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With the exhaust air from ventilation and air conditioning installations escaping into the open, the heat content is also lost which fresh air from outside obtains at considerable expense of energy and technical equipment. The heat content, on the other hand, consists of about equal proportions of sensible heat and latent heat which is associated with the moisture content of the air. In order to regain the heat content of the escaping air so as to be able to use it again - and this is becoming increasingly important with rising energy costs - heat exchangers are necessary which remove the heat content from the exhaust air and transfer it to the fresh air from outside. With the high proportion of latent heat, this energy exchange is only effective if the latent heat can also be regained. For this purpose it is essential to have exchange surfaces which store and transfer both heat and moisture. To achieve this they must come into contact with the exhaust air stream and the fresh air stream alternately. Technically, this is done in a simple way by resolving rotor-like storage material. But a rigid separation of the air streams is no longer possible. Even if it is known that there are very highly developed sealing elements between the fixed and moving parts, the question whether particles from the exhaust air can get into the newly introduced outside air through the rotating storage material still gains in importance in certain types of usuage. For example, this is of importance for hospitals, especially in the operation areas in which air conditioning is desirable for 24 hours daily on hygienic grounds, but also in schools and offices where the present normal practice, for economic reasons, of recirculating air is to be avoided to stop the transference of infections pathogens and odours. In various places, experiments have been carried out earlier with heat exchangers consisting of asbestos board and with rotating storage material coated with lithium chloride and a particularly high degree of exchange achieved. The experiments are published and had the result that nothing was transferred which could give rise to objection. Yet these experiments have been criticised from various points of view.

The microflora of the air in university lecture rooms was examined with the sedimentation method. For this purpose blood agar (20 degrees C and 37 degrees C), saline agar and Endo plates were used. The germ counts identified correlate to the size and utilization of the rooms. The lowest germ count was determined in the room used for practical work, which was followed by the lecture room. The newly built lecture room contained the highest colony count. The floor ventilation furnishes an explanation for this. Most of the colonies were Gram-positive cocci (micrococci, indifferent streptococci, sarcins). Aerobic spore-forming organisms were found to be present in smaller quantities than cocci. Fungi, mainly, moulds grew better at a temperature of 20 centigrade. The microbiological monitoring of air forms an important part of environmental protection. Great importance can be attached to a stardization of the air germ count in the prevention of airborne infections.