London's clean air zone rapidly restores children's stunted lung growth

In 2019, London introduced a policy that would become a natural experiment in children's health. The Ultra Low Emission Zone, or Ulez, required older, more polluting vehicles to pay a daily charge to drive in central London—a measure that would later expand across the entire city. What researchers found in the years that followed surprised even the scientists conducting the work: children whose lungs had been stunted by years of breathing London's dirty air began to recover with remarkable speed.

A team led by Queen Mary University of London tracked more than 3,400 primary school children over five years, starting in the year before Ulez took effect. They measured the same children annually, testing how much air they could forcibly exhale in a single second after taking a deep breath. They also compared London children to a control group of similar age and background in Luton, a less polluted city with a comparable mix of pollutants. The initial results were stark: London children's lungs were measurably smaller in capacity than their Luton peers.

But something unexpected happened. By the end of the study period, the gap had closed. The London children's lungs had grown faster than normal—accelerating to catch up with the Luton group until their lung capacity reached nearly identical levels. "I was absolutely stunned when I first saw the results," said Prof Chris Griffiths, a senior author on the study. "The speed of catch up was surprising and impressive." In practical terms, the improvements meant London children could now run without getting winded as easily, or blow out the same number of candles as their peers in cleaner air.

The scale of the shift was significant. The proportion of London children with clinically impaired lung capacity—a measure suggesting lung damage that causes coughs or breathlessness—fell from 14 percent to 9 percent over the study period. In Luton, where smaller-scale pollution measures had been implemented, the figure dropped from 9 percent to 7 percent. The researchers also measured nitrogen dioxide exposure, the harmful gas that comes primarily from motor vehicles. Children in London experienced faster reductions in their exposure to this pollutant than children in Luton, suggesting the lung improvements were directly tied to the air quality gains from Ulez.

The findings matter because air pollution damages developing lungs in ways that can persist for life. Children are especially vulnerable—their lungs and immune systems are still forming, and when they play outside, they breathe closer to the ground where exhaust fumes concentrate. Stunted lung growth increases the risk of asthma, heart disease, diabetes, and premature death. The case of Ella Adoo-Kissi-Debrah, a nine-year-old Londoner who died from an asthma attack in 2013, underscored the stakes. In 2020, she became the first person in the UK to have air pollution officially listed as a cause of death.

Independent researchers offered measured praise for the work while noting important caveats. Prof Anna Hansell at the University of Leicester, who was not involved in the study, called the findings "carefully conducted" and noted that the comparison between London and Luton was particularly valuable because it controlled for general air pollution improvements happening across the country—and crucially, it showed the findings were not simply an artifact of the Covid-19 pandemic, which occurred during the study period. Other experts suggested that increased walking and cycling to school in London after Ulez came in might have contributed to some of the health gains.

Yet the researchers themselves cautioned against complacency. Dr Helen Wood, the study's lead author, noted that air pollution in both London and Luton remains above World Health Organization guideline levels. "There is still work to be done," she said. Griffiths added that the findings carry global weight: more than a billion children live in cities worldwide, most in polluted environments. "That's why these are important findings with global relevance," he said. The study, published in the Lancet Public Health, involved researchers from universities across the UK and the United States, and it offers what scientists believe is the strongest evidence yet that ambitious clean air zones can reverse some of the damage pollution inflicts on developing bodies.