Moulds spread by releasing tiny seed-like particles called spores into the air. Warm and wet indoor conditions help mould to grow. This sometimes results in more moulds being present in indoor air than in outdoor air. Biological contaminants can come from both outside and inside the home. Sources include house dust mites, animal dander, dust, pollen, cockroaches and moulds, all of which can cause an allergic reaction.
Some organisms can contaminate water sources, and become airborne through humidifiers.
Not all moulds cause health problems. Some moulds and other biological contaminants can cause allergic symptoms such as wheezing, sneezing, coughing, and nose, throat and eye irritation. Less commonly, breathing in large amounts of mould spores can cause a serious allergic reaction called 'hypersensitivity pneumonitis'. Large differences between inside and outside temperatures can cause water to condense on the insides of windows. Leaks from pipes or from the roof, or a damp concrete basement floor can also cause wet areas where moulds can grow.