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Health Professional: Mold Frequently Asked Questions

The following questions are organized by topical area. Numeric references at the end of each answer are the source documents. References are found at the end of the answer list. The information found in the following Frequently Asked Questions was developed through a joint effort between the Wisconsin Department of Health Services and the Wisconsin Section of the American Industrial Hygiene Association, whose contributions are kindly acknowledged. We encourage you to explore the information provided. If you would like more information, contact the Division of Public Health, Bureau of Environmental and Occupational Health, PO Box 2659, Madison, WI 53701-2659, telephone 608-266-1120, or email the Division of Public Health webmaster.

How can mold affect me?

Health effects associated with airborne mold exposure are allergic reactions, eye and respiratory irritation, infection and toxicity. About 10% of the population is allergic to one or more types of mold. Many of these people will be affected by outdoor as well as indoor exposures to mold.

Respiratory mold infection (growth in the lungs) can occur, but is rare. Occurrence is limited almost exclusively to immune-compromised patients, including those with transplants, people receiving chemotherapy, individuals with AIDS, newborn infants, etc. Toxicity is related to the ability of some molds to produce mycotoxins. There is not sufficient evidence to link health effects to indoor exposure to airborne mycotoxins, although ingestion of moldy food with mycotoxins has resulted in illness. Anyone suspecting they are ill from mold exposure should seek treatment and advice from a medical doctor. It should also be noted that indoor air quality can be influenced by other factors, such as other organisms; solvents, pesticides, and other chemicals; and gases such as carbon monoxide. Such factors may warrant consideration in some cases. If you experience health symptoms, you should see a physician (3,14).

When is mold considered toxic?

The term "toxic mold" is misleading. Molds may produce substances called mycotoxins that modify their environment. Some of these substances are useful as antibiotics; but others are potentially harmful, especially if eaten. However, there is little evidence that breathing mycotoxins in mold-contaminated buildings represents a health hazard. The American College of Occupational and Environmental Medicine states, "Current scientific evidence does not support the proposition that human health has been adversely affected by inhaled mycotoxins in the home, school, or office environment." One mold, Stachybotrys, is frequently singled out. Although it was initially thought to be related to cases of a lung disease called idiopathic pulmonary hemosiderosis among infants, the U.S. Centers for Disease Control and Prevention (CDC) has stated, "In a further review of our first investigation, CDC reviewers and an external panel of experts determined that there was insufficient evidence of any association between exposure to S. atra or other toxic fungi and idiopathic pulmonary hemosiderosis in infants." Current guidance of federal and professional organizations is that mold growth should be controlled in an appropriate manner, regardless of the type of mold (1,3,6,12,14).

Can mold affect people with asthma?

A person with asthma who is sensitive to molds could have an asthma attack triggered by either indoor or outdoor exposures. With respect to allergy, the American College of Occupational and Environmental Medicine (ACOEM) states, "While indoor molds are well-recognized allergens, outdoor molds are more generally important." A physician should be consulted if mold exposure may be a concern.

For people with asthma, a common health strategy is to avoid exposure by minimizing the amount of dust in the home. Humidity control is also very important. ACOEM supports indoor moisture control and the broad array of indoor respiratory challenges it affects. Moisture control is also strongly supported by the National Academy of Sciences as outlined in its report, Damp Indoor Spaces and Health (3,14).

How can airborne fungal infections be prevented in hospitals?

Controlling infection rates requires that hospital management assures that proper planning and dust control practices are in place. This includes:

  • Involving infection control staff in construction and maintenance planning
  • Managing traffic patterns to control dust
  • Using wet methods and HEPA vacuuming for cleaning critical care areas
  • Cleaning and maintenance of humidification and filtration systems
  • Maintaining proper pressure differentials in protective environments

Exposure of the small but growing population with impaired immune systems is a concern. Molds can cause infections in susceptible people, particularly in hospital settings. Cases of hospital acquired respiratory fungal infection have been associated with construction activity, contaminated air filters, air conditioning systems and open windows. Suggested acceptable levels of airborne mold in protective patient environments are very low. In highly filtered environments such as bone marrow transplant wards, concentrations no greater than 15 colony forming units per cubic meter (CFU/m3) 0.1 CFU/m3 for Aspergillus fumigatus have been proposed.

For those with depressed immune systems, it is important to minimize exposure to dust both at the hospital and at home. The same principles applied to controlling dust in the hospital also apply at home. Care should be taken to minimize dust using wet cleaning methods and HEPA vacuuming. In particular, exposure to soil and plant materials such as compost should be avoided as they can have large numbers of mold. For more information on controlling mold and related infections in health care environments, please see the Guidelines for Environmental Infection Control in Healthcare Facilities, Recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC). (8)

What is a safe level of mold in air?

