The average West-European spents around nineteen hours a day indoors. Office workers spend eight of these hours at their desks. In summer especially, htis isn't the ideal situation. Even if an office is air-conditioned, it can often be hot and uncomfortable. And in many cases the air conduits turn out to be a health hazard - if airconditioning systems are not maintained regularly, bacteria and fungus spores can be spread through the air.

The Fraunhofer Institute for Surface Technology and Biochemical Engineering in Stuttgart has developed a component to humidify the air in air-conditioning systems. It is a membrane contactor containing a bundle of heir-fine tubes made of semi-permeable membranes. The space between the tubes is filled with water. When air passes through the tubes, it is moistened through the pores in the membrane. The required humidity level can be regulated via several factors: the surface area of the membrane, the temperature and the volume of air. It is thus possible to create the ideal ambient atmosphere, and germ-free too, because the membrane tubes also act as sterile filters - only water molecules are capable of penetrating the membrane.

The membrane contactor has also another application: it produces sparkling water from tap water. In this case it is fitted to the tap. What passes through the tubes is water, rather than air as in an air conditioning system. Bottled carbon dioxide diffuses into the water through the pores. As a result carbonic acid is added into the tap water as it flows from the mains. The price of this contactor is roughly DM 250, which makes it affordable to private households. "We envisage further applications for the membrane contactor in future", says Norbert Stroh, an engineer at the IGB, "for example in aerobic water purification or as a pollution filter for carbon dioxide"