What Is Mold?

Mold exists everywhere and is a natural part of the environment. There are many types of molds and all need water and food to grow. Outdoors, mold plays an important role in breaking down dead organic material like fallen leaves and dead trees, but indoors, it can cause property damage and health problems. Indoor mold can be problematic if it is left to grow in large areas, like in carpets or walls. Excessive mold indoors indicates there is too much moisture in the building.

Where Does the Moisture Come From?

Indoor environments can be prone to moisture problems, which, if left unchecked, can lead to mold growth. Moisture indoors can show up as a liquid – like from a leaky pipe or roof - or as water vapor in the air. Everyday activities like cooking, bathing, and breathing can add moisture to the air.

The warmer the air temperature is, the more water vapor it can hold. When the air cools, it can’t hold as much water vapor, so the excess moisture condenses on cold surfaces. This is why you sometimes see window condensation on cold days. Too much moisture can lead to mold growth.

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

  • 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.
  • Improper ventilation of clothes dryer.
  • Use of a humidifier.
  • Outdoor humidity.
  • Condensation or moisture build-up in humidifiers, dehumidifiers, air conditioners and drip pans under refrigerator cooling coils.

Key signs of a moisture problem:

  • Musty odor
  • Moisture on cold surfaces
  • Water stains that signify water leaks or condensation

How Mold Impacts Health

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.

Exposure to mold has been linked to:

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

How to Protect Against Mold

The best way to control mold indoors is to control the sources of moisture. That means preventing excess moisture as well as fixing new problems quickly. Problems arise when materials remain wet, especially long enough for mold to grow.

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 to reduce moisture and prevent mold:

  • Fix all leaks quickly. You may need assistance from a plumber or contractor.
  • Clean and repair roof gutters regularly.
  • Keep indoor humidity levels below 50 percent.
  • Increase air movement and ventilation.
  • Run exhaust fans in the bathroom when bathing.
  • Cover pots and use exhaust fans when cooking
  • Keep appliance drip pans clean.
  • Be sure downspouts are directed away from the building.
  • Regularly and thoroughly clean places where molds are likely to grow.

How to Clean Up Mold

Surface mold, like the small patches that appear in the shower, can easily be cleaned with a stiff brush and a solution of warm water and mild soap or detergent.

For larger areas impacted by mold, follow these steps:

  • 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 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 or about 3 ft. by 3 ft. If you are unable to determine the extent of the problem, contact a professional.
  • Remove, clean or discard affected materials.
    • For affected areas less than 10 square feet, use a mild detergent and warm water to get rid of the mold. Do not use bleach as it can irritate the lungs. Dry the surface completely.
    • Materials that cannot be cleaned or are too damaged to reuse should be discarded. Porous materials, like ceiling tiles, likely cannot be cleaned thoroughly and must be discarded.
    • At minimum, wear an N-95 mask (available at hardware stores), disposable gloves and goggles during clean-up.

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

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

Page last updated: November 2, 2023

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