Updated: Apr 12, 2022
Dampness contributes indirectly to asthma and allergy risks. Increase in humidity in the building encourages proliferation of moulds, bacteria, house dust mites, and other micro-organisms some of which are toxin producers or allergens. Dampness also results in increased emission of chemical pollutants from degrading building materials.
Mould spores are generally everywhere. Nutrients are also available in form of organic dust or cellulose found on most building surfaces. In most cases the limiting factor to mould growth in a building is moisture. Moulds have a minimum level of dampness (relative humidity) below which growth is limited. Generally relative humidity below 65% restricts mould growth.
Dampness is also essential for the survival of house dust mites. Since house dust mites do not drink free water, they absorb it from the air and the environment. House dust mites feed on human skin scales (flakes), pollen, moulds, bacteria, animal dander, and skin scales of birds. These food sources have to be moist for mites to utilize them.
Therefore, the food consumption of mites increases (and hence their population) at high relative humidity. It has been estimated that human beings shed dead skin at a daily rate of 0.5-1.0g per person and several thousand mites can survive for months on just 0.25g, so moisture and not food would be the limiting factor to mite proliferation. House dust mites survive best at relative humidity of 70-85% and temperatures of 24-27 °C.