Introduction to Molds

September 1, 2007
Introduction to Molds

Molds live in the soil, on plants, and on dead or decaying matter. Outdoors, molds play a key role in the breakdown of leaves, wood, and other plant debris. Molds belong to the kingdom fungi. Unlike plants, they lack chlorophyll and must survive by digesting plant and other organic materials for food. Without molds, our environment would be overwhelmed with large amounts of dead plant matter.

Molds produce tiny spores to reproduce, just as some plants produce seeds. Mold spores can be found in both indoor and outdoor air, and on indoor and outdoor surfaces. When mold spores land on a damp spot, they may begin growing and digesting whatever they are growing on to survive. Since molds gradually destroy the things they grow on, one can prevent damage to building materials and furnishings, as well as save money, by eliminating mold growth.

Moisture control is the key to mold control. Molds need both food and water to survive; but since molds can digest most things, water is the factor that limits mold growth. Molds often will grow in damp or wet areas indoors. Common sites for indoor mold growth include bathroom tile, basement walls, areas around windows where moisture condenses, and near leaky water fountains or sinks. Common sources or causes of water or moisture problems include roof leaks, deferred maintenance, condensation associated with high humidity or cold spots in a building, localized flooding due to plumbing failures or heavy rains, slow leaks in plumbing fixtures, and malfunction or poor design of humidification systems. Uncontrolled humidity also can be a source of moisture leading to mold growth, particularly in hot, humid climates.

Health Effects and Symptoms Associated With Mold Exposure

When moisture problems occur and mold growth results, building occupants may begin to report odors along with a variety of health problems, such as headaches, breathing difficulties, skin irritation, allergic reactions, and aggravation of asthma symptoms. All of these symptoms may be associated with mold exposure.

All molds have the potential to cause health effects. Molds produce allergens, irritants, and, in some cases, toxins that may cause reactions in humans. The types and severity of symptoms depend on the types of mold present, the extent of a person’s exposure, the age of the individual, and his or her existing sensitivities or allergies.

Specific reactions to mold growth can include the following:

  • Allergic reactions. Inhaling or touching mold or mold spores may cause allergic reactions in sensitive individuals. Allergic reactions to mold are common and can be immediate or delayed. They may include hay fever–type symptoms, such as sneezing, runny nose, red eyes, and skin rash (dermatitis). Molds spores and fragments can produce allergic reactions in sensitive individuals whether the mold is dead or alive. Repeated or single exposure to mold or mold spores may cause previously non-sensitive individuals to become sensitive. Repeated exposure has the potential to increase sensitivity.
  • Asthma. Molds can trigger asthma attacks in people who are allergic (sensitized) to molds. The irritants produced by molds also may worsen asthma in non-allergic (non-sensitized) people.
  • Hypersensitivity pneumonitis. This condition may develop following either short-term (acute) or long-term (chronic) exposure to molds. The disease resembles bacterial pneumonia and is uncommon.
  • Irritant effects. Mold exposure can cause irritation of the eyes, skin, nose, throat, and lungs, and sometimes can create a burning sensation in these areas.
  • Opportunistic infections. People with weakened immune systems (immune-com-promised or immune-suppressed individuals) may be more vulnerable to infections by molds (and can be more vulnerable than healthy persons to mold toxins). Aspergillus fumigatus, for example, has been known to infect the lungs of immune-compromised individuals. These individuals inhale the mold spores, which then start growing in their lungs. Trichoderma also has been known to infect immune-compromised children. Healthy individuals are usually not vulnerable to opportunistic infections from airborne mold exposure. However, molds can cause common skin diseases, such as athlete’s foot, as well as other infections.

This article has been reprinted in part with permission from the U.S. Environmental Protection Agency (EPA). For expanded information, please visit