians and pundits opposed to the Ulez have cynically sought to turn questions of public health into a crude culture war,” Khan wrote. “All the mainstream political parties, including my own, were opposed. In recent years, though, those critics have gone quiet.”

“Today, though, it’s clear that we’re winning the battle against toxic air in the capital, and it’s London’s children who are reaping the rewards.”

Impaired lungs down by a third as London children ‘catch up’

The study recruited 1,664 children in London and 1,750 in Luton between June 2018 and April 2019, all aged six to nine, from 84 primary schools. Luton, a town 32 miles northwest of London with a similar pollutant mix and demographic profile but no clean air zone, served as the control site, with researchers measuring lung function at annual school visits in both cities.

Before the ULEZ took effect, children in London had significantly smaller lungs than their peers in Luton, and were breathing more than double the levels of nitrogen dioxide, the pollutant most closely tied to vehicle exhaust.

Children are especially vulnerable to the toxic effects of dirty air. They breathe faster than adults and more often through their mouths, their heads sit closer to exhaust pipes, and their still-developing lungs absorb lasting damage from long-term exposure, raising their risk of asthma, chronic obstructive pulmonary disease, heart disease, diabetes and premature death well into adulthood.

Over the five years since the low emission zone’s introduction, London children’s exposure to NO₂ fell twice as fast and more than twice as much as in Luton.

As the haze of exhaust from ageing diesel cars, trucks and London’s ageing buses thinned, children’s lung growth accelerated by nearly 5% per year over the comparison group, closing the gap entirely. By the study’s end, lung capacity in the two cities had reached parity. London’s children had caught up.

The share of London children with clinically impaired lung function, damage severe enough to cause coughing and breathlessness, fell by more than a third over the study period, dropping from 14% to 9%. In Luton, where only smaller-scale air quality measures were underway, the decline was 2%.

“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,” said Helen Wood, research fellow at Queen Mary and the study’s lead author.

Complementary research by Imperial College London published in June found reducing air pollution lowers premature deaths across the board, not just in children. Premature deaths linked to air pollution in the capital fell by roughly 40% between 2019 and 2024 as the ULEZ expanded, researchers found, with emergency cardiac hospital admissions falling 9.3% and overall admissions dropping 5.1%.

Deaths attributable to air pollution fell from an estimated 6,400-8,000 in 2019 to 3,800-5,100 in 2024, while London met legal limits for nitrogen dioxide for the first time in 2024, Imperial’s study found. 

“If we want to improve the lives of children living in high-traffic urban environments, we need bold and ambitious measures,” said Ian Mudway, associate professor in the School of Public Health at Imperial College London and joint senior author. “Our data demonstrates that clean air zones can be an effective public health intervention to prevent damage to developing lungs.”

Together, the growing collection of studies and data in London trace a clear story: air pollution was damaging children’s lungs, a policy was introduced to cut it, the air got cleaner, and both children’s lung growth and adult survival rates improved.

Before the ULEZ, researchers at King’s College London had calculated that at the pace air quality was then improving, the city would take nearly two centuries to bring NO₂ within legal limits. The zone got there 184 years ahead of schedule.

“We must not be complacent,” Wood added. “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.”

Nearly the entire global population lives above those guideline levels, which the WHO tightened in 2021 in recognition of evidence that no level of air pollution is safe to breathe. Dirty air is linked to more than eight million deaths per year, according to the State of Global Air, making it the second leading risk factor for death worldwide.

A decade of evidence

The findings cap a research arc stretching back more than a decade. A predecessor study by the same Queen Mary team, known as EXHALE and published in The Lancet Public Health in 2019, had established that London children exposed to higher levels of traffic-related NO₂ had measurably smaller lungs, with those in the most polluted areas showing 5-10% less lung capacity than peers elsewhere.

London’s original Low Emission Zone, rolled out from 2008, had produced only small improvements in air quality, and EXHALE found no evidence those gains were reaching children’s lungs. The zone’s modest progress, researchers found, showed “no evidence of a reduction in the proportion of children with small lungs” despite the cleaner air.

“Interventions that deliver larger reductions in emissions might yield improvements in children’s health,” researchers wrote at the time.

