Ten years after "Better Health for London" named air pollution alongside housing and employment as one of the capital’s most stubborn drivers of ill health, London air quality has measurably improved — but the evidence behind that improvement is more layered than a single before-and-after chart suggests. The Ultra Low Emission Zone is the policy instrument most credited with the change, and a decade of monitoring data from Transport for London, the Greater London Authority, and several university research groups now lets residents ask a sharper question than whether ULEZ "worked": which respiratory outcomes has it actually shifted, by how much, and where do the researchers still disagree.
What the Monitoring Network Shows About Pollution Levels
The headline figures on London air quality are the least contested part of the evidence base. A 2025 Transport for London and Greater London Authority analysis found nitrogen dioxide (NO2) levels 27% lower across London than they would have been without ULEZ and its expansions, with central London down 54%, inner London down 29%, and outer London down 24%. Separate GLA-commissioned research measured a 26% fall in NO2 emissions within the original zone (around 5,000 tonnes) since 2019, and a 23% fall (roughly 13,500 tonnes) across London as a whole. The University of Bath’s Institute for Policy Research reported an 18.4% reduction in central London NO2 in 2019 against a 2016–18 pre-ULEZ baseline, and the Department for Environment, Food and Rural Affairs confirmed London met the legal nitrogen dioxide limit in 2024 for the first time. Particulate matter (PM2.5) tells a similar story: exhaust emissions from cars and vans in outer London were estimated to be 31% lower in 2024 following the ULEZ expansion, and GLA-commissioned work found PM2.5 within the original zone down 19%. Across the monitoring network as a whole, air quality improved at 99% of sites since 2019, with London improving faster than the rest of England. Not every study agrees on the pace: a 2021 Imperial College London analysis found NO2 fell by less than 3% on average in the scheme’s first weeks, situating ULEZ within a longer downward trend rather than crediting it with an isolated shock.
The Harder Link: Air Pollution and Hospital Admissions
Connecting cleaner air to fewer hospital admissions is where the evidence base gets genuinely contested, and that is worth stating plainly rather than smoothing over. Imperial College London researchers found that measures to cut central London air pollution, including ULEZ and the earlier T-charge, were linked to a 6.2% reduction in the yearly trend for respiratory disease admissions — but once the analysis adjusted for changes in a comparison area outside the scheme, the effect shrank to 2.7% and was not statistically significant. That is a genuinely useful finding, not a null result to discard: it tells us the population-level signal is real in direction but still too noisy to isolate cleanly from other trends in respiratory illness. A separate University of Bath analysis of the earlier Low Emission Zone reached a firmer number, estimating 12 fewer respiratory admissions per 10,000 people in Greater London compared with the rest of England, alongside an 8% decrease in conditions such as asthma and bronchitis. The same Bath analysis found a 9-prescription reduction per 1,000 patients for respiratory infections, saving the NHS an estimated £74 per 1,000 registered patients, and a 4.5% reduction in long-term health problems overall. Reading these findings side by side is itself instructive: two credible institutions, using different methods and different time windows, arrive at different confidence levels for the same underlying question. That is what an honest monitoring evidence base looks like.
Children’s Lungs: The Clearest Signal So Far
The strongest and most consistent findings in the evidence base concern children, whose developing lungs are more sensitive to ambient pollution than adult lungs. King’s College London research has linked pollution exposure to reduced lung capacity in London children, raising their risk of asthma and bronchitis. Asthma + Lung UK reported that between 2017 and 2019, air pollution was responsible for 1,700 hospital admissions for asthma among London children — 7% of all childhood asthma admissions in the capital — and identified Brent, now fully inside the ULEZ boundary, as the borough with the highest rate of child asthma admissions in London. More recent evidence points toward improvement: the CHILL cohort study, involving Imperial College London and presented at the 2025 European Respiratory Society Congress, found that children in London experienced faster lung function growth after ULEZ implementation than a comparison group in Luton, with an interquartile decrease in NO2 associated with a 16.5ml-per-year increase in lung function growth (FEV1). Separately, ULEZ has been linked to a 30% reduction in children admitted to hospital with asthma in inner London. Taken together, this is the part of the evidence base doing the most work to justify continued investment in monitoring and in the clean-air agenda more broadly.
Reading the Evidence Base: What Each Institution Actually Found
Because different bodies publish different metrics on different timelines, it helps to see the findings side by side rather than as a single verdict:
| Institution / study | What it measured | Key finding |
|---|---|---|
| GLA / TfL (2025) | NO2 across London | 27% lower than without ULEZ; central London 54% lower |
| University of Bath (IPR) | NO2 in central London, 2019 | 18.4% reduction against 2016–18 baseline |
| Imperial College London | Respiratory hospital admissions | 6.2% reduction in yearly trend; 2.7% after adjustment, not statistically significant |
| University of Bath | Low Emission Zone respiratory admissions | 12 fewer admissions per 10,000 people vs. rest of England; 8% fall in asthma/bronchitis |
| Imperial College (CHILL study) | Children’s lung function growth | NO2 decrease associated with 16.5ml/year higher FEV1 growth |
| Asthma + Lung UK | Childhood asthma admissions, 2017–19 | 1,700 admissions attributed to air pollution; Brent highest borough rate |
| Imperial College London | Air-pollution-associated deaths, 2019–24 | Estimated reduction of approximately 40% |
Read across the table, the pattern is consistent with what a careful population-health inquiry would expect: the physical measurements (NO2, PM2.5) show large, well-replicated reductions; the clinical outcome measurements (hospital admissions, prescriptions) show real but smaller and sometimes statistically fragile effects; and the paediatric evidence is the most consistently positive of the three categories. None of that undermines the case for London air quality monitoring — if anything it is the argument for it. Effects this size, moving in this direction across multiple independent research groups, are exactly the kind of signal that a sustained monitoring network is built to detect and that a single year of data would miss entirely.
Why This Belongs in a Population-Health Record
This is the kind of evidence review the original 2014 inquiry was built to encourage: not a press-release verdict on a single policy, but a standing habit of reading the monitoring data, the clinical outcome data, and the disagreements between researchers as part of the same picture. Air pollution was never treated as an isolated environmental issue in that report — it sat alongside housing, employment, and primary care as one of the wider determinants shaping who gets sick and who stays well across London’s boroughs. Ten years on, the ULEZ evidence base illustrates why that framing still holds: the clearest respiratory health gains are showing up first in children, in the boroughs with the highest prior burden, which is precisely where a population-health approach would expect to look for early returns on a clean-air policy. As London’s monitoring network continues to accumulate data, the more interesting question is no longer whether air quality has improved — the readings are clear on that — but which respiratory conditions, in which boroughs, will show the strongest signal next.