In fact, molds have important roles in the environment and in living systems. In soil molds play a crucial part in decomposition of organic matter and in making nutrients available to plants. Molds and mildews can be very destructive to materials on which they grow, and are usually unwelcome and unhelpful in homes. Where colonies are extensive they can also produce enough spores, and by-products to be harmful to health. Many of the by-products of mold and mildew are irritating to skin, eyes and respiratory tracts.
Some molds produce true allergic sensitization and allergic reactions in susceptible people.
Some molds produce toxic by-products that could be harmful to skin, and poisonous if ingested or inhaled in quantity. Mold and mildews are harmful at least to the materials on which they grow, and they usually produce objectionable odors, stains and discolorations. If mold conditions are allowed to exist for long in a wood structure, the wood can quickly become weak and rotten. Fabrics and paper can be seriously damaged or destroyed in days by damp, moldy conditions. If molds grow extensively, they may produce enough airborne irritants to cause coughing and cold-like symptoms.