There is no specific number that defines either safe or unsafe mold exposure. Some experts have proposed airborne mold guidelines; however, none of these have been adopted by regulatory agencies. Mold concentrations outdoors vary greatly with respect to time, species, and amount. Exposure to airborne mold outdoors, where levels often exceed thousands of spores per cubic meter, is considered safe for the general public. Except in buildings with extensive mold growth, the amount of mold found in indoor air is usually much less than what is found outdoors. For people with allergies to mold, however, there may be no practical level of exposure, either indoors or outdoors, that would not create discomfort or harm. It is therefore wise to remove and prevent indoor mold growth.

In extremely dusty environments, health effects have been observed in agricultural workers exposed to extremely high mold levels. In rare cases, very low levels of certain kinds of mold have been shown to affect people with severe immune deficiency, such as bone marrow transplant patients.

When should I evacuate because of mold?

Evacuation in response to mold should be rare. There is no established level of airborne mold that is accepted as unsafe for the general population. Those cases where evacuation may be warranted include spaces undergoing mold removal activity, and spaces that are occupied by sensitive populations, such as infants, elderly, the immune-suppressed, and those with medically confirmed symptoms related to mold exposure. It is generally recommended that exposure to mold indoors be minimized. However, it must accepted that mold exposure is inevitable in the world in which we live. We are exposed to many of the same mold species indoors as we are outdoors. Ultimately the decision to vacate a space should rest with the individual occupant, parent or guardian and be based on the amount of exposure, individual sensitivity, and advice of a medical doctor. Communication from building owners to all occupants is very important to arriving at appropriate decisions (1).

Does the type of mold determine the cleanup plan?

No. Current guidance from federal agencies and professional organizations is that mold growth in indoor environments should be controlled in a proper manner regardless of the type of mold (1,3,4,5,6).

Is mold growth on roof sheathing in my attic a problem?

It is not uncommon to find mold on the underside of roof sheathing. Usually this is the result of insufficient venting, such as venting bathroom or laundry exhaust into the attic, and/or insufficient peak or soffit vents. Moisture-laden warm air that enters the attic rises, contacts the cold sheathing and condenses. If this happens often enough it can affect the structural integrity of the roof through warping or rot. Although it may be possible for mold-contaminated attic air to enter the house in some cases, the function and integrity of the roof should be the greater concern. Consult with a home energy or roofing specialist on a proper ventilation strategy.

How can I tell if remediation (cleaning) has been effective?

The source of moisture responsible for the mold growth must have been corrected. No visible mold or related odors should remain in the work area. There should also be no debris present. For projects where extensive mold growth was identified, work should have been done under containment conditions (a plastic enclosure under negative pressure evident by the inward movement of plastic walls). The work area should have been thoroughly cleaned using wet methods such as wet wiping with a detergent solution and by vacuuming with a HEPA vacuum. Sampling is generally not recommended due to the high variability inherent in mold levels (4,5,9,14).

How do you test for mold?

While testing can be useful in some cases, such as for conducting health-related research or investigating fungal species-specific health effects, deciding what to do about mold should be based primarily on visual assessment, knowledge of the building structure, and the history of water damage in the building. There are many methods of assessing mold exposures; all of them have limitations. Further, mold levels within a structure are highly variable and a large sample number is required to obtain meaningful data. Some research suggests there may be less variability in data for components of microbiological cell walls than for entire organisms, but questions about how well such data correspond to true exposure remain. More information on sampling can be found by consulting the references listed or by consulting with an EMLAP accredited laboratory.

How many samples are necessary to find a mold problem?

The proper number and location of air samples is often a matter of debate and depends on the questions to be answered, strength of desired conclusions and on cost. Mold levels vary greatly with season, temperature, humidity and time of day. This variability requires that multiple samples be collected at each location to be sure the observed difference is real and not just due to chance. A report recently released by the National Academy of Sciences addresses the difficulties associated with sampling. "Thus, because only sparse data are available on variation of exposure to biologic agents in the home environment, it is not possible to recommend how many samples should be taken to produce an accurate assessment of the risk-relevant exposure. However, there is a strong suggestion that airborne concentrations are characterized by high variability over time, an indication that one sample per home is unlikely to be sufficient even when acute health effects are being considered, because variations in exposure occur over very short periods." Sampling plans, when used, should be designed by an experienced professional to answer specific questions. The design should be statistically robust and the goals and strength of the plan should be adequately defended prior to proceeding. More information on sampling can be found by consulting the references listed or by consulting with an EMLAP accredited laboratory (5,6,7,14).

Why are outdoor mold counts important?

