Molds are simple, microscopic organisms that can grow virtually anywhere, both in homes and outdoors. Along with mushrooms, yeasts, and mildew, molds are classified as fungi. Molds typically consist of a network of threadlike filaments that infiltrate the surface on which the mold is growing. Molds reproduce by releasing spores, which are lightweight and small enough to travel through the air. Spores can resist dry, adverse environmental conditions, allowing them to outlive the mold that produced them.Molds play an important ecological role in breaking down dead organic matter and returning nutrients to the environment. They require moisture and food to grow, and they typically thrive in warm, moist environments. Moisture is the key factor determining mold growth in the home, influencing both the types of mold present and the extent of mold colonization. A variety of materials found in the home, including insulation, wallpaper, glues used to affix carpet, backing paper on drywall, dust, and dirt, can serve as a food source for mold. Mold colonies can go dormant under adverse conditions and revive when favorable conditions return.

Mold growth often appears as green, gray, black, brown, or other discoloration. Eventually, mold growth results in the breakdown of the substrate. More than 1,000 types of molds have been found in US homes.

Exposure and Health Impacts

People are exposed to mold on a daily basis. Most exposures in the home occur when occupants inhale spores or mold fragments, which are components of household dust. They also may be exposed when their skin comes into contact with mold-contaminated materials.

Most people are unaffected by exposure to moderate amounts of mold. However, mold exposure can cause allergic reactions in some people. Approximately 6-10 percent of the general population, and 15-50 percent of persons who are genetically prone to develop allergies (atopic individuals), are allergic to mold, according to the National Academy of Sciences. The most common symptoms include runny nose, eye irritation, coughing, congestion, and exacerbation of asthma in persons who have the disease. At this point, it is unclear whether mold can cause individuals to become asthmatic. For more information on asthma and allergies, see Asthma, Allergies, and Respiratory Illnesses.

Some types of mold produce toxic substances known as mycotoxins, which can cause health problems when they are inhaled, absorbed through the skin, or ingested. One mold species may produce a number of different mycotoxins; conversely, one mycotoxin may be produced by several different types of mold. Mycotoxin production varies depending on environmental conditions such as moisture level, temperature, and substrate content. As a general matter, toxin-producing molds have higher water requirements than most household molds, so they thrive indoors only under wet conditions.

Although the health impacts of exposure to mycotoxins in the home are not well studied, adverse health effects have been observed in occupational settings and in animal studies. Of course, health impacts vary depending on the mycotoxin at issue and the nature of the exposure. Skin rashes, fatigue, dizziness, flu-like symptoms, nausea, respiratory and eye irritation, immuno-suppression, birth defects, lung inflammation, and cancer have been associated with exposure to mycotoxins. Persons exposed to high levels of mold toxins, e.g., mold remediation workers or farm workers, may be at risk for organic toxic dust syndrome (OTDS) or hypersensitivity pneumonitis (HP). ODTS may occur after a single, heavy exposure to mycotoxins, and usually carries with it fever, respiratory, and flu-like symptoms. HP is an immunological disease caused by repeated, high-level exposures to the same agent, and can result in permanent lung damage.

Mold exposure also may lead to infections such as fungal pneumonia in persons with compromised immune systems.

Mold Assessment

The most reliable way to identify a mold problem is through visual inspection. According to experts with the Building Science Corporation, “If you see mold or you smell mold, you have mold.” Since mold requires water in order to grow, looking for water or moisture problems is usually the best way to locate mold. This may require looking behind walls or ceilings, under furniture, in crawlspaces and basements, or behind cabinets and toilets. While assessing mold contamination, workers should wear gloves and eye protection and a respirator. They also should take steps to ensure that large amounts of mold are not released into the home from concealed areas, by misting moldy surfaces before disturbing them or using a HEPA vacuum attachment when cutting mold-contaminated surfaces, for example.

Prevention and Control

Although health-based standards for mold currently do not exist, it is generally accepted that no one should live or work in an indoor environment beset by extensive mold growth. Since mold requires moisture to grow, mold problems can be prevented by solving moisture problems quickly and effectively. Moisture in the home may be caused by poor ventilation, excess condensation (due to humidifiers or unvented clothes dryers, for example), water leaks, or floods. In the case of a flood or leak, mold growth can be prevented if water-damaged materials are dried and cleaned and/or removed within 24-48 hours. Additional preventative measures include regularly checking plumbing and promptly repairing leaks; maintaining relative humidity below 60 percent; venting clothes dryers; and installing exhaust fans in kitchens and bathrooms vented to the outside. For more information on addressing moisture problems in the home, see How to Control Moisture.

If mold is obviously present, the first step in controlling the problem is to assess the extent of the contamination. The Environmental Protection Agency (EPA) has developed mold remediation guidelines for schools and commercial buildings, and the New York City Department of Health (NYC) has created assessment and remediation guidelines for fungi in indoor environments. Both of these guidelines recommend remedial measures and precautions calibrated to the amount of mold present.

As a general matter, the goal of mold remediation is to remove or clean contaminated materials in a manner that prevents mold and contaminated dust from escaping the work area, while protecting the workers performing the remediation. The underlying water or moisture problems must be addressed prior to or during remediation; otherwise, mold growth will recur.


Despite the flurry of activity around the country to pass laws relating to mold, legislation on the problem remains in the nascent stages. Currently, there are no health-based standards for mold exposure. The EPA and NYC guidelines set forth recommendations for safe assessment and remediation of mold contamination, but they are not legally binding. The laws being considered, and in some cases adopted, address a few common themes. Some laws seek to establish committees or task forces to study the issues surrounding mold. Other laws have sought to implement licensing schemes for mold inspectors and/or remediators. Some laws under consideration have addressed insurance issues, while others have sought to require disclosure of mold during sale or lease transactions. In some cases, legislatures have focused on indoor air quality issues in schools and public buildings.

Several states also have considered adopting more comprehensive mold legislation, modeled in some cases on California’s Toxic Mold Protection Act, which requires the state’s Department of Health Services (DHS) to convene a task force to consider the feasibility of adopting exposure limits to mold in indoor environments (and to adopt standards if feasible). The Act also directs DHS to adopt practical standards to assess the health threat posed by mold, develop remediation guidelines, and assess the need for standards covering mold assessment and remediation professionals. Landlords are required to provide written disclosure of known mold contamination to tenants prior to entering into a lease and to provide a DHS brochure on mold. However, these requirements do not become effective until after the standards are adopted and DHS creates a brochure. City attorneys, as well as code enforcement and public health officials, are authorized to enforce the Act, which has gone largely unimplemented due to lack of funding. In New York State, two bills have been introduced that mirror the California Act.

See Action Agenda for more information about legislation and regulations.

Sources and Additional Information:

Affordable Comfort –

Building Science Corporation –

Canada Mortgage and Housing Corporation –

Environmental Health Watch, Moisture Audit of Residential Structures –

Environmental Protection Agency, Mold Remediation in Schools and Commercial Buildings –

T. Platts-Mills, J. Vaughan, M. Carter, and J. Woodfolk, Journal of Allergy and Clinical Immunology, “The Role of Intervention in Established Allergy: Avoidance of Indoor Allergens in the Treatment of Chronic Allergic Diseases,” pp. 787-804 (November 2000).

US Dept. of Housing and Urban Development, Healthy Homes Issues: Mold, External Review Draft, Version 2, October 2, 2001 –