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Controlling Indoor Mold

The key to mold control is moisture control

Tiny mold spores are all around us, both indoors and outdoors. Mold spores travel easily through the air and begin to grow indoors when moisture is present. There is no practical way to eliminate all mold and mold spores from the indoors, so the best way to control mold growth is to control indoor moisture. When indoor conditions are just right, mold spores can grow and become a problem. By taking important steps, you can prevent and control mold growth inside your home.

Mold spores need three things to grow:

  1. Moisture
  2. A nutrient source (i.e., wood, paper, or other materials)
  3. The right temperature

Of these three conditions, the most important to control is moisture. Indoor mold growth is really a sign that moisture is present. If indoor moisture is controlled, mold will not grow.

A blue home flying the American flag in the front porch with wooden fence

Controlling moisture is key! Find the moisture source, repair it, and clean what you can.

Fixing the moisture source

Since moisture is essential for mold growth, do all you can to quickly identify and fix any source causing too much indoor moisture. Common household problems that lead to indoor moisture issues include:

  • Roof leaks.
  • Leaking pipes or plumbing fixtures.
  • Condensation due to high indoor humidity.
  • Indoor flooding.

After all moisture and water problems have been fixed, clean the moldy area and keep the area dry.

If you cannot identify the moisture source, or if you are dealing with a large mold and water problem, consider a professional home inspection. Visit our Wisconsin Mold Contractor's page for a listing of indoor air consultants and mold remediation contractors.

Testing for mold

The Wisconsin Department of Health Services does not recommend testing for mold because:

  • Federal standards or limits for airborne mold concentrations or mold spores do not exist.
  • Mold spores are everywhere around us, indoors and outdoors.
  • Mold testing can be expensive.

If you see or smell mold, it is present. In any situation, your approach should be to find the moisture source, fix it, and clean what you can.

Preventing mold growth

Important actions can be taken to prevent indoor mold from becoming a problem:

  • Keep indoor spaces well ventilated and dry. Air conditioners and dehumidifiers can help.
  • Keep indoor humidity levels below 50%.
  • Clean bathrooms often and keep surfaces dry. Run the bathroom ventilation fan during and after showers.
  • Promptly fix water leaks.
  • Clean up and dry your home fully and quickly (within 24-48 hours) after any flooding event.

Frequently asked questions

Molds are everywhere and have existed for millions of years. Molds are fungal organisms that are essential for the breakdown of dead plant and animal matter. Molds come in a variety of colors including white, brown, green, and black. They reproduce by developing microscopic particles called spores which are not visible to the naked eye, and are easily carried through the air. When spores land in a moist environment with an adequate food source, mold begins to grow.

Ease of growth: Most molds grow naturally outdoors. Molds can be easily brought into buildings through open windows and doors, ventilation, and air conditioning systems, as well as on pets, clothing, or shoes. Once indoors, molds grow very easily, only needing moisture and a food source. Damp paper products, lint, ceiling tiles, drywall, wallpaper, insulation, carpeting, fabric, upholstery, and wood can all serve as good nutrient sources for mold growth.

Health effects: People with asthma may be more sensitive to mold in the air. Other susceptible populations may include children and infants, the elderly, or people with existing respiratory conditions or a weakened immune system (for example, those undergoing chemotherapy or who have HIV/AIDS). Symptoms of mold sensitivity are non-specific and may include nasal stuffiness, eye irritation, shortness of breath, wheezing, and in rare cases, mold infections in the lung.

To date, no scientific evidence exists to positively link residential exposure to mold with specific toxic effects. The majority of human exposures to mold toxins have occurred from ingesting moldy food. Some occupations, such as farming and working in greenhouses, where high levels of mold can be encountered, present a risk for workers.

Given a source of moisture, mold can grow just about anywhere. Moisture control and air filtration are necessary to control mold growth.

