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. Tightly sealed buildings, re-circulating the same indoor air and not having enough fresh air entering the building can also add to the problem. Areas of mould growth appear on damp surfaces, such as window sills or on the inside of poorly insulated outside walls.
Moulds also grow on, or under, water-damaged surfaces such as tiles and carpets.
This can lead to mould growing inside the wall itself, although this is uncommon. Use a humidistat control on bathroom fans. Vent clothes dryers to the outside. Ventilate attic and crawl space to prevent moisture build up. If the crawl space is bare earth, cover it with plastic.