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A few drops of water can tell you whether it's mold or mineral deposits

White Mold or Something Else? How To Identify and Handle White Growth

White fuzz on a basement wall may be mineral salts, not mold. How to tell the difference, when white growth on wood is serious, and what to do.

9 min read

Mold can be white. The EPA notes that "molds come in many colors including white." But white growth isn't always mold. On concrete and masonry basement walls, a white, fluffy deposit is often mineral salts left behind as water evaporates. NDSU Extension says these deposits dissolve easily in water and are harmless to health, though they do signal moisture moving through the wall. White fuzz on drywall, wood, fabric, or stored items is more likely mold, and should be handled like any other mold. White strands or fan-shaped growth on wood in a crawl space can be a decay fungus that damages framing, which calls for a professional. White mold isn't safer or more dangerous than other colors. The CDC says all molds should be treated the same with respect to potential health risks and removal.

Mold or Mineral Deposits? A Simple Test

The white powder or crust that forms on basement walls and floors is often efflorescence. The Department of Energy's Building America Solution Center explains that it forms when water drawn through masonry evaporates, "leaving behind a residue of salts, minerals, and other materials."

NDSU Extension notes that these salts "often take on a white, fluffy or moldy appearance in cracks in the floor or wall." To tell the difference:

  1. Put a few drops of water on it. NDSU says soluble salt deposits "are dissolved easily by water," so this is one way to tell whether it's salt or mold.
  2. If water doesn't work, try a few drops of vinegar. Some mineral deposits, such as carbonates, don't dissolve in water but can be removed with a dilute acid like vinegar, according to NDSU.
  3. If neither dissolves it, and it's fuzzy or growing, treat it as mold.

NDSU says salt deposits are "harmless from a human health standpoint." But they're still a warning sign. University of Minnesota Extension lists efflorescence among the signs of a basement moisture problem. The same moisture can lead to mold on nearby materials. NDSU notes that moisture seeping through concrete walls and floors can cause mold on walls, carpeting, and items stored in the basement.

Mold needs something organic to feed on. NDSU says mold grows on materials such as paper, dirt, wood, and soap scum. So mold on a concrete wall is usually growing on dust, dirt, paint, or something stored against it.

White Growth on Wood: Mold or Decay?

This distinction matters most for your home's structure.

Surface mold. The USDA Forest Products Laboratory's *Wood Handbook* says mold and stain fungi affect the strength of wood only slightly, and that surface growth on softwoods "often can easily be brushed or surfaced off." The Forest Products Laboratory's guide to mold and mildew on wood says mold causes no structural damage to wood, other than discoloration. It adds that mold in basements and attics is typically caused by high humidity and poor ventilation.

Decay fungi. Decay is different. The Wood Handbook says fresh surface growth of decay fungi may appear as "fan-shaped patches..., strands, or root-like structures that are usually white or brown in color." Signs the wood itself is decaying include:

  • White rot: the wood may look whiter than normal and feel spongy.
  • Brown rot: the wood may crack across the grain, shrink, collapse, or crumble to powder.

The Wood Handbook says serious decay only occurs when wood moisture content is above about 30 percent. Humid air alone won't wet wood enough for significant decay, though it can allow mold. So decay usually means liquid water from a leak, soil contact, or poor drainage.

Water-conducting fungus. University of California Cooperative Extension describes Meruliporia incrassata, often called "poria," as the most common water-conducting fungus found in buildings in North America. It forms root-like strands that can carry water from the soil into a structure. Risk factors include a very humid crawl space, siding that extends below grade, and cracks in the foundation or slab. Repairs require cutting off the strands from their water source and replacing damaged wood. The Wood Handbook notes that the common term "dry rot" is misleading, because wood must be damp to decay.

If you find white fans or strands on framing, or wood that feels soft or crumbles, call a qualified contractor experienced with wood decay. Fix the moisture source before replacing wood.

White Growth in the Attic

White or dark growth on the underside of roof sheathing usually points to moisture from inside the house. The Building America Solution Center says bathroom and kitchen exhaust fans that vent into the attic or crawl space can cause mold growth and wood damage as moist air condenses on cold rafters, trusses, and sheathing. Its guidance on attic condensation warns that repeated condensation can lead to mold and even structural rot, and recommends indoor humidity between 30 and 50 percent during the heating season.

Make sure fans vent all the way outdoors, and have a contractor look at attic ventilation and air leaks if growth is widespread.

