A mold is a type of fungus: a plant-like organism that does not use photosynthesis to produce food, but instead obtains it directly from its surroundings. Molds can display themselves in a variety of colors, such as the familiar green covering often seen on bread or fruit, but this is due only to the reproductive part of the organism — the fungal equivalent of flowers and fruit. The main part of the fungus consists of a network of thin, transparent filaments known as hyphae, which is much less visible.
These fungi can grow on a variety of materials, including soil, decaying plant parts, food, fabrics, and damp walls. They reproduce by releasing huge numbers of tiny spores, which are very tough, and can withstand drying and freezing. Although mold can be a nuisance, and occasionally a health hazard, they can also be useful; they are essential for the breakdown of dead organic matter in the natural environment, they are a valuable source of antibiotics and drugs, and are employed in the production of some foods.
Humans have lived with molds for millennia, but in a modern domestic setting, they can cause problems. The black type that forms on damp surfaces such as bathroom walls is unsightly, and may cause damage to the structure. When large quantities of spores become airborne, they can cause a variety of unpleasant symptoms. Some people are allergic to these fungi, and may experience coughing and sneezing, eye irritation, and skin rash. People with asthma, however, may experience more severe problems, such as breathing difficulties.
The spores are constantly present in the atmosphere, and little can be done about them; however, they do not pose any direct threat at the low concentrations normally found in outdoor air. It is when they are produced in large amounts in an enclosed, indoor space that they may have health effects. While spores can survive harsh conditions, the molds themselves require moisture and some degree of warmth to grow. Damp areas in the home are ideal for them, especially if they have poor air circulation and receive limited amounts of natural light, so they are common in bathrooms, basements and attics. When spores land on a damp, indoor surface, they can germinate and form mold that spreads rapidly.