There is a high level of uncertainty in government’s model for assessing pollution levels, which may mean that further areas of poor air quality might be missed by the programme. Government uses a national model to identify areas that are likely to be breaching air quality limits, and uses this to direct local authorities to take action. The national model does not directly use the results of monitoring by local authorities; instead, it uses a national network of monitoring stations. The model is regularly updated to take account of differences to local modelling, however government estimates that overall uncertainty in the model remains at the ‘allowable limit’ of +/- 30%. There is no formal margin of tolerance to identify local authorities for inclusion in the programme, and some local authorities are concerned this may result in an unfair situation whereby areas with high levels of NO2 pollution are not required to take action due to the national model not predicting a breach. Recommendation: As part of the Treasury Minute response, the government should set out how it will satisfy itself that all areas in exceedance of pollution limits have been identified and included in the programme, taking into account the high levels of uncertainty associated with the national model.