The kids in central London were still breathing, but their lungs were falling behind. Researchers had long known that dirty air slows lung growth — what nobody had proven was that cleaning the air could fix it. Now a study published in The Lancet Public Health shows it can, and the evidence is written in the breaths of thousands of children.
Scientists at Queen Mary University of London followed more than 3,400 children aged 6 to 9 over several years, tracking their lungs as London's Ultra Low Emission Zone — or ULEZ — rolled out. The ULEZ charges drivers of the most polluting vehicles to nudge them toward cleaner transport. For researchers, that created what they call a natural experiment: a rare chance to watch a whole city change.
To make the test fair, they compared the central London children to a similar group in Luton, a town with no clean-air zone. That let them see what the policy actually did, separate from other factors.
The numbers tell the story. Nitrogen dioxide, a gas closely tied to car exhaust, fell roughly twice as fast in central London as in Luton — dropping about 3.8 micrograms per cubic metre each year in London versus 1.8 in Luton. Both cities improved; London just did it dramatically faster.
And the children's lungs moved with the air. Researchers measured how much air each child could forcefully exhale in one second, a standard test of lung capacity. Before the ULEZ, London children's lung growth lagged behind Luton's. As the air cleaned up, that changed. London children's lung function grew by an average of 233 millilitres per year, against 223 in Luton. After four years, the gap had vanished entirely — the London children caught up.
Perhaps the most striking figure: the share of central London children with impaired lung function fell from 14 percent to 9 percent over that same stretch.
Why childhood lungs matter so much: they aren't fully developed at birth. They keep growing through childhood and into early adulthood, and those years shape a person's respiratory health for life. Kids whose lungs don't fully develop face higher lifetime risks of asthma and other conditions. This is the first study to connect a clean-air zone directly to faster lung growth — a change you can measure in children's bodies, not just in pollution readings.
The ULEZ hasn't been an easy victory. Its cameras have been vandalized or stolen, according to the Metropolitan Police, and the 2023 expansion across all London boroughs was challenged in court by several local councils before the High Court let it proceed. The science is now adding weight to one side of that fight — the side that says cleaner air changes real, measurable lives.