Mold: Frequently Asked Questions
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 Wisconsin Division of Public Health, Bureau of Environmental and Occupational Health, PO Box 2659, Madison, WI 53701-2659, (608)
266-1120, or email the webmaster at
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.
| Assessment | Remediation
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
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, chemotherapy, AIDS, neonates,
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 other factors can influence indoor air
quality such as other organisms, chemicals such as solvents and
pesticides, or 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. Initially thought related to cases of a lung
disease called idiopathic pulmonary hemosiderosis among infants, the U.S.
Centers for Disease Control 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).
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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
- Involving infection control staff in construction and maintenance
- Managing traffic patterns to control dust,
- Using wet methods and HEPA vacuuming for cleaning critical care
- Cleaning and maintenance of humidifcation and filtration systems,
- Maintaining proper pressure differentials are maintained in
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 healthcare
environments, please see the
Guidelines for Environmental Infection
Control in Healthcare Facilities, Recommendations of CDC and the
Healthcare Infection Control Practices Advisory Committee (HICPAC).
(exit DHS) (8)
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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
well as 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 the advice of a medical
doctor. Communication from building owners to all occupants is very
important to arriving at appropriate decisions (1).
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Does the type of mold determine the clean up plan?
No. The 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).
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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 large sample number is required to
obtain meaningful data. Some research suggests that there may be less
variability in data for components of microbiological cell walls, than for
entire organisms, but questions regarding 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. You may also review a white paper on the subject produced by
the American Industrial Hygiene Association at
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).
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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 (www.aaaai.org)
(exit DHS) 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).
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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. For more information on remediating commercial and school
properties, see the EPA or NYC guidance
on mold remediation (4,5).
What are appropriate control methods for mold remediation?
There are a number of options available to clean up a mold condition,
depending on the size and type of surfaces affected. Most important is the
need to control dust associated with the clean-up 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
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).
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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 (non-porous) 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. (See DHS fact sheet, "Mold
in Your Home: Cleaning Options"). There is no evidence that bleach is
more effective than detergents in removing mold from non-porous 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 re-growth (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. (exit
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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
There are no national or state recognized mold certifications. There
are a number of credentialing programs that provide evidence of health and
safety competency including the American Board of Industrial Hygiene and
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 is 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
As in any business, it is important to ask for a statement of
qualifications and a client reference list. In particular, consumers
should evaluate the professionalís familiarity and understanding of the
references listed below. Asking to see evidence of professional
development, participation in professional organizations and obtaining
customer references is also appropriate.
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Where can I find information on mold?
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.
2 OSHA Technical Manual, Section III, Chapter 2 http://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_2.html
3 Adverse Human Health Effects Associated with Molds in the
Indoor Environment, American College of Occupational and Environmental
Medicine, 2002 http://www.acoem.org/guidelines.aspx?id=850
4 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
http://www.epa.gov/mold/mold_remediation.html (exit DHS)
5 Guidelines on Assessment and Remediation of Fungi in Indoor
Environments, New York City Department of Health & Mental Hygiene,
Bureau of Environmental & Occupational Disease Epidemiology , Jan.
http://www.nyc.gov/html/doh/downloads/pdf/epi/epi-mold-guidelines.pdf (exit DHS; PDF 71KB)
6 Bioaerosols, Assessment and Control. American Conference of
Governmental Industrial Hygienists, 1999
7 Field Guide for the Determination of Biological Contaminants in
Biological Samples. AIHA Biosafety Committee
8 Guidelines for Environmental Infection Control in Healthcare
Facilities, Recommendations of CDC and the Healthcare Infection Control
Practices Advisory Committee (HICPAC) http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5210a1.htm
9 IICRC Reference Guide for Professional Mold Remediation (IICRC
10 EPA- Ozone Generators that are Sold as Air Cleaners: An
Assessment of Effectiveness and Health Consequences (http://www.epa.gov/iaq/pubs/ozonegen.html)
11 Addressing Indoor Environmental Concerns During Remodeling,
Ventilation for Homes, US Environmental Protection Agency. http://www.epa.gov/iaq/homes/hip-ventilation.html
12 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 http://www.cdc.gov/nceh/airpollution/images/moldsci.pdf
13 American Industrial Hygiene
Association, The Environmental Microbiology Proficiency Analytical Testing
14 Damp Indoor Spaces and Health (2004), Board on Health
Promotion and Disease Prevention, Institute of Medicine, http://www.nap.edu/books/0309091934/html/
15 Assessment, Remediation,
and Post-Remediation Verification of Mold in Buildings, AIHA Guideline 3-
2004, American Industrial Hygiene Association
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June 26, 2013