What Is Toxic Mold?

Damage caused by damp on a wall in modern house
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Mold growing on your wall could be one of a number of species.

Although no one knows for sure, we could be sharing the planet with as many as 300,000 different species of mold. It's a safe bet that the air you're breathing right now contains mold spores, and if all of them were harmful, no one would be free from perpetual respiratory distress. Fortunately, only some molds are harmful, and the ones that have earned the designation "toxic" produce harmful mycotoxins as a product of metabolism. The one typically of most concern to homeowners is Stachybotrys chartarum, commonly known as toxic black mold, but it's just one of several mold species that can cause respiratory problems for sensitive people. Most of these species are allergens, but one — Aspergillus fumigatus — is perhaps even more dangerous than Stachybotrys. It's important to keep all mold under control.

What's "Toxic" About Toxic Mold?

The designation "toxic mold" is something of a misnomer, because by itself, no mold qualifies as a toxin. Some fungi species that can become airborne, such as Aspergillus and Candida, are pathogens, but molds by themselves are generally allergens at worst. What makes a particular mold toxic is the waste products of metabolism, which are called mycotoxins. When these natural poisons become airborne and you inhale them, they can create symptoms similar to inhaling a pesticide or a heavy metal. However, it's important to note that, although this potential exists, it isn't certain that's it's an actual danger in homes in which Stachybotrys (the classic toxic mold) is growing. The mycotoxins produced by Stachybotrys might merely cause ordinary allergic reactions and inflammation similar to that caused by the spores of other allergenic species. Even if they aren't actual poisons, though, allergens can pose a serious health problem.

On the Lookout for Mold

Mold and dirt on window
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Mold thrives in the condensation that collects on window frames.

Stachybotrys is fairly easy to identify. It's called black mold because it's greenish-black. It feeds on cellulose, so you'll often find it growing on wood and drywall paper. Besides cellulose, Stachybotrys, like all molds, needs moisture. It often grows in moist areas, such as basements, dark closets, bathrooms, and poorly ventilated corners.

The mold you see may not be Stachybotrys, however. It could be any one of several other species that commonly grow in homes. Many of these species are also allergens, and some, like Alternaria infectoria — the most common indoor species — look very similar. Alternaria is the species most likely responsible for the discoloration around your windows. It can cause asthma-like symptoms and grows quickly. Other allergenic species you might see include Cladosporium herbarum (in drapes and other fabrics), Mucor racemosus (in HVAC ducts) and Aspergillus, which can produce carcinogenic mycotoxins.

Because molds can look so much alike, they can be difficult to identify, and the Centers for Disease Control and Prevention recommends that you don't even try. Testing molds is lab work, and no matter what type it is, you'll want to get rid of it.

How to Control Mold

Person Hand Cleaning Moldy Wall
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Physically remove mold with a soap solution.

Every mold remediation program, whether it's a cleanup after a flood or a routine cleanup of a bathroom, starts with eliminating moisture. Turn up the heat, increase air circulation using fans, if necessary, and wipe up any standing water or persistent condensation with a rag or a sponge. If the moisture was caused by a plumbing or roof leak, stop the leak. Once the affected area has dried out, try to keep the humidity in that area below 50 percent.

Once the area has dried out, the best way to remove mold is to physically scrub it. You can use a solution consisting of 1 part household bleach to 10 parts water, but a soapy water solution works just as well, and it's easier on your hands. Bleach has a high surface tension that prevents it from penetrating porous surfaces like wood and drywall, so while it kills surface mold, it doesn't kill the roots.

The keys are to remove as much of the visible mold as possible and to keep the area dry to prevent it from growing again. As an alternative to bleach, try a natural disinfectant, such as vinegar, hydrogen peroxide, tea tree oil and pine oil. When mold is growing in problem areas, such as the framing behind a wall, dusting the affected area with borax or boric acid is a good way to prevent the mold from coming back.