The regional press ad in the Evening Standard, from the Mayor of London, for a proposed Ultra Low Emission Zone (ULEZ) in central London was challenged by Clean Air in for its claim that: "Introducing the zone in 2020 will encourage the use of newer, cleaner vehicles to reduce vehicle pollution by half".

The not-for-profit organization said it understood that the information was based solely on computer modelled projections of reductions in exhaust emissions only and excluded other major polluting elements.

TfL said that, when assessing the likely impact of future schemes, they used modelled projections, given that monitoring data of actual pollution levels was only available for present and past years. They said that that was a recognised standard approach for assessing the impact of future proposals of that type. They believed the projections had been produced in a robust way using established modelling methodologies and bespoke modelling of how drivers would respond to the scheme.

In response, the ASA upheld the claim. It acknowledged that, “when attempting to assess the impact of a proposal such as the ULEZ, there would be some inherent uncertainty, and that modelling was likely to be seen as an acceptable way of predicting the likely outcome” but said that it considered that the claim that vehicle pollution would be reduced by half was likely to be “interpreted by consumers as relating to all types of vehicle pollution, unless it was made clear that it related only to certain types”.

Because the claim related only to NO2 and NOx vehicle emissions, and excluded, for example, the figures for CO2, and total PM10 and PM2.5 emissions, the ASA concluded that the claim was misleading and said that consumers would expect carbon dioxide (CO2) to be included in the figure.

As a result, TfL were told to ensure that it made clear in future that claims about reductions in vehicle pollution referred to specific types of pollution rather than vehicle pollution as a whole.