The ventilation rates stated in the ASWL for non-domestic buildings (i.e. 10 l/s/person for non-smoking areas, 20 l/s/person for smoking areas) equate to an EN 13779 indoor air category of IDA2. Therefore, the filter selection needs to be based on IDA2 as a minimum. Annex A.3 of EN 13779 sets out the methodology for determining the filter class required based on the IDA category and the quality of outdoor air (classified ODA1-ODA3 – 3 representing the most polluted air). The classification of outdoor air (ODA) is also detailed in EN 13779.