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This information was reviewed and approved by Carah Santos, MD (12/5/2022).

Molds are a kind of fungus, the most common found on earth. There are 1.5 million species of mold, making up 25% of the earth’s biomass. However, only 80 species are known to trigger allergic reactions.

Many types of molds (microscopic fungi) live in our environment. Mold grows in indoor and outdoor areas that are warm, dark and/or moist. Molds reproduce and grow by sending tiny spores into the air. Inhaled spores cause allergy and asthma symptoms. Examples of allergy symptoms include itchy eyes, runny nose and rash.

Mold grows easily in most humid, warm conditions, but it also can occur in conditions that are cool and, rarely, even dry. Mildew is a common mold found indoors. Mold spreads by way of spores, which are like tiny airborne seeds. Most mold in the home originates outdoors. Mold spores can waft in or be carried in on clothes. Mold finds hospitable ground and colonizes in wet basements, humid crawl spaces, and anywhere leaks, high humidity or condensation are present.

The microscopic spores that mold uses to spread and reproduce are the primary culprits in mold allergies. When inhaled, they can cause respiratory allergic symptoms and, potentially, other health problems. Although mold growth isn’t a reason to panic, large areas of mold growth may require a special cleanup in order for a residence to be habitable.

Factors That Influence Mold Growth

The amount of mold growing inside your home depends on these factors:

  • How much moisture is or was in the area
  • How long the area stayed wet
  • How much air circulates in the area to dry the moisture
  • How many times the area has been wet
  • How much sunlight comes into the area

It may not be possible to eliminate all mold spores from your home environment, but reducing moisture can make the environment unsuitable for mold to thrive.

Reduce Mold Exposure 

Once an allergy has been identified, your health care provider may recommend medications or therapies to control symptoms. The next step is to decrease or eliminate exposure to the allergen. This is called environmental control. Evidence shows that allergy and asthma symptoms may improve over time if the recommended environmental control changes are made. Here are some environmental control measures to consider:

  • In the bathroom
    • Use an exhaust fan, or open a window to remove moisture after showering.
    • Wipe down the damp surfaces after showering.
    • Wash bathrooms with a mold-preventing or mold-killing solution at least once a month.
  • In the kitchen
    • Use an exhaust fan to remove water vapor when cooking.
    • Discard spoiled foods immediately.
    • Empty the garbage daily.
    • Empty water pans below self-defrosting refrigerators frequently.
  • Around the home
    • Remove moldy stored items.
    • Vent the clothes dryer outside.
    • Remove leaves, clippings and compost from around your house.
    • The person with a mold allergy should avoid cutting grass and raking leaves or wear a face mask for these activities.
    • Keep the indoor moisture low. The ideal humidity level is 30-40 percent. In warm climates, use a dehumidifier or central air conditioning to control the temperature inside your house. Central air conditioning reduces indoor humidity. Clean the dehumidifier regularly.
    • Avoid using humidifiers. Humidifiers and vaporizers will increase humidity in the room and create a favorable environment for mold growth. Additionally, they can inadvertently spread mold spores throughout your house. If you must use a humidifier, prevent mold growth with proper humidifier care.


Health Risks of Mold

Is mold dangerous? There have been many news reports concerned with mold toxicity, linking mold to memory loss, lethargy and even hemorrhage in infants. “Toxic mold syndrome” remains controversial and unproven, but experts agree that it’s best to limit exposure to molds.

Though rare, mold-induced infections do occur and are especially dangerous to people with weakened immune systems.

The most likely adverse reaction to mold is a respiratory allergic reaction in mold-sensitive people. These reactions are similar to other respiratory allergies, causing sneezing, watery eyes, nasal discharge and congestion.

People in certain professions may suffer disproportionately from mold allergies because of increased mold exposure. These include farmers, dairy workers, loggers, bakers, greenhouse employees and others.

Just like other allergies, mold allergies can be responsible for asthma and its potentially serious health consequences. In fact, 21 percent of current U.S. asthma cases are attributable to mold allergies. Households with a very high mold burden are much more likely to have a child with asthma.

Aside from the health risks, mold can directly damage anything from a small corner of a room to an entire house. Regularly testing for mold is not only a way to protect your family’s health but also a way to protect your real estate investment.

Proper Humidifier Care

A neglected humidifier can be a major source of mold and mold spores. Learn how to keep a humidifier clean and reduce or eliminate this common allergen.

CAUTION: Use of a humidifier or vaporizer can increase dust mite and mold growth. Frequent use of a humidifier or vaporizer is not recommended for people who are allergic to these allergens. Routine cleaning of the humidifier or vaporizer is very important, as anyone can develop a lung condition known as humidifier lung (hypersensitivity pneumonitis).


  • Empty reservoir.
  • Wash reservoir completely with hot soapy water.
  • Rinse well under running tap water. Be careful that no water gets into the motor.
  • Fill reservoir to appropriate level. Distilled water is preferable, but tap water may be used.



  • Clean the equipment in an area away from the person with a lung problem.
  • Empty the reservoir.
  • Clean the reservoir with hot soapy water. Rinse well.
  • Add two cups of white vinegar and enough water to enable the humidifier or vaporizer to create a mist. Allow the machine to run 30 minutes in a well-ventilated area. Alternatively, take outside and run through a solution of half water and half bleach. Run through plain water thereafter, and return inside.
  • Rinse the equipment well.
  • Operate the humidifier or vaporizer for 3 minutes with clean tap water.
  • Pour out the tap water and fill to the appropriate level. Distilled water is preferable, but tap water may be used.

When the reservoir water level is low, always discard water remaining in the humidifier before refilling with distilled water.

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