In most instances, indoor mold levels will closely parallel outdoor levels, both in type and amount. In naturally ventilated buildings such as many homes, air from the outside enters the building whenever the windows or doors are open. Mold is present outdoors and can enter buildings in many ways, such as when windows and doors are open and people are entering and leaving. Depending on the effectiveness of the building air cleaning devices, outdoor fungi can be removed somewhat from the airstream. The extent to which such a reduction occurs depends on the efficiency of the filtration system and how "open" the building is to the outdoors. Outdoor counts will vary greatly and may in turn cause similar variation in indoor levels. Because of this variability, it can be difficult to differentiate true difference between outdoor and indoor samples without taking a large number of samples. Soil and plant materials are major sources of airborne mold. Studies indicate that outdoor fungal levels vary greatly by region, season, weather conditions, and air movement. According to data published by the American Academy of Asthma, Allergy and Immunology (, outdoor mold counts for major U.S. cities regularly exceed 10,000 spores per cubic meter of air during much of the year. (6,7)

What is an acceptable level of mold in surface dust?

Surface or bulk material sampling is often used to determine if mold is present on surfaces or materials such as carpeting and textiles. However, this may not always be necessary. Often, HEPA vacuuming or laundering may be sufficient to clean surfaces where mold spores had accumulated. When mold growth occurs on the surface, professional cleaning or replacement may be required. Methods of sampling surfaces include tape samples (or tape-lift samples), swab samples, and vacuum samples. As with air samples, guidelines have been offered to help define "normal" levels of mold particles in surface dust. However, these numbers can be misleading and care must be exercised in the interpretation of sample data. The weight of the sample can also affect the result obtained. In addition to mold per unit weight and mold per unit area, total dust per unit area should also be recorded. Perhaps of greatest importance is evaluation of the microbial flora reported in the sample data. This can provide insight into the moisture conditions of the building. A mycologist may be consulted for assistance with data interpretation. More information on sampling can be found by consulting the references listed or by consulting with an EMLAP accredited laboratory (1,2,5,6,7,14).

How can I respond to a mold problem?

The first step is to identify and repair the moisture problem. Mold will not grow unless sufficient moisture is present. Small amounts of mold growing on visible surfaces can usually be easily cleaned by the homeowner. Care must be taken to control dust related to the cleaning and repair efforts. Larger amounts of mold may require more extensive evaluation, repair or replacement, and dust control. Professional assistance may also be necessary. For more information for homeowners, see the DHS basic mold information page.

What are appropriate control methods for mold remediation?

Several options for cleaning up a mold condition are available, depending on the size and type of surfaces affected. Most important is the need to control dust associated with the cleanup activity. Dust should be controlled using damp cleaning methods and by using HEPA vacuuming. HEPA refers to High Efficiency Particulate Air, meaning that the vacuum filter is capable of removing particles that are 0.3 um (micron: one millionth of a meter) in diameter at 99.97% efficiency. Typical vacuum filters will not capture spores as efficiently and may further disperse them in air. When the size of the area with visible mold growth is large or when sensitive people (defined above) are nearby, containing the work area in a plastic enclosure is appropriate. The air inside the enclosure should be actively exhausted to the outdoors by placing the enclosed environment under negative pressure with respect to the rest of the room or building. This means if there are any leaks in the enclosure, that air will move from the cleaner areas outside the enclosure into the enclosure, and minimize air movement in the opposite direction. More specific guidance is offered by the references below and should be followed (4,5).

How should I handle carpet, drywall and other porous materials that got wet?

When porous items such as drywall and carpet get wet, they should be dried within 48 hours or discarded. Porous items or surfaces are those that can soak up water easily. They include drywall, clothing, textiles, upholstered furniture, leather, paper goods, and many types of artwork or decorative items. Many soft materials can be a food source for mold. When these materials soak up water, moisture can be retained for extended periods, increasing the potential for mold growth. Foam carpet pads can retain moisture long enough to support mold growth. In addition to flooding, extended periods of high interior humidity can lead to mold growth. This is sometimes indicated by a musty odor. If sewage or gray water is involved, the materials should be discarded. For assistance on structural restoration from water damage, consult a professional familiar with the guidelines of the Institute for Inspection, Cleaning and Restoration Certification (IICRC) (4,5,9).

How can settled spores be removed from porous items?

In environments where the porous materials have not been wet, but there has been extensive visible mold growth on nearby building surfaces, removing settled mold spores is possible using various cleaning methods. HEPA vacuuming is a good method to consider. If the materials show visible mold growth or are subjected to high humidity for extended periods, cleaning is much less effective. Unless the item has high value, disposal is often the most cost-effective solution (4,5,9).

How can I clean mold on hard surfaces?

