London’s air may not be as clean as official Defra figures suggest, with local council monitoring showing parts of the capital are still breaching legal nitrogen dioxide limits despite years of ULEZ.
According to the Daily Mail, the discrepancy comes from the way national and local air pollution data is collected.
Defra assesses air quality using modelling and measurements from around 200 Automatic Urban and Rural Network stations across the UK, with London treated as one large region and covered by 15 AURN stations.
But London boroughs operate a far denser network of local pollution monitors, including highly accurate automatic monitors and hundreds of nitrogen dioxide diffusion tubes read manually each month.
Analysis of those local figures suggests more than half of London boroughs are still recording illegal levels of nitrogen dioxide, a toxic gas largely emitted from diesel vehicles.
The legal annual average limit for nitrogen dioxide is 40 micrograms per cubic metre of air.
A Defra report last year claimed London fell wholly within that limit for 2024, a finding Sadiq Khan hailed as a “historic milestone” after years of defending his £12.50-a-day Ultra Low Emission Zone.
But the newspaper reports that at least 18 areas of the capital are still recording annual average levels above the legal limit at individual monitoring stations.
One station in Romford recorded an annualised average almost twice the limit in 2024, as did several stations in the City of London, where ULEZ was introduced seven years ago.
Professor Frank Kelly, Head of Imperial College London’s Environmental Research Group, told the Mail: “When you look at a lot of these other monitors that aren’t in Defra’s AURN, we’re exceeding legal limits. The monitors run by local authorities are looked after in the same way. They’re pretty much the same as the AURN – there’s no issue there, really.”
The findings matter because ULEZ has been presented by City Hall as a major success in cleaning up London’s air.
The scheme charges drivers of older, non-compliant vehicles £12.50 a day and was expanded across all London boroughs in 2023, it generated a record £219m last year, compared with £215m in 2024.
Asthma + Lung UK, which analysed the figures with the Healthy Air Coalition, said the government may be understating the true level of air pollution.
Andrew McCracken from the charity said: “Where there are differences in monitoring approaches these must be urgently addressed so that communities across the UK can have confidence in the information they receive, and pollution hotspots are not overlooked.”
He added: “The 12 million people in the UK who live with lung conditions… need data they can trust to protect themselves from dangerous levels of exposure to this invisible threat.”
Susan Hall, Leader of City Hall Conservatives, accused Khan of “cherry-picking figures to suit his agenda” and said he should explain whether London’s air is really within legal limits.
A spokesperson for the Mayor of London said: “London’s ULEZ, the world’s largest clean air zone, has been a huge success in cleaning up London’s air, with harmful roadside NO2 concentrations now 24 per cent lower in outer London compared to without the ULEZ in place.”
But Professor Kelly warned the UK legal limit is still far weaker than the World Health Organization recommendation.
He said: “It doesn’t matter if some places are in the limit and some aren’t legal. The point is everywhere is still illegal and having an impact on people’s health.”
Copyright © 2026 Energy Live News LtdELN