White Fuzz on Houseplant Soil

White fuzz on potting soil is usually harmless to the plant. UF/IFAS Extension explains that many of these fungi feed on organic matter in the potting mix and are favored by high moisture, high humidity, and poor drainage. Let the soil dry more between waterings and make sure the pot drains well.

Don't confuse it with salt buildup. Clemson Extension says white to gray crusty deposits on the soil surface or pot rim are a sign of salt buildup, not fungal growth. If small yellow mushrooms pop up in a pot, Penn State Extension says they won't harm a healthy houseplant, but they are poisonous to pets and children, so remove them.

"White Mold" in the Garden

In gardening, "white mold" is the name of a plant disease. University of Minnesota Extension explains that it's caused by the fungus Sclerotinia sclerotiorum, which infects more than 400 plant species and forms fluffy white growth on stems. It's a garden problem, not the same as mold growing inside your home.

White Mold on Food, Clothing, and Leather

Food. Some cheeses are made with mold on purpose. The UC Master Food Preserver Program notes that "some cheeses such as Brie and blue cheese are made with mold." For unexpected mold, it advises discarding moldy food except hard cheese and firm fruits and vegetables, which can be trimmed by cutting at least 1 inch below the mold.

Clothing and leather. University of Missouri Extension describes mildew as "a thin, black, or sometimes white" growth that attacks cotton, linen, silk, wool, leather, wood, and paper in damp, poorly ventilated spots. NC State Extension suggests brushing mold off leather and then cleaning it with water and a non-phosphate detergent, and notes that water- and mold-damaged upholstered furniture is seldom salvageable. For more cleaning methods by material, see mold vs. mildew.

Is White Mold Dangerous?

White mold isn't less of a concern because it's pale, and it isn't more dangerous either. NIOSH says the color of mold "does not necessarily indicate that it is more or less dangerous," and the CDC says all molds should be treated the same with respect to potential health risks and removal.

The CDC notes that mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash in sensitive people. People with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease may get lung infections from mold. For more, see what kind of mold is dangerous in a house.

How To Remove White Mold Safely

  1. Fix the moisture. The EPA says mold will most likely return if you don't fix the water problem.
  2. Size it up. The EPA says most people can clean an area smaller than about 10 square feet themselves.
  3. Protect yourself. The CDC recommends at least an N95 respirator, gloves, and goggles. People with asthma, allergies, lung disease, or weakened immune systems shouldn't do the cleanup.
  4. Clean hard surfaces with detergent and water and dry them completely. If you use bleach, use no more than 1 cup per gallon of water, and never mix it with ammonia or other cleaners.
  5. Remove porous materials that can't be cleaned, such as moldy ceiling tiles or carpet.
  6. Don't paint over it. The EPA says paint applied over moldy surfaces is likely to peel.

Testing usually isn't needed. The EPA says sampling is unnecessary in most cases when mold is visible. Call a professional for large areas, hidden growth, decay in structural wood, or mold that keeps returning. See who to call for a mold inspection.

Prevent it: keep indoor humidity below 60 percent, ideally 30 to 50 percent, as the EPA recommends. See what humidity level helps prevent mold.

Frequently Asked Questions

Is white mold less dangerous than black mold?

No. Color doesn't tell you how dangerous a mold is. NIOSH says color doesn't necessarily indicate risk, and the CDC says to treat all molds the same.

How can I tell if the white stuff on my basement wall is mold?

Put a few drops of water on it. Mineral salt deposits dissolve easily in water, according to NDSU Extension. If water doesn't work, try vinegar. Deposits that won't dissolve and look fuzzy or spread over time are more likely mold.

Does efflorescence mean I have a water problem?

It means water is moving through the concrete or masonry and evaporating. That moisture can raise humidity and lead to mold on nearby materials. Check gutters, downspouts, and grading, and consider a dehumidifier.

Is white fuzzy growth on crawl space wood serious?

It can be. Light surface mold doesn't weaken wood much, but white fan-shaped patches or root-like strands can be a decay fungus. Probe the wood for softness and have a professional inspect it, especially if the crawl space is damp.

Is white fuzz on my houseplant soil harmful?

It's usually a harmless fungus feeding on organic matter in the potting soil, encouraged by overwatering. Let the soil dry out more between waterings. Remove any mushrooms if you have pets or young children.

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