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.

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.

Health Impacts
Sources of Mold
Testing for Mold
Reducing Exposure
Steps to Remove Mold
More Information

Health Impacts

Mold is a serious health hazard in the home environment, as it produces allergens, irritants, and in some cases, potentially toxic substances. Further, mold can trigger respiratory problems such as asthma in vulnerable and allergic populations. Therefore, preventing and eliminating mold problems is a crucial part of ensuing quality housing conditions.

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.

Sources of Mold

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.

Testing for Mold

In addition to preventative measures, visual inspections for mold should be performed periodically for the early detection of potential problems. The most reliable way to identify a mold problem is through visual inspection.

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.

Mold may be clearly visible or it may be hidden under furniture and carpets, in cabinets, and in crawlspaces or attics. When assessing mold problems in the home environment, it is important to know such potential hiding places and visually inspect all likely areas that are reasonably accessible. In some cases, mold will not be discovered even after searching typical hiding places, but a musty odor or related health problems will indicate a mold problem. In these instances, mold may be hidden on the backside of such materials as drywall, wallpaper, paneling, and carpet pads or inside wall cavities and ductwork. Investigation of such hidden mold problems is more complicated as actions such as peeling off wallpaper may disturb the mold and cause widespread dispersal of mold spores. Expert assistance may be required for such disruptive actions.

Use the right safety equipment during inspections. 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.

Special sampling techniques or tests may be useful. Because all molds should be treated similarly (safely removed, while addressing underlying moisture problems), there is no need to identify mold by type prior to remediation. However, bulk or surface sampling may be helpful in identifying specific mold contaminants in connection with a medical evaluation or in confirming the presence of mold if a visual inspection is unclear. Bulk sampling involves removing and collecting visible mold from surfaces, while surface sampling involves wiping a surface or stripping it with tape to collect specimens.

Airborne fungal testing is rarely appropriate but may be useful if, for example, building occupants are experiencing symptoms that seem to be mold-related, and a visual inspection and sampling have failed to locate mold. Airborne testing does not provide reliable data on the average mold content in a home—instead, it provides a “snapshot” of mold levels, which vary considerably over the course of hours, days, weeks, and months. Airborne fungal tests also are expensive and there currently are no standards for determining whether measured fungal concentrations are safe. Extensive airborne testing should be reserved for specialized cases, such as when health problems persist in a complex building environment with no discernable source of the problem. In such a setting, expert assistance should be engaged.

The Alliance has informative materials and tools for low-cost assessments of mold and moisture, in English and Spanish, from its past project, the Community Environmental Health Resource Center.

Reducing Exposure

Controlling mold problems in the home environment is largely dependent on controlling the level of moisture in the home, because mold cannot grow without moisture. Further, excessive moisture in the home is cause for concern as it can also cause or contribute to structural home damage and other housing hazards to human health such as cockroaches, dust mites, and peeling lead paint.

Fortunately, there are ways to prevent and control excessive moisture, and therefore mold growth, in the home environment—both practical measures for residents as well as precautionary measures during construction or renovation. Please visit the Moisture page for additional information such as practical tips, telltale signs, and likely sources of moisture both inside and outside the home.

The New York City Department of Health & Mental Hygiene and the U.S. Environmental Protection Agency have developed separate but complementary guidelines for assessing and remediating mold in indoor environments. These guidelines contain detailed recommendations on the appropriate remediation activities for varying sizes and locations of mold contamination in various structures.

The NYC Guidelines contain five levels of mold remediation protocols. The most basic techniques apply to areas of 10 square feet or less, and include training workers on safe cleanup methods; protecting workers with disposable respirators, gloves, and goggles; vacating people from the work area; suppressing dust; removing and disposing of contaminated items that cannot be cleaned; and final cleaning of work areas and work area-egress locations. In addition to containing more protective measures such as containment for larger mold problems, the Guidelines also address remediation of HVAC systems.

EPA advises that if the moldy area is less than 10 square feet in total size, non-professionals can usually manage the cleanup by following some basic precautions and procedures. Larger jobs may require the services of a contractor who should have prior experience cleaning up mold.

Steps to Remove Mold

When a manageable mold problem is identified in the home environment, the following are some basic steps that individuals, landlords, and homeowners can take to remove the mold:

  • Remediate the moisture source immediately. Mold cannot grow without water; therefore, controlling underlying moisture problems must be an integral part of removing mold.
  • Wear gloves, goggles, and appropriate respiratory protection during all mold remediation activities. Both EPA and New York City recommend the N-95 respirator available on the Internet and at most hardware stores for approximately $12-25.
  • Take photos of the moldy surfaces. These may be useful in the future should the need to document the problem arise.
  • Determine if it is possible to clean the moldy area or not. Non-porous and semi-porous materials (e.g. metals, glass, hard plastics, wood, and concrete) can generally be cleaned and reused. Porous materials (e.g. fabrics, ceiling tiles, insulation, wallboard) may be cleaned, but it is preferable that they be removed and thrown away, as it is extremely difficult to ensure complete removal of the mold.
  • Remove belongings from the clean-up area.
  • Clean the moldy area as soon as possible with either a detergent/soapy water solution or a baking soda and vinegar solution. Thoroughly dry the area and immediately dispose of all sponges or rags used in both the cleaning and drying process. Chemicals such as chlorine bleach are not recommended for routine mold cleanup.
  • When finished cleaning the visible mold area, clean all nearby surfaces and scrub or vacuum the floor.
  • Make sure the area is well ventilated until all surfaces are dry.
  • Regularly check the area for signs of recurring water damage and new mold growth. If the mold returns, it may indicate that the underlying water problem has not been appropriately addressed.


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.

To find out more, see the Alliance's pages on selected state and local legislation and existing federal policy regarding mold and other hazards.

More Information

Building Science Corporation

- A US Department of Energy, Building America Consortium

- Building Design by Climate Guides

Canada Mortgage and Housing Corporation - Moisture and Mold

Environmental Health Watch - Moisture Audit of Residential Structures

New England Asthma Regional Council - Building Guidance for Healthy Homes

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).

Puget Sound Energy - Controlling Moisture in Your Home

University of Minnesota Extension Service and North Dakota State University Extension Service -

U.S. Environmental Protection Agency

A Brief Guide to Mold, Moisture, and Your Home
Mold Remediation in Schools and Commercial Buildings

U.S. Department of Housing and Urban Development - About Mold and Moisture

Asthma No Attacks Hotline: 1-866-NO-ATTACKS (1-866-662-8822)

General IAQ Hotline (IAQINFO): 1-800-438-4318
Sponsored by EPA, this hotline provides general information on indoor air quality and related pollutants.

Su Familia (Your Family) Helpline: 1-866-SU FAMILIA or 1-866-783-2645
The National Alliance for Hispanic Health sponsors this helpline to offer Hispanic consumers free, reliable, and confidential health information in Spanish and English and help navigate callers through the health system.

Learn More