DfT told us that it takes account of the uncertainty in the model when identifying local authorities requiring action, and that it worked with local authorities to update and reopen the model as required based on local monitoring.25 However Defra and DfT subsequently wrote to us to explain that there is not a formal margin of tolerance; instead, the national modelling is continually updated to reflect the latest evidence, and additional local authorities can be brought into the programme wher...

DfT told us that it takes account of the uncertainty in the model when identifying local authorities requiring action, and that it worked with local authorities to update and reopen the model as required based on local monitoring.25 However Defra and DfT subsequently wrote to us to explain that there is not a formal margin of tolerance; instead, the national modelling is continually updated to reflect the latest evidence, and additional local authorities can be brought into the programme where new exceedances are identified.26 16 Q 14 17 Q 23 18 Qq 20, 57 19 C&AG’s Report, paras 2.5, 2.9 20 C&AG’s Report, paras 2.9, 2.10; UK Air Website 21 C& Type: conclusion | Number: 19 | Response status: not_addressed