Sadiq Khan and Transport for London misled Londoners with some claims about the benefits of the Ulez, the advertising watchdog ruled on Wednesday.
It said the Greater London Authority or TfL breached its rules on three occasions, including with the suggestion that “one of the most polluted places in London is inside your car”.
TfL was also wrong to have said that “most air pollution related deaths actually occur in Outer London areas” and that the Ulez had resulted in “almost a halving of levels of nitrogen dioxide” in central London.
But it was acceptable for it to say that “London has an urgent air quality problem”, that the Ulez expansion was being introduced to “help clear London’s air” and to highlight the link between air pollution and dementia.
The rulings relate to TV, radio and newspaper adverts placed by the mayor and TfL ahead of the Ulez expansion to the Greater London boundary last August.
TfL will have to ensure that any future Ulez adverts explain the source of data used to justify claims about the clean air zone’s effectiveness.
None of the adverts were banned from future use, and nor has the ASA given any opinion on the merits of the Ulez.
However the ruling is likely to infuriate Mr Khan as it will enable Ulez critics to pick apart some of the arguments he used to justify his decision to expand the zone Londonwide.
The ASA ruled that a GLA radio advert broadcast between last February and March, in which it was said that “one of the most polluted places in London is inside your car”, had not been adequately substantiated and was “likely to materially mislead”.
The TfL adverts appeared between January and June and attracted 503 complaints.
The ASA did not uphold three of the five grounds of complaint – where it was stated that London has an urgent air quality problem, that the Ulez expansion would help clear the air, and that research showed a link between toxic air and an increased risk of developing dementia.
However the ASA found a claim that the first Ulez zone, which launched in April 2019, had reduced NO2 by almost a half in central London was “likely to mislead” and breached its rules on misleading advertising and in other areas.
This was because TfL’s calculations were based on hypothetical predictions of what would have happened had the Ulez not been introduced, rather than on actual readings from roadside air monitoring devices.
The claim that “most air pollution related deaths actually occur in Outer London areas” was also “likely to mislead”.
This was because the number of pollution-related deaths attributed to outer London – 2,896 at the lowest estimate, and 3,321 at the highest - was “based on modelled estimates” rather than actual deaths.
The ASA report said: “We considered that listeners were likely to understand from the claim, which referred to ‘actual’ deaths, that the evidence supporting the claim was not estimated or modelled in any way. Clear qualifying information to explain the basis of the claim was therefore required.”
A TfL spokesperson said: “We are disappointed that the Advertising Standards Authority has upheld the complaints made against some of our advertisements. We believe that the information, which was based on robust scientific evidence, was clearly presented.
"The ASA did not challenge the science. Its ruling centres around a minor technical point in some ads. We will take this into account when drafting the wording and referencing in any future adverts.
“Scientific analysis based on modelled scenarios and estimates is standard practice in the scientific community and central government."
Tony Devenish, City Hall Conservatives environment spokesman, said: "Sadiq Khan and TfL have been caught out making misleading and unsubstantiated claims about the benefits of Ulez to Londoners. This shouldn’t surprise anyone.
"Sadiq Khan’s own report said the Ulez expansion would do little to tackle air pollution."