There is no way to eliminate all molds from your home, but here are some steps you can take to reduce the opportunities for mold growth:

  • Usually when mold growth is found, it is related to a moisture or water problem. The source of moisture must be addressed as well as clean up of the mold growth.
  • After any flooding incident, make sure to completely dry out any affected areas of the house.
  • Do not rely on evaporation alone to adequately dry an area that's been flooded. Providing good air circulation and using dehumidifiers are important for fast, adequate drying.
  • Carpets that cannot be dried in 48 hours should be removed and discarded. In general, carpets that have been contaminated with flood or sewer water should be replaced.
  • Repair cracks in basement walls. Moisture can seep in, creating a moist environment conducive to mold growth.
  • A dehumidifier or air conditioner should be used to reduce indoor moisture levels during humid times of the year.
  • Always clean up spills and fix leaks in plumbing promptly and thoroughly. Mold growth can begin within 24 to 48 hours after a water leak.
  • Regularly clean and empty the drip pans in your air conditioner, refrigerator, and dehumidifier.
  • Typically, it is not necessary to use humidifiers in homes. Normal activities such as cooking, showering, and breathing add adequate moisture to the air.
  • If you have a rental property, consider including a provision in the tenant's lease requiring the prompt reporting of any leaks, floods, or mold growth. If you are renting, promptly report any leaks, flooding, or mold growth.
  • Be sure the home has adequate ventilation, including exhaust fans in the kitchen and bathrooms. Let exhaust fans run during, and 20 minutes after, bathing or showering. Consider installing exhaust fans with humidistat controls that shut off automatically when the moisture is adequately ventilated from the area.
  • Use mold inhibitors, which can be added to paints. Be sure and read the application instructions carefully. If not used according to instructions, mold inhibitors can cause health problems for some individuals.
  • Do not carpet bathrooms.

There are other indoor air contaminants that can occur in a home, causing health problems similar to mold exposure. Exposure to tobacco smoke, pet allergens, carbon monoxide from faulty furnaces and hot water heaters, and poor sanitation can all cause health problems for residents. If you are experiencing adverse health symptoms, it is important that you see your physician for a proper evaluation and diagnosis. Your physician may refer you to a specialist, such as an allergist, for additional tests. If you seem to feel better when you are away from your home for several hours, this may be an indication that there is a contaminant in your home.

If you and your physician suspect an indoor air quality issue is causing your symptoms, and you are unable to identify and clean up the source of the contamination, call your local health department for more information or for help on where to turn next. They can either help you directly or refer you to an indoor air quality specialist.

If you can see mold growth or smell a musty odor, you have a mold problem. However, it is recommended that you do a thorough inspection to determine the source of the mold growth.

Testing: If you can see or smell a mold problem, testing or sampling is typically not necessary. It is more important to identify and remediate the moisture source and clean up the mold, than to spend the time and money on testing. In addition, there are no agreed-upon, health-based standards for human exposure to mold or mold spores. Testing is only indicated in rare instances and should be discussed with an indoor air quality specialist.

Cleanup: Small areas of mold growth on nonporous surfaces can usually be cleaned by the removal of the gross mold buildup, followed by application of a simple bleach solution. The affected area(s) should be allowed to dry thoroughly. Extensive clean up of large areas may require the use of a mold or water cleanup service and the removal of mold-contaminated surfaces (including drywall, carpeting, or wood flooring). For more detailed information, visit our Cleaning Mold In Your Home page.

Seeking professional help: If you suspect you have an indoor air quality problem, contact your local health department. Health department staff may be able to do an in-home inspection, or can refer you to an indoor air quality specialist. The Wisconsin Department of Health Services has established guidelines for inspectors and contractors involved in mold detection and remediation efforts. For more information, visit Tips on selecting an IAQ consultant or mold contractor.

Complaints: If you have a complaint about the quality of work or other contractor issues, contact the Department of Agriculture, Trade and Consumer Protection at 1-800-422-7128.


For more information, contact your local health department, or call the Wisconsin Department of Health Services at 608-266-1120.

Last revised September 28, 2022