Defra and DfT also wrote to us to explain that local monitoring is not directly fed into Defra’s national model in real time, particularly as many local measurements do not meet the data requirements for inclusion in the national NO2 compliance assessment. Instead, they review the model results against local NO2 assessments and use any discrepancies to prioritise improvements to the national NO2 compliance assessment. They also told us that an uncertainty of around +/-30% is not unusual for a model of this scale and complexity, and that their model meets the uncertainty requirements for assessing compliance with the annual mean NO2 limit value under the Air Quality Standards Regulations.27 However, the NAO highlighted that this is the limit of what is permitted.28 Working with local government