After months of debate, Sadiq Khan has expanded the Ultra-Low Emission Zone (ULEZ) to cover the whole of London.

Vehicles that don’t meet emissions standards will pay £12.50 a day to drive in the zone, or face a fine.

Transport for London (TfL) says nine out of ten cars driving in outer London are already ULEZ compliant and the policy will improve air quality in the capital.

But research by the mayor’s own office and TfL predicts the latest expansion will only have “minor” and “negligible” effects on two key types of air pollution.

And, based on the mayor’s data, FactCheck calculates that the expanded ULEZ would add just 13 minutes to the life expectancy of the average Londoner in 2023.

Meanwhile, the evidence supporting the early phases of ULEZ has sparked a row between rival groups of researchers at one of Britain’s top universities.

Let’s take a look at the evidence we have so far – and what we still don’t know.

Did the early ULEZ in central and inner London improve air quality?

The first ULEZ area covered central London and took effect in April 2019.

A 2020 report from Sadiq Khan’s office looking at the first ten months of the scheme said the central London ULEZ was responsible for a 37 per cent reduction in nitrogen dioxide (NO2), a key air pollutant, compared to an imagined scenario in which no ULEZ had been introduced.

The good news continued in February 2023, when the mayor’s office reported on the expanded ULEZ, which since 2021 had stretched across inner London between the North and South Circular roads.

It linked ULEZ to significant falls in major pollutants: NO2 concentrations down 46 per cent and nitric oxide (NOx) emissions down 26 per cent.

But a 2021 report from researchers at Imperial College London’s Centre for Transport Studies appeared to cast doubt on this initial success.

Their analysis, which was peer-reviewed by an unnamed expert and published in the journal Environmental Research Letters, said the central London ULEZ had reduced NO2 concentrations by just under 3 per cent on average. That’s a lot less impressive than the 37 per cent the mayor’s office reported.

And they found “insignificant” effects on particulate matter levels – meaning it wasn’t possible to attribute any fall in this pollutant to the scheme.

The mayor’s office strongly rejected these findings – and continues to do so. As does another team of scientists at Imperial College London: the Environmental Research Group.

The head of that team, Professor Frank Kelly, told FactCheck that the paper by his colleagues in the Centre for Transport Studies “only looked at a small window of time after the ULEZ was introduced”.

He suggested that it didn’t account for the fact that “many vehicle owners had upgraded their vehicles prior to the introduction of the actual ULEZ” in order to avoid a separate Toxicity Charge scheme, which came in at the end of 2017.

Though, on this, he and his colleagues may actually agree. The Transport Studies paper suggested that “it is the combined effects of several policies that have led to improvements in air quality”.

Nevertheless, Professor Kelly recommended we “avoid” the Transport Studies paper, “as it is now widely recognised to be poor quality”. However, we found four other teams of researchers who have cited it in peer-reviewed journal articles.

Professor Kelly directed us instead to the February 2023 report published by the mayor’s office. That report thanks Dr Gary Fuller, who is also a member of Professor Kelly’s Environmental Research Group, for providing “peer review support and comments” on its methodology.

Professor Kelly’s own team has also faced controversy on this issue.

The Telegraph reported in August that it had seen private emails between Professor Kelly and Shirley Rodrigues, Sadiq Khan’s deputy in charge of the environment, in which she reportedly described herself as “deeply concerned” about Imperial’s Transport Studies paper.

Professor Kelly reportedly replied that he was “totally dismayed” and would be “very happy to provide the Greater London Authority with support required as you move to mitigate the damage”, according to the Telegraph. The broadsheet claims that the academic also “sent a statement to Ms Rodrigues to check”.

Since 2021, the Environmental Research Group he leads has received over £800,000 in funding from the Greater London Authority, which is run by the mayor. Sadiq Khan’s political opponents have recently suggested that this, along with the reported emails with the deputy mayor, calls into question the Group’s research findings.

A spokesperson for Sadiq Khan told the Telegraph in August that “it is right – and standard practice across government – that we commission experts to carry out research to inform the work we do”.

Professor Kelly and the Environmental Research Group at Imperial are, the spokesperson said, “some of the world leading academic institutions looking at air quality”. They said of the mismatch between Professor Kelly’s team and the Centre for Transport Studies research: “It is commonplace for academic experts to disagree with how other academic studies are interpreted, as was the case here.”

We’ve not seen the emails the Telegraph says it reviewed.

But whatever the controversy surrounding Professor Kelly’s team, one of his criticisms of his colleagues in the Centre for Transport Studies is correct: their findings use data covering just a few weeks after the ULEZ was introduced, whereas the mayor’s office measured changes after ten months of the scheme. That could give the mayor’s office research an edge.

And there is other research – not from either team at Imperial – that suggests the central London ULEZ scheme did produce more marked improvements in air quality.

Scientists at Queen Mary and University College London concluded that the initial ULEZ was linked to a 20 per cent reduction in NOx levels.

