Recent data from local councils suggest that the Ultra Low Emission Zone (ULEZ) in London has not significantly improved air quality as claimed by Mayor Sadiq Khan. Despite the introduction of the £12.50-per-day charge, aimed at reducing pollution from diesel vehicles, more than half of the city’s boroughs are still experiencing nitrogen dioxide (NO2) levels above legal limits.
Last year, Khan praised the ULEZ for allegedly bringing NO2 levels within legal boundaries citywide for the first time. However, upon analyzing official air quality figures, it becomes evident that many areas continue to struggle with illegal levels of this harmful gas.
The legal threshold for NO2 is an annual average of 40 micrograms per cubic meter of air. A report from the Department for the Environment, Food, and Rural Affairs (Defra) last year suggested that London’s air quality fell within this limit for 2024. Yet, at least 18 locations across the capital still report annual averages exceeding this standard at specific monitoring sites.
Particularly concerning are findings from Romford, where an annualized average nearly double the acceptable limit was recorded in 2024. Similarly, several monitoring stations in the City of London, where ULEZ was first implemented seven years ago, report comparable breaches of the legal NO2 threshold.
However, at least 18 areas of the capital are still recording annual average levels above this at individual monitoring stations.
One in the centre of 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.
The findings come after the Mail reported that last year alone, ULEZ generated a record-high income of £219million, compared to £215million in 2024.
The charity Asthma + Lung UK, which analysed the figures together with the Healthy Air Coalition, claims that the government is understating the levels of air pollution.
Andrew McCracken, from the charity, says that while ULEZ has been ‘vital’ for reducing pollution people, especially those with lung conditions, deserved full transparency about the air they are breathing.
Critics of ULEZ claim its benefit has been overstated after analysis of local air pollution monitors found the air in some parts of London is still above legal limits for nitrogen dioxide
‘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 told the Mail.
‘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 Mr Khan of ‘cherry-picking figures to suit his agenda’, blaming the pursuit of low-traffic neighbourhoods for creating gridlock and fuelling emissions on main roads.
She added: ‘It’s vital that Sadiq Khan, having told us all the air is clean in London because of ULEZ, tells us if this is not the case.’
The data discrepancy has arisen from how Defra records nationwide air quality statistics compared to local authorities.
Its researchers break Britain up into 43 huge regions, reporting NO2 levels using modelling and measurements from around 200 Automatic Urban and Rural Network (AURN) stations. London is considered a single region, and has 15 AURN stations.
But the capital’s boroughs each have their own, far denser network of pollution monitors which could provide a more detailed picture of local air quality.
These are made up of scores of highly accurate automatic monitors similar to those used by Defra, and hundreds of NO2-absorbing ‘diffusion tubes’ that are read manually each month and provide a more general, less accurate pollution reading.
Experts say local level data, particularly that from the automatic monitors, is likely to be reliable – but Defra will not use it because it does not meet its strict criteria on quality and where the monitors are sited.
Professor Frank Kelly, one of the world’s leading air quality experts and head of Imperial College London’s Environmental Research Group, said: ‘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.’
Mr Khan hailed London’s clean air status as a ‘historic milestone of meeting the legal limits for nitrogen dioxide’ – but Professor Kelly has pointed out that the NO2 limit for the UK, 40ug/m3, is four times higher than the WHO recommendation.
Labour pledged to introduce a Clean Air Act in its 2024 manifesto which would see the UK meet this standard – of which nothing has been said since.
Prof Kelly added: ‘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.’
NO2 levels on some roads in the City of London – where ULEZ was introduced seven years ago – are still above legal limits, according to local council figures (pictured: Upper Thames Street, where an illegal level of NO2 was recorded)
Sadiq Khan – who debuted a new shaven look this week after undertaking a pilgrimage to Mecca – has credited ULEZ with bringing NO2 levels to within legal limits
Join the discussion
Is ULEZ really making London safer or just hitting drivers’ wallets for little real change?
City Hall backs Defra’s stance on only using AURN monitors, while at the same time boasting online that London is home to one of the world’s ‘most comprehensive air quality monitoring networks’ – largely made up of monitors it is ignoring.
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.
‘Air quality data from Defra shows that London met legal limits for nitrogen dioxide for the first time in 2024.
‘We know the damage that air pollution can cause to the health of Londoners so we always look at all the data available in the capital to get a clear picture of the work that still needs to be done.
‘All Londoners are now breathing much cleaner air, but there is still more to do and the Mayor will continue to take action as we build a greener, fairer London for everyone.’
A Defra spokesperson said: ‘Poor air quality robs people of their health and costs the NHS millions in extra treatments for lung conditions and asthma.
‘That is why the government remains committed to improving air quality to deliver benefits for public health, the environment, and the economy.
‘We have set new air quality targets to cut exposure to harmful particles by nearly a third by 2030, improving the lives across the country.
‘Alongside this, we are taking steps to reform areas like simpler industrial permits to reduce emissions, and tightening standards for new wood burning appliances to help reduce health impacts.’
