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Molds are simple, microscopic organisms, present virtually everywhere, indoors and outdoors. Molds, along with mushrooms and yeasts, are fungi and are needed to break down dead plant and animal material and to recycle nutrients in the environment. Because molds grow by digesting organic material, they gradually destroy whatever they grow on. Molds can grow on surfaces or objects in buildings. Mold growth on surfaces can often be seen in the form of discoloration: frequently white, gray, brown, or black but also green and other colors. They may be visible or, if inside walls or building structures, not visible to you.
For molds to grow and reproduce, they need only a food source—any organic matter, such as leaves, wood, paper, or dust—and moisture, which does not have to be liquid water but can be just a damp material or surface. Because organic matter is always available, moisture or dampness in buildings is thus the limiting factor determining whether mold can grow. Molds can usually grow whenever enough moisture is available.
Common sources of moisture that may lead to indoor mold problems include:
- Leaky roof
- Sprinkler spray hitting a building
- Plumbing leaks
- Overflow from sinks or sewers
- Damp basement or crawl space
- Steam from showers or cooking
- Wet clothes hung to dry indoors
- A clothes dryer that exhausts air indoors
Prevention is important to avoid mold problems. Inspect your home, school, or workplace regularly for signs and sources of indoor moisture and mold.
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Molds release tiny spores and even smaller particles that travel through the air. Everyone inhales some mold every day without apparent harm; however, molds can cause allergy, irritation or inflammation, or rarely, infection. Allergic reactions are the most well-recognized responses to inhaling mold spores, and some people are more sensitive to the effects of dampness mold. However, it is becoming increasingly clear that individuals not sensitized to mold may experience health effects.
We know that dampness and mold cause health effects both in allergic and non-allergic people. If you can see mold, water damage, or moisture, or smell mold, there is at least some increased health risk. The more extensive or severe the dampness and mold, the greater the risk of health effects. We do not know whether specific types of mold are responsible, or whether bacteria or chemical emissions related to dampness also cause some of the health problems. The health effects consistently associated with indoor dampness and molds include:
- Causation of new asthma
- Asthma attacks in those who already have asthma
- Allergic rhinitis (sneezing, congested nose, or runny nose)
- Upper respiratory symptoms, such as stuffy or congested nose or sinuses, sore throat, or irritated nose or throat
- Lower respiratory symptoms, such as wheezing, difficulty breathing, shortness of breath, or cough
- Respiratory infections such as acute bronchitis
- Eye irritation (burning, watery, or reddened eyes)
- Eczema and skin rashes or irritation
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CDPH in its "Statement on Building Dampness, Mold, and Health" (updated 2016) to states:
- CDPH has concluded that the presence of water damage, dampness, visible mold, or mold odor in schools, workplaces, residences, and other indoor environments is unhealthy.
- We recommend against measuring indoor microorganisms or using the presence of specific microorganisms to determine the level of health hazard or the need for urgent remediation. Rather, we strongly recommend addressing water damage, dampness, visible mold, and mold odor by
a) identification and correction of the source of water that may allow microbial growth or contribute to other problems,
b) the rapid drying or removal of damp materials, and
c) the cleaning or removal of mold and moldy materials, as rapidly and safely as possible, to protect the health and well-being of building occupants, especially children."
CDPH's position is based on the current consensus among scientists and medical experts (cited in the Statement) that
- Visible water damage, damp materials, visible mold, and mold odor indicate an increased risk of respiratory disease.
- The traditional methods used to measure mold exposure do not reliably predict health risks.
- The differentiation of some molds as "toxic molds" that are especially hazardous to healthy individuals is not justified by available evidence.
- The most important steps in dealing with indoor dampness or mold are to identify the source of moisture and take the necessary steps to make repairs to stop them, dry or remove damp materials, and clean or remove moldy materials.
Another CDPH document provides more detailed information and links on cleanup for dampness and mold:
CDPH Mold or Moisture in My Home: What Do I Do? (October, 2016)
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Dampness in your home living spaces has long been listed as a condition making a home substandard to a code inspector. As of January 1, 2016, mold is also a condition that makes a home substandard. The owner of a rental property cited as by a local (city or county) code inspector substandard is required to repair the substandard condition.
The California Department of Consumer Affairs provides advice for tenants on mold issues in “California Tenants - A Guide to Residential Tenants' And Landlords' Rights And Responsibilities”:
"... the presence of mold conditions in the rental unit that affect the livability of the unit or the health and safety of tenants" may be a way in which the implied habitability of a unit is violated and that a tenant may be able to claim a breach of the implied warranty on the basis of documented contamination.
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The 2001 Toxic Mold Protection Act (SB 732, Ortiz) directed the California Department of Health Services (now Department of Public Health or CDPH) to determine the feasibility of establishing health-based permissible exposure limits (PELS) for indoor mold. If that were possible, the CDPH was also directed to create programs to develop guidelines for mold assessment, clean-up, and disclosure in residences. However, CDPH responded in 2005 ("Report to the California Legislature on Implementation of the Toxic Mold Protection Act of 2001") that available evidence did not support the establishment of science-based PELs for indoor molds at that time. This view was reaffirmed in the Department's July 2008 update. To date, as of 2015, the evidence on this question has not changed the CDPH position.
However, CDPH also said in 2005 that it “agrees with other building and health professionals that indoor dampness, water intrusion, or fungal growth should always be eliminated in a safe and efficient manner." This advice was expanded in CDPH Statement on Indoor Dampness and Mold (revised 2016), based on the increased availability scientific information.
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Additional reliable resources on mold are available from
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CDPH Info Sheets on Mold and Dampness