The verdict, however, came with a caveat. EXHALE measured a different group of 8-9 year-olds each year, a series of snapshots that could reveal the damage but never followed any single child long enough to know whether it could heal. 

With ULEZ’s announcement, the team saw an opportunity – the new policy offered the chance to run a stronger experiment by recruiting a single group of children in the year before the zone launched and measuring the same lungs annually as the policy took hold. That study became CHILL, the Children’s Health in London and Luton study published this week.

In a city of 12 million people, the science was only part of the story. In 2013, nine-year-old Ella Adoo-Kissi-Debrah, who lived 25 metres from the South Circular Road, one of London’s busiest and most heavily congested arterial routes, died following a severe asthma attack after 27 hospital admissions in three years.

A landmark 2020 coroner’s ruling found air pollution made a “material contribution” to her death, making her the first person in the world to have it listed as a cause of death.

Her story became the clarion call for local action, with grassroots groups of parents, teachers and doctors springing up across London to make public demands for action on air quality heard.

Mums for Lungs, founded in Brixton in 2017 by parents alarmed that their borough exceeded annual safe pollution limits within the first days of each year, campaigned for the ULEZ expansion, drove diesel surcharges in Lambeth and won School Streets closures around pick-up and drop-off times that cut NO₂ by up to 23%.

“This shows that cutting air pollution doesn’t just help Londoners being born today,” Khan wrote. “It can help children whose lungs have already been stunted by toxic air to recover, proving that it’s never too late to act.”

More than 325 low-emission zones now operate across Europe, from Germany and Italy to France, Belgium, Spain and the Netherlands, with over 500 planned on the continent.

Yet the scientific evidence for the health and economic benefits they provide has lagged far behind their spread, leaving leaders like Khan to fight bruising political battles over congestion charges, scrappage costs and the burden on lower-income drivers armed with little more than the intuition that cleaner air must be healthier.

Slowly, that research is starting to catch up. A study comparing Belgian cities with and without clean air zones, published last year, found that the zones in Antwerp and Brussels accelerated declines in all major pollutants and reduced socioeconomic disparities in exposure.

In Madrid, the low-emission zone’s introduction in 2018 has been linked to improvements in both air quality and student academic performance. A 2023 systematic review in The Lancet Public Health found the strongest evidence for cardiovascular benefits, linking the zones to fewer hospital admissions for heart problems and stroke.

The cost of clean air

Even as the science strengthens, a formidable hurdle stands before cities hoping to follow London’s lead: the price tag.

The city’s entire bus fleet was retrofitted or replaced – part of an ongoing multi-billion pound effort to create a fully net zero fleet – growing London’s zero-emission fleet from just 30 buses in 2016 to more than 3,000 today, one of the largest in Europe. 

The vehicle scrappage scheme, providing subsidies for people to replace polluting vehicles no longer allowed under ULEZ rules, alone cost  £160 million. A further £33 million has flowed through the Mayor’s Air Quality Fund to more than 100 local projects since its launch.

Few cities outside Europe and North America can hope to match that investment. Sub-Saharan Africa received less than 1% of global outdoor air quality funding from governments and development finance institutions in 2023, a total of $12 million for the entire region. That’s less than a tenth of what London spent on its scrappage scheme alone, and less than the city spends in a single year operating its reference-grade air monitoring stations. Air pollution killed an estimated 1.2 million people across the continent in 2021.

To add pollution to inequality, successful ULEZ schemes and broader vehicle emissions standards at national and European levels actually fuel the import of air pollution into developing cities.

A UN Environment Programme report found that 14 million used vehicles were exported from wealthy nations to developing countries between 2015 and 2018, more than half to Africa, with up to 80% failing to meet minimum emissions standards.

“Over the years, developed countries have increasingly exported their used vehicles to developing countries,” Inger Andersen, executive director of the UN Environment Programme, said of the study. “Because this largely happens unregulated, this has become the export of polluting vehicles.”

Some cities are trying. Jakarta is electrifying its bus fleet and building the evidence base for a low-emission zone of its own, while Accra and Nairobi have established their first air quality monitoring networks.

Yet for most of the world, following London’s path will be neither quick nor cheap. What changed this week is that cities around the world now have evidence to show a vital point: low-emission zones work. 

Image Credits: Aron Van de Pol, Ashleigh Joy, CC.