What is mold? Why it grows indoors and how to stop it

What is mold and how does it get into my home?

Mold is a type of fungus that occurs naturally indoors and out. It reproduces by releasing tiny spores that drift through the air and settle on surfaces.

Mold spores are everywhere, and their presence has nothing to do with poor hygiene. They come in through windows and doors and on clothes and pets, and it's virtually impossible to keep them out of your home.

What you can do is stop them from growing. Mold needs three things:

1. Spores: always present.
2. Something organic to feed on: wood, plaster, paper, dust, fabric, and most other materials in a home.
3. Moisture: the one thing you can control. You can't keep spores out, but managing humidity and fixing leaks stops them from growing into mold.

Where does mold grow in a home?

Mold grows wherever it finds moisture. Common places include:

• Bathrooms: around showers, on the ceiling above them, and along sealant and grout around tubs.
• Kitchens: under and behind sinks, around dishwashers, behind appliances, and in window corners.
• Bedrooms: on exterior walls behind beds and wardrobes, and around poorly sealed windows where condensation forms overnight.
• Basements and crawl spaces: where ground moisture and poor ventilation combine.
• Behind or beneath furniture and flooring: cupboards or sofas against cold walls, under carpets, behind headboards. Air doesn't circulate, so moisture gets trapped.
• Around any leak: a dripping pipe or failed roof flashing will grow mold faster than any humidity problem.

Why do cold outside walls grow mold even when the room feels dry?

Warm air holds more moisture than cold air. When warm indoor air meets a cold surface, such as an uninsulated exterior wall or a cold window pane, it cools and its relative humidity rises sharply, sometimes to the point of condensation. Furniture against that wall blocks air movement, keeping the surface colder and damper than the rest of the room. That's why mold so often appears behind wardrobes and beds.

Is mold a common problem?

Yes. A widely cited 2007 analysis published in Indoor Air estimated that about 21% of current US asthma cases, roughly 4.6 million people, could be attributed to dampness and mold in the home, at a national cost of about $3.5 billion a year. More recently, a 2024 meta-analysis of 32 studies found that living in a moldy home during childhood was associated with a higher risk of later developing asthma: 53% higher in case-control studies and 15% higher in cohort studies.

What humidity level prevents mold?

The EPA recommends keeping indoor relative humidity below 60%, and ideally between 30% and 50%. The EPA also advises drying water-damaged areas and items within 24–48 hours, because in most cases mold won't grow if materials dry within that window.

Mold risk depends on how humid the air is, how cold the surfaces are, and how long damp conditions last. Short spikes from a hot shower or cooking won't cause mold. Humidity that stays high for days or weeks in a poorly ventilated room is what lets it take hold. Learn more in our guide to ideal indoor humidity levels.

How does mold affect your health?

According to the EPA, damp and moldy homes are linked to:

• Coughing, wheezing, and shortness of breath
• Worsening asthma and more frequent attacks
• Allergic symptoms such as sneezing, a runny or blocked nose, and itchy eyes
• Throat and skin irritation

People with asthma, allergies, or weakened immune systems, as well as infants and older adults, are more affected. If your symptoms ease when you leave home and return when you come back, it's worth checking for damp or mold. Mold and dust mites can also make pollen allergies feel worse.

Is black mold more dangerous?

"Black mold" usually refers to Stachybotrys chartarum. Despite its reputation, the CDC says molds that produce mycotoxins, like Stachybotrys chartarum, should be treated the same as other molds that grow indoors. Any indoor mold should be removed and the moisture problem fixed.

How do I get rid of mold?

Cleaning mold off a surface is simple. If you don't fix the dampness behind it, though, it will come back.

1. Find and fix the moisture source first. Look for leaky pipes, failed sealant around a shower or sink, a broken bathroom or kitchen exhaust fan, or blocked ventilation. Dry any wet materials within 24–48 hours.
2. Clean small areas yourself. The EPA says you can usually handle an area smaller than about 10 square feet (roughly 3 ft by 3 ft). Wear gloves, eye protection, and a mask, and ventilate the room.
   • Scrub hard surfaces with detergent and water, then dry them completely.
   • Shake fabrics outdoors, then wash them on the hottest setting the fabric allows and dry them thoroughly.
   • Never mix bleach with cleaners that contain ammonia; the fumes are toxic.
3. Don't paint over mold. The EPA advises against painting or caulking moldy surfaces: clean and dry them first. Paint applied over mold is likely to peel, and the mold can grow back.
4. Call a professional for large or complex jobs: areas over 10 square feet, damage from contaminated water, or mold inside wall cavities, insulation, or under flooring.
5. Throw away badly affected porous materials such as carpet, ceiling tiles, and drywall. Mold grows into these and can't be fully cleaned out.

For full guidance, see the EPA's Mold cleanup in your home.

How to stop mold from coming back

Preventing mold comes down to controlling moisture and keeping air moving:

• Ventilate wherever water is produced. Run the bathroom fan during and after every shower, use the range hood when cooking, and put lids on pans.
• Dry laundry outside or in a well-ventilated room. Drying clothes indoors releases a lot of water into the air; if you must, run a dehumidifier.
• Leave a gap behind furniture. Pull beds, sofas, and wardrobes a few inches away from exterior walls, and avoid overfilling closets on exterior walls.
• Use a dehumidifier in problem rooms, especially basements and bathrooms.
• Wipe away morning condensation from windows and sills, and open a window.
• Keep heating steady in winter. Letting rooms get cold and then heating them hard causes more condensation than a constant, moderate temperature.

How does an air quality monitor help prevent mold?

Mold develops after damp conditions persist for days or weeks, so tracking humidity and temperature continuously shows you a problem long before you see it on a wall. You'll quickly spot a bedroom that regularly sits above 60% or a basement that never drops below 70%.

• The Airthings View Plus tracks humidity and temperature alongside radon, particulate matter, CO₂, and VOCs, and can alert you when humidity stays too high.
• The Airthings Wave Enhance is a lower-cost way to track humidity, temperature, CO₂, and VOCs room by room.
• If you're concerned about damp and radon in your basement, the Airthings Corentium Home 2 monitors radon alongside temperature and humidity.

To understand how humidity affects your home and health, read our guide What is indoor humidity?, or learn how to test indoor air quality at home.

Sources

• US EPA: A brief guide to mold, moisture and your home
• US EPA: Mold and health
• US EPA: Mold cleanup in your home
• US CDC / NIOSH: Mold, testing, and remediation
• Mudarri & Fisk (2007), Public health and economic impact of dampness and mold (Indoor Air)
• Childhood asthma and mould in homes: a meta-analysis (Wiener klinische Wochenschrift, 2024)