After the source of moisture has been controlled, visible mold growth on hard (nonporous) surfaces can be scrubbed using detergent and water. Some stains may still be seen on the surface after cleaning. Various strengths of bleach solutions have also been recommended for disinfecting, but proper safety precautions should be taken to prevent skin, eye and respiratory damage when bleach is used. There is no evidence that bleach is more effective than detergents in removing mold from nonporous surfaces. The exception to this is when the water damage is related to sewer backups, where there is a concern for infectious disease transmission. Following cleaning, the surfaces should be rinsed and thoroughly dried to reduce the potential for more mold. The underlying cause of the mold growth (water or moisture) should be corrected to prevent regrowth (4,5,9).

Do ozone generators work for mold?

Ozone generators are not recommended for mold control. Ozone is a reactive form of oxygen that is toxic to living cells. Ozone levels sufficient to kill microorganisms are predictably irritating to the eyes, nose, throat and lungs and can aggravate asthma. In some cases where ozone has been used for mold control, reported levels were high enough to present a serious health hazard in occupied spaces. While ozone can kill microorganisms, it does not remove allergens from air, even if they are dead. Ozone generators or other types of air cleaning devices do not address the underlying reasons why mold is growing on building materials or other articles in your home. If there is visible mold growth, it should be properly cleaned up or the affected materials should be removed. Many ozone-generating devices are aggressively marketed for improving indoor air quality. If you have a complaint to register about the use or marketing of such a device, call the Wisconsin Department of Agriculture, Trade and Consumer Protection at 800-422-7128. For more information about ozone, air cleaners and indoor air quality, see the EPA website.

Who is qualified to do mold work?

Individuals involved in mold assessment and remediation must commit to following standards of care. A number of federal agencies and professional organizations have produced helpful references that help define standards of care for mold practice. (See the references section of the frequently asked questions.)

There are no national or state recognized mold certifications. A number of credentialing programs provide evidence of health and safety competency, including the American Board of Industrial Hygiene and the American Society of Safety Engineers. Credentials are not necessarily an indicator of mold work proficiency. They are an important measure of general competency and professionalism, but must be combined with professional education and experience relevant to the essential body of information that defines the standards of care. Mold assessment and remediation are a multi-disciplinary process. Simply being able to test for mold is of little benefit if other tasks such building inspection, thorough investigation, and data interpretation are not properly performed.

As with any business, it is important to ask for a statement of qualifications and a client reference list. In particular, consumers should evaluate the professional’s familiarity with and understanding of the references listed below. It is also appropriate to ask to see evidence of professional development, participation in professional organizations, and customer references.

The following references are important to consult when considering the proper method to assess and respond to mold conditions.

  1. Report of Microbial Growth Task Force, American Industrial Hygiene Association, May 2001. OSHA Technical Manual, Section III, Chapter 2
  2. Adverse Human Health Effects Associated with Molds in the Indoor Environment, American College of Occupational and Environmental Medicine, 2002 Mold Remediation in Schools and Commercial Buildings, United States Environmental Protection Agency, Office of Air and Radiation, Indoor Environments Division (6609-J) EPA 402-K-01-001, March 2001
  3. Guidelines on Assessment and Remediation of Fungi in Indoor Environments, New York City Department of Health & Mental Hygiene, Bureau of Environmental and Occupational Disease Epidemiology , November 2008 (PDF)
  4. Bioaerosols, Assessment and Control. American Conference of Governmental Industrial Hygienists, 1999 Field Guide for the Determination of Biological Contaminants in Biological Samples. AIHA Biosafety Committee Guidelines for Environmental Infection Control in Healthcare Facilities, Recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC)
  5. IICRC Reference Guide for Professional Mold Remediation (IICRC S250) EPA- Ozone Generators that are Sold as Air Cleaners: An Assessment of Effectiveness and Health Consequences (
  6. Addressing Indoor Environmental Concerns During Remodeling, Ventilation for Homes, US Environmental Protection Agency.
  7. Statement for the Record Before the Subcommittees on Oversight and Investigations and Housing and Community Opportunity Committee on Financial Services United States House of Representatives State of the Science on Molds and Human Health Statement of Stephen C. Redd, M.D. Chief, Air Pollution and Respiratory Health Branch National Center for Environmental Health Centers for Disease Control and Prevention, U.S. Department of Health and Human Services For Release on Delivery Expected at 2:00 PM on Thursday, July 18, 2002 (PDF)
  8. American Industrial Hygiene Association, Environmental Microbiology Proficiency Analytical Testing (EMPAT) Program
  9. Damp Indoor Spaces and Health (2004), Board on Health Promotion and Disease Prevention, Institute of Medicine
  10. Assessment, Remediation, and Post-Remediation Verification of Mold in Buildings, AIHA Guideline 3- 2004, American Industrial Hygiene Association

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Last revised October 18, 2022