The physical and also to some extent physiological facts which underlie the quality of the air we breathe-primarily exchange and humidification of the air in rooms - are presented to make clear what hygienic significance is attributable to these climatic factors. It is essentially a matter of the air in densely populated rooms, particularly under the complicating conditions of the cold and cool seasons. The ways which can be adopted to make the indoor climate tolerable by economic and technically simple means are shown. For this, the co-operation of all offices of the civil service is absolutely necessary to promote the quality of this vital condition of the air in rooms.