London ULEZ expansion will prevent 1,300 cases of asthma per year

London’s Ultra-Low Emissions Zone was expanded on 29 August. The aim is to reduce air pollution in outer London and protect people’s health. But it can seem unclear just how big an effect that has, and that can make it seem like a cash grab instead of a step to address a public health crisis. That’s why we’ve conducted analysis to understand the situation and its impact on health.

Using our new STRIAD:AIR tool, we modelled how air pollution changes and how a range of health conditions – including asthma, diabetes, stroke, heart disease and lung disease – are caused over the year. We’ve used hyperlocal data on populations, so we can show this for as little as 600 households.

We estimated how one of those conditions would be affected by air pollution – asthma. Asthma is a lifelong condition that occurs most commonly in children. While most people with asthma live long and healthy lives, it can lead to severe complications (as in the tragic case of Ella Adoo-Kissi-Debrah in 2013). It’s highly likely that, similar to inner London, the ULEZ will lead to a fall in nitrogen dioxide (the air pollutant most closely related to traffic) of 21% a year after it’s introduced.

We estimated that there will be 1,367 fewer cases of asthma after a year of reduced nitrogen dioxide concentrations (95% CI 606 – 1,733). This means that 773 young people (under 20) won’t develop asthma in that year.

Our modelling indicates that he borough with the greatest fall will be Croydon, with 100 fewer cases of asthma following the ULEZ introduction – 57 of those among young people. Other highly affected boroughs are likely to include Barnet (87 fewer cases), Hillingdon (87 fewer cases), Enfield (83 fewer cases), Bromley (81 fewer cases), and Redbridge (78 fewer cases).

Asthma isn’t just a health issue, but also has knock-on effects on NHS spending. Using NHS budgeting data, we estimate that the reduction in cases caused by ULEZ expansion would save more than £1.6 million each year from diagnoses alone across the zone – without including the costs of ongoing treatment (which lasts a lifetime). Of that amount, £1.2 million would be saved due to young people avoiding asthma.

Air pollution has huge effects on health, and that has knock-on effects on wealth and wellbeing for society. When you look at how these changes affect lives, it’s clear how important it is to take steps to make our air quality better.

Fall in cases by borough. Only areas in the expansion zone are coloured.

Notes:

We used the following methodology to derive these estimated values.

We drew on the London Atmospheric Emissions Inventory (LAEI) to estimate concentrations of nitrogen dioxide across the city. The LAEI was last estimated for 2019 – we used our STRIAD:AIR modelling (and modelled estimates for 2025) to estimate 2024 values. This means our estimates take into account other changes in air pollution (such as the expected increase in the use of electric cars).

We estimate a 21% fall in nitrogen dioxide in the expanded ULEZ zone after one year. This is in line with the fall after the ULEZ was rolled out in inner London[1] (and considerably less than the fall in central London after one year of 46%).

Data on asthma incidence was licenced from the Institute for Health Metrics and Evaluation. STRIAD:AIR modelled the expected incidence of asthma in over 2,000 data zones within the ULEZ expansion area, then applied known values for increased asthma incidence from the scientific literature[2] to determine the percentage of disease attributed to NO₂ at concentrations with and without the ULEZ.

We have modelled the estimated healthcare costs for all incidences of air-polluted related asthma using 2021 data from NHS digital. The total costs of all incidences for each ULEZ scenario were modelled using both the NHS digital data, and the estimated 2024 attributable asthma incidences from the STRIAD:AIR model.

Full information borough-by-borough is below. Those boroughs with low numbers (e.g. Southwark, Newham) generally have few data zones included within the ULEZ expansion zone.

More information on STRIAD:AIR is available here.

Cases by borough

[1] Inner London ULEZ One Year report (2023)

[2] Health outcome data sourced from Khreis et al, 2017 (DOI: 10.1016/j.envint.2016.11.012)