Moulds are micro-organisms that produce thousands of tiny particles called spores as part of their reproductive cycle. Actively growing mould colonies are usually visible as colourful "woolly" or "slimy" growths. They can be virtually any colour—red, blue, brown, green, white, or black. When disturbed by air movement or contact, moulds release their spores into the air. Given the right environmental conditions, these spores can go on to form other mould colonies.

Mould becomes inactive or dormant if moisture is not present. Inactive mould is often dry and powdery or crusty and, like actively growing mould, can be readily disturbed by air movement or contact, causing mould particles and spores to become airborne. Mould can remain inactive for many years but will grow once moisture returns