Mold works non-stop to keep the planet going by breaking down organic matter — but we still don’t want it in the house, and for awfully good reason. Mold can trigger rashes, headaches, allergies and asthma attacks, according to the Mayo Clinic. Certain types of mold can even cause brain infections and sepsis (blood poisoning).

Whether it’s black, brown, green or pink, experts agree you should get rid of it. “Any visible mold should be removed, no matter what its color or species,” says Tiina Reponen, PhD, professor of environmental health at the University of Cincinnati. “In a healthy building, you don’t have visible mold.”

Related: The Top Places Mold Hides in Your Home

Like most fungi, molds grow best in damp conditions — think bathrooms and basements. If the spores find a moist surface to land on, they grow.

Although “toxic mold” is a misnomer, according to the Centers for Disease Control and Prevention (CDC), the agency notes some molds do produce toxic substances called mycotoxins.

Here, a color guide to molds commonly found in the house.


If you see green mold, it could be just about any type of unwelcome fungus. There are more than a hundred thousand types of mold — and thousands of species of green mold, according to Neil Kao, MD, an allergist in private practice in Greenville and Spartanburg, South Carolina. So what does the color green tell you? Not much.

Olive-green, brown, grey or black

These are common molds in the Cladosporium genus. Outdoors, they lurk on plant leaves. Indoors, they're often found on walls and insulation and can grow on damp carpet, too. They are linked to skin, eye and sinus infections. Very rarely, they can cause brain infections (fungal meningitis), according to the CDC.

Blue, green, or white

These molds, also common, belong to the Penicillium genus. You’re right if you think that a type of this mold was used to make penicillin many years ago. It’s usually found on food and walls. If you’re sensitive to mold, you may develop an allergic reaction to it.

Yellow, green or black

These may be Aspergillus molds. According to the CDC, people breathe in these molds every day, usually without getting sick. But those with existing lung problems or weakened immune systems may develop aspergillosis. This illness might entail coughing, wheezing and sinus inflammation. But aspergillosis can also be more serious, especially in people with existing lung problems or weakened immune systems, causing cavities in the lungs or forming "fungus balls" (ick) in them. This is known as chronic pulmonary aspergillosis.

Black or grey

These could be Alternaria, which is most common as an outdoor mold, growing around damp, dusty areas, soil and plants. But it has made its way indoors. In one study, Alternaria was found in more than 90 percent of house dust samples. Exposure to it may boost the risk of asthma.


The pink “mold” often seen in the bathroom in the form of a slimy, pinkish discoloration on sinks and tubs is actually bacteria, not mold. Specifically, it's Serratia marcescens. It thrives on soap and shampoo residues and is linked to urinary tract and respiratory infections.

Don’t obsess about getting infected from pink bathroom slime, though. S. marcescens usually enters the urethra through catheters or the lungs through respirators, according to an article on the website of Scientific American magazine. It has also contaminated soap solutions in hospitals and contact lens cases, infecting the cornea of some contact lens wearers.


This mold, of the Stachbotrys genus, is the infamous “black mold” that some news reports have linked to severe health problems, including memory loss and lung bleeding. It's less common than the molds described above — and possibly less dangerous than news reports would have you believe. According to the CDC, Stachbotrys has not be proven to cause either memory loss or lung bleeding. It prefers to live on high-cellulose, low-nitrogen surfaces, which include drywall, gypsum board, paper, dust and lint that is regularly exposed to moisture. The CDC notes, "Growth occurs when there is moisture from water damage, excessive humidity, water leaks, condensation, water infiltration, or flooding. Constant moisture is required for its growth."

Related: What To Do After a Flood

See mold? Get rid of it

You really can’t tell for sure what kind of mold you have by looking at it, and the CDC doesn’t recommend mold testing, since you should get rid of any type of mold you see in the house. (According to the CDC, most mold can be removed by cleaning with a solution of no more than 1 cup of bleach in 1 gallon of water.)

Reponen agrees with the CDC. “Don’t waste time trying to figure out the species,” she advises. “If there’s mold, there’s moisture, and you need to find out where it’s coming from and fix the problem.”

She notes that young children are more susceptible to mold because their lungs are developing and they are breathing more air relative to their body mass. In a study published in the Journal of Allergy and Clinical Immunology in 2012, Reponen and her colleagues found infants exposed to mold in their homes have a higher risk of developing asthma by age 7.

Kao sees a lot of patients with mold allergies. “The most common reaction to mold is an allergic reaction, like hay fever,” he says. But, he says, serious respiratory problems or lung infections can also occur, especially in people with weak immune systems. If you suspect you are having an allergic reaction or other health problems related to mold, see your doctor.

Meanwhile, eliminate mold wherever it crops up, and take steps to prevent it from getting a foothold in your home . If your house has a moldy smell but you don’t see anything, it may be in the ductwork or inside your walls. Hire a professional to look for the source.

Related: Is a Hidden Hazard Lurking in Your Washer or Dryer?

Mary Purcell is a freelance writer and health researcher in Piedmont, Calif., with expertise in policy analysis. She has a master's degree in Latin American studies from Georgetown University.