It is a known fact that a relative humidity level below 40 % can lead to the drying up of mucous membranes, thus increasing the susceptibility to infectious colds, as dry air has a negative impact on the cleaning function of the inner surfaces of the airways.

A higher level of relative humidity, in the range between 40 % and 60 %, has a positive influence on the comfort of the indoor climate for several reasons: It reduces the concentration of respirable dust in the air, activates the skin’s defences against microbes, lowers the life expectancy of bacteria and viruses, reduces unpleasant odours and prevents the troublesome build-up of electrostatic charges in the room.

However, humidity exceeding 70 % is usually perceived as uncomfortable, presumably because the absorption of oxygen in the blood is reduced in warm, humid conditions. Cold, damp air appears to increase the incidence of rheumatic disorders. It is of particular concern that levels of humidity above 70 % can lead to the rapid growth of mildew in closed rooms.