Where does mold come from?

Mold exists everywhere. However, without sufficient moisture, mold cannot grow.

How can dampness and mold impact health?

Dampness may show up in visible moisture, like leaks or as high humidity. Dampness creates the environment for mold to grow. Dampness can cause harm even when mold is not present. Excessive moisture also promotes the growth of dust mitescockroachesbacteria and viruses, which can impact health.2

Exposure to mold can trigger allergic reactions and asthma symptoms in people who are allergic to mold. However, even without mold, dampness indoors causes asthma attacks and other upper and lower respiratory problems. Anyone—with or without allergies—may experience irritation of the eyes, skin, nose, throat and lungs when exposed to airborne mold particles.

Dampness and mold have been linked to:

  • Worsening of asthma
  • Coughing
  • Wheezing
  • Nasal congestion
  • Sore throat
  • Sneezing
  • Rhinitis.2

Where does the moisture come from?

Water from many sources within a home or building can cause moisture problems or mold. Some of the sources of moisture include breathing, washing clothes, showering and cooking. Some sources of moisture are impossible to stop, so all indoor spaces need to be well-ventilated. Problems arise when materials remain wet, especially long enough for mold to grow. If you discover a water problem, fix it quickly.

Moisture may build up indoors in many different ways. These include:

  • Flooding or leaks that allow rainwater indoors.
  • Poorly connected plumbing and leaky pipes.
  • Continually damp carpet (may occur if carpet is installed on poorly ventilated floor).
  • Inadequate exhaust of bathrooms and kitchens.
  • Outdoor humidity.
  • Condensation or moisture build-up in humidifiers, dehumidifiers, air conditioners and drip pans under refrigerator cooling coils.

How can dampness and mold be prevented?

The best way to control dampness and mold indoors is to control the sources of moisture. That means preventing the problem from developing as well as fixing new problems quickly. Common problem areas include air-conditioning units; basements, attics and crawl spaces; bathrooms; humidifiers and dehumidifiers; and refrigerator drip pans. Here are some key steps:

  • Keep indoor humidity levels below 50 percent.
  • Fix all leaks.
  • Increase air movement and ventilation.
  • Run exhaust fans in the bathroom and kitchen to reduce moisture.
  • Keep appliance drip pans clean.
  • Regularly and thoroughly clean places where molds are likely to grow.

Watch out for these signs to identify a dampness problem:

  • Musty odor;
  • Moisture on hard surfaces; and
  • Water stains that signify water leaks or condensation.

For more information on preventing dampness and mold, visit the U.S. Environmental Protection Agency (EPA) website.

How can dampness and mold problems be fixed?

  • Identify and eliminate the water problem. The most critical step is to identify and eliminate the source of the water that is creating a damp environment. You may need technical assistance from a plumber or contractor to determine what needs to be fixed or changed.
  • Determine the extent of the problem. EPA recommends calling for professional help to clean any mold growth that covers more than 10 square feet. If you are unable to determine the extent of the problem, contact a professional, who can help you protect your family.
  • Remove, clean or discard affected materials. To get rid of mold, wash it off materials that can be effectively cleaned, such as hard surfaces. Use detergent and warm water. Dry the surface completely. If the materials cannot be cleaned or are too damaged to reuse, discard them. Porous materials, like ceiling tiles, likely cannot be cleaned thoroughly and must be discarded.
  • Protect occupants and workers during the cleanup process. At minimum, wear an N-95 mask (available at hardware stores), disposable gloves and goggles.

If water build up was caused by sewage, floods or other contaminated water, call a professional.

For more information on guidelines for safely cleaning up mold, visit the EPA's mold clean up site.

Page last updated: May 17, 2023

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