And experts at the University of the West of England and Erasmus University Rotterdam found “statistically significant” reductions in NO2 levels inside the central London ULEZ in the first 90 days of its introduction – compared to no changes at “control” sites outside the ULEZ area. That could mean it was the ULEZ that caused or contributed to these changes, though we’d need more research to say for certain.

Will expanding ULEZ to outer London improve air quality?

So far, we’ve looked at the evidence for the effects of the central London and inner London ULEZ. But what do we know about the new outer London zone?

The mayor’s own impact assessment, published in 2022, predicts that expanding ULEZ to outer London will cause only a “minor reduction” of 1.3 per cent in the average Londoner’s exposure to NO2 – and “negligible reductions” (0.1 per cent) in exposure to particulates.

There’s a more upbeat London-wide projection on NOx emissions. Though even those are only expected to see a “moderate” change, with ULEZ set to reduce average emissions by 5.4 per cent across the capital.

We put it to TfL that this meant the evidence supporting the latest expansion of ULEZ was much weaker than it was for the early phases. TfL told FactCheck that it was important to consider how the policy will affect the absolute amount of air pollutant emitted, not just the percentage change.

For example, a 5.4 per cent reduction in NOx emissions doesn’t sound like much compared to the 35 per cent fall attributed to the central London ULEZ. But because outer London is so big, it works out at a larger reduction in absolute terms (362 versus 240 tonnes of nitric oxide).

But while it’s true the absolute reduction in air pollutants is greater in the outer ULEZ than the central and inner zones, we should remember that these benefits are also spread out over a much larger population.

The real-world impact of the scheme is perhaps best demonstrated when we look at the effects on Londoners’ health.

The mayor’s impact assessment estimates that expanding ULEZ to outer London will cut the number of hospital admissions caused by air pollution by 26 this year. (To be clear, that’s not 26 per cent – but 26 individual admissions averted.) Of course, any reduction in hospital admissions is a good thing – but it works out at only 1.2 per cent fewer admissions than we would have seen had ULEZ not been expanded.

Then there’s life expectancy. The mayor’s office expects 214 “life years” will be saved in 2023 thanks to the expanded ULEZ that would otherwise have been lost to NO2 exposure. That’s not the same as saving 214 people from death – it’s the additional number of years that Londoners as a whole can expect to live as a result of the policy.

We calculate that, spread across the city’s population of 8.8 million, expanding ULEZ to outer London could add just 0.0089 days – or 13 minutes – to the average Londoner’s life expectancy this year. (We ran this calculation past an independent statistician.)

Responding to this point, the mayor’s office noted that the same report says the expanded ULEZ “would continue to deliver air quality and health benefits for a number of years beyond 2023”, but that these haven’t been modelled in the research. In other words, there could be more life expectancy gains in future years, but they haven’t calculated what those might be. The mayor’s report says that any health benefits would be lower in future years because cars and other vehicles would have been improving anyway, even without ULEZ.

The mayor’s office also noted that the life expectancy benefits wouldn’t be spread out equally – so some individual Londoners might experience greater gains. (Though by the same token, some would experience smaller gains, if any at all.)

As part of a longer statement, a spokesperson for the mayor told FactCheck: “The science is clear – the impact of the ULEZ expansion will be transformative. It will mean five million Londoners breathing cleaner air, and reduce toxic NOx, CO2, [particulate] emissions from vehicles. It is the largest clean air zone in the world.”

They added: “The ULEZ expansion is part of a wider package of bold measures we are taking to clean up the air across London, from electrifying our bus fleet, to delivering the Superloop to outer London, to more electric taxis, to installing electric vehicle chargers.”

Does ULEZ expansion reduce carbon emissions in London?

It’s not the primary goal of ULEZ expansion but reducing greenhouse gases is second on the list of policy objectives in the mayor’s impact assessment.

However, that same research concludes that widening the scheme to outer London is “estimated to have a negligible beneficial impact on carbon emissions”.

Though again, the mayor’s office stressed the need to consider the absolute reduction in carbon emissions, which is expected to be 27,000 tonnes in outer London in 2023.

FactCheck verdict

Reports from Sadiq Khan’s office, as well as some peer-reviewed research, suggest that the ULEZ in central London has been linked to improvements in air quality. Though it’s also been suggested that these could be a result of a combination of policies beyond ULEZ.

According to the mayor’s own impact assessment, widening ULEZ to outer London will only have “minor” and “negligible” benefits in terms of nitrogen dioxide and carbon particulates exposure, which are two key measures of air quality.

There’s a more upbeat prediction on nitric oxide emissions – the third main pollutant – though even those are only expected to see a “moderate” reduction of just 5.4 per cent, according to the mayor’s office research.

TfL and the mayor’s office stress that the absolute change in emissions is still greater in the outer London expansion compared to previous ULEZ phases.

Though the benefits are spread across a larger population, too. Using the mayor’s own data, we estimate that the outer London ULEZ would add 13 minutes to the average Londoner’s life in 2023.

The mayor’s spokesperson told FactCheck: “the science is clear – the impact of the ULEZ expansion will be transformative”.

Additional reporting by Helen Johnson.