The road from science to practice was never a highway downhill. That is the case of ‘climatisation’or ‘air-conditioning’. Practice required “rules of thumb”,“calculation tools”, etc. to make life more ‘fluid’for practitioners. Yet, if proper solutions cannot be without scientific explanations, every science advance affects the current culture and practice provoking resistances from the installed interests.

This paper aims at contributing to hopefully situate the case of the quality of air indoors at the beginning of the 21st century, a time of a permanent celebration for the key political move of EU banning tobacco smoke in public buildings in December 2004. On the wake of that milestone many advances to better health and comfort conditions indoors have happened, eventually without receiving yet due attention. The attempt is to comment on those and to elaborate on the rational for ventilation as a function of climate, density of occupation and, of course, in relation with the air in the city and the pollution sources indoors.