"Black mold" is one of the most searched phrases about mold, and one of the most misunderstood. It usually refers to a specific mold, Stachybotrys chartarum, but people use it for almost any dark growth they find at home. The good news is that the practical answer is the same either way: find the water, fix it, and remove the growth safely.
This guide explains what black mold is, why it grows, how it differs from other dark molds, and what to do if you find it.
What is black mold?
Stachybotrys chartarum is a greenish-black mold. According to the CDC, it grows on materials with a high cellulose and low nitrogen content, such as fiberboard, gypsum board, paper, dust, and lint, and it needs constant moisture to grow. In a house, that usually means the paper face of drywall, ceiling tiles, wood, cardboard, or paper that has stayed wet for a while.
The name "black mold" is not a scientific category. Many common indoor molds are dark, and the EPA reminds people that mold comes in many colors, not just black. Stachybotrys is less common indoors than many other molds, but it is not rare where water damage has gone unnoticed.
Is all black mold "black mold"?
No. Dark spots on shower grout, window frames, or bathroom ceilings are very often other surface molds or mildew, not Stachybotrys. Several common indoor molds are dark green, brown, or black. You cannot tell the species by looking, and it rarely matters for what you do next. The CDC's guidance is that it is not necessary to determine what type of mold you have, because all molds should be treated the same way with respect to health risks and removal.
Our guides on what black mold looks like and mold vs. mildew go into how to tell dark growths apart.
What causes black mold in homes
Mold spores are present in all indoor and outdoor air. They start to grow only when they land on something wet. Stachybotrys in particular needs materials that stay wet, which is why it is usually a sign of an ongoing water problem rather than a one-time spill. Common causes include:
- A slow leak from a supply line, drain, or toilet that soaks drywall from behind
- A roof or window leak that wets ceiling drywall or wall cavities
- Flooding or storm water that was not dried quickly
- Condensation on cold surfaces, such as around air conditioning ducts and lines
- A clogged air conditioning condensate drain that overflows into the closet or ceiling
- Long periods of high indoor humidity, especially in a closed-up house
The EPA's rule of thumb is to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth. Black mold from water damage usually means that window was missed.
Where black mold shows up in a house
Because it needs wet, cellulose-based material, Stachybotrys tends to appear:
- On the back side of drywall behind sinks, tubs, showers, and toilets
- On ceilings under roof leaks or overflowing air conditioning pans
- On the lower part of walls after flooding
- In basements and crawl spaces with water intrusion
- Under carpet and pad that stayed wet
- Around windows that leak or collect condensation
The EPA notes that mold can also grow on hidden surfaces, such as the back of drywall, wallpaper, paneling, the top of ceiling tiles, and the underside of carpets and pads. A musty smell with no visible source often means growth is hidden. See signs of mold in your house.
Is black mold dangerous?
Any indoor mold, including Stachybotrys, can cause health problems for some people, such as a stuffy nose, sore throat, coughing, wheezing, burning eyes, or skin rash, according to the CDC. People with asthma, mold allergies, chronic lung disease, or weakened immune systems are at greater risk.
Black mold's "toxic" reputation comes from its ability to produce mycotoxins and from reports in the 1990s linking it to serious illness. The CDC's position is that the hazards of molds that may produce mycotoxins should be considered the same as other common molds, and that a causal link between toxic mold in homes and rare conditions has not been proven. Our guide is black mold dangerous? covers the evidence in more detail. If you think mold is affecting your health, talk to a doctor.
Do you need to test for black mold?
Usually not. The EPA says that in most cases, if mold growth is visible, sampling is unnecessary, and no federal limits exist for mold or mold spores. Knowing the species rarely changes the plan, because every mold is handled the same way: fix the moisture and remove the growth.
Testing can make sense when you suspect hidden growth, need documentation for an insurance claim or a home sale, or want to confirm that remediation worked. See what a mold inspection involves and our mold testing page.
How to get rid of black mold
The process is the same as for any mold:
- Stop the water. Fix the leak, drain, or humidity problem first, or the mold will return.
- Size up the area. The EPA says homeowners can often clean areas smaller than about 10 square feet. Remember that growth on the back of drywall is often larger than what shows on the front.
- Clean or remove. Hard surfaces can be scrubbed with detergent and water and dried. Porous materials with mold growth, such as drywall and ceiling tiles, usually have to be cut out and bagged.
- Protect yourself. Wear an N-95 respirator, gloves, and goggles, and keep others out of the area.
- Call a professional for larger jobs. Areas over about 10 square feet, mold inside walls or insulation, and mold in the air conditioning system need containment and equipment.
Do not paint over black mold, and do not rely on spraying bleach on drywall or wood; the EPA does not recommend biocides as a routine part of cleanup, because dead mold can still cause allergic reactions. See DIY mold removal vs. hiring a professional and what is mold remediation.
As for price, cost depends on size and location, not color. National cost guides put typical professional remediation at about $1,100 to $3,400; see our mold remediation cost guide. If the mold followed a sudden leak, read does homeowners insurance cover mold before you pay out of pocket.
Black mold in Florida homes
Florida's heat and humidity, heavy air conditioning use, and storm season make long-wet materials common. Homes left closed up with the air conditioning off, condos where leaks travel between units, and houses with roof damage after a storm are typical places for dark mold on drywall. Florida requires a DBPR license for most mold remediation over about 10 square feet. See our Florida mold guide and local pages such as Fort Lauderdale and West Palm Beach.
Next step
If you own your home and the black mold covers more than a small patch, sits on drywall or insulation, or keeps coming back, call the number on this page to be connected with an independent local mold removal company for an assessment and a written quote.