There's several quality level for interior air in EN 15251 : category I to III. The reference guide doesn't enjoin a specific category, as you can see in this extract :
"To determine the ventilation rate: Use Table B.1 in Annex B of CEN Standard EN 15251:2007 to find the appropriate percentage of dissatisfied building occupants and select Category I, II, or III based on the building design and applicable local codes."
We hesitate to select the lower category (III). Do you have any feedback about this ?