Mould is made from microscopic fungi that can form extremely rapidly and create large patches known as spores. You've almost certainly seen mould growing on food that has gone past its sell-by-date or in wet, dusty areas, but mould can also thrive within your air conditioning system. How can mould grow in air conditioning systems? It might sound odd to say that mould can grow within an air conditioning system. After all, a clear stream of air is regularly passed through the ducts, most ducts are made from metal or plastic, and there shouldn't be any water in the system.