Black Mold in Your House: Is It Dangerous? What to Do

The short answer

"Black mold" usually refers to Stachybotrys chartarum, a greenish-black mold that grows on wet, cellulose-rich materials such as drywall paper and wood. But many common molds are black too, and the CDC advises treating all molds the same when it comes to health risks and removal.
Any mold can cause stuffy nose, wheezing, and red or itchy eyes or skin, and people with asthma, allergies or weakened immune systems are more sensitive. The CDC says no test proves a link between Stachybotrys and particular health symptoms.
Testing is usually unnecessary. The CDC doesn't recommend routine mold sampling, and there are no set standards for acceptable mold levels in a home. If you can see or smell mold, remove it and fix the moisture source, whatever the species.
The EPA suggests a homeowner can usually clean an area under about 10 square feet (roughly 3 feet by 3 feet). Larger areas, mold inside walls or HVAC, mold after sewage, or anyone with health concerns call for a professional.

What people mean by black mold

Most of the time, "black mold" means Stachybotrys chartarum. It needs material that stays wet for a long time, such as drywall paper, ceiling tile or wood after a slow leak or flood, which is why it's a sign of a serious moisture problem. But other common molds, including species of Cladosporium, Alternaria and Aspergillus, can also look black or dark green, and they're far more common on bathroom grout, window sills and damp basements.

The species matters less than the moisture. Mold won't grow without water, and it will come back after cleaning if the leak, condensation or humidity isn't fixed.

Health effects

Most people: mold can cause stuffy nose, sore throat, coughing or wheezing, burning eyes and skin rash.
People with allergies or asthma: reactions can be stronger, and mold can trigger asthma attacks.
People with weakened immune systems or chronic lung disease: are at higher risk of infection and should avoid moldy areas and cleanup work.

If you have symptoms you think are related to mold, see a doctor. Mold remediation companies can remove mold, but they can't diagnose health problems.

Can you clean it yourself?

The EPA's guidance for homeowners uses a size threshold of about 10 square feet. Below that, cleanup is usually a do-it-yourself job if the mold isn't from sewage and no one in the house is at higher risk.

Fix the water first. Repair the leak, dry the area, and lower humidity below 60% (ideally 30% to 50%).
Protect yourself. Wear an N95 respirator, gloves and goggles without vent holes.
Scrub hard surfaces with detergent and water, then dry them completely.
Remove porous materials that are moldy, such as drywall, ceiling tile, insulation and carpet. Bag them before carrying them through the house.
Don't paint or caulk over mold. It keeps growing underneath and the paint will peel.

When to call a remediation company

The growth covers more than about 10 square feet, or keeps coming back.
It's inside walls, under floors, in the attic or crawl space, or in the heating and cooling system.
It followed a sewage backup or floodwater.
Someone in the house has asthma, a weakened immune system, or symptoms you think are mold-related.

Professional remediation under the IICRC S520 standard sets up containment with plastic sheeting and negative air pressure so spores don't spread, removes contaminated materials, cleans with HEPA vacuums, and dries the area. Ask the company whether it also fixes the moisture source or whether you'll need a separate contractor.

When mold testing makes sense

You smell mold but can't find it, and want to narrow down where it is.
You want clearance testing after a professional remediation, done by an independent assessor rather than the company that did the work.
A landlord, buyer, insurer or court requires documentation.

Frequently asked questions

Is black mold dangerous?

All indoor mold growth should be removed, but black-colored mold isn't automatically more dangerous than other mold. Molds can cause allergy-like symptoms and asthma attacks, and people with weakened immune systems or chronic lung disease can develop infections. The amount of growth, how long you're exposed and your own health matter more than the color.

How do I know if it's black mold?

You can't tell the species by looking at it, and in most cases you don't need to. Treat any visible mold the same way: fix the water problem, then clean or remove the moldy material.

Should I get a mold test?

Usually not. The CDC doesn't recommend routine sampling, because there are no set standards for acceptable mold levels and results can't tell you whether someone will get sick. If mold is visible, testing doesn't change what you need to do. Testing can help in specific cases, such as tracking down a hidden source behind a persistent smell, or clearance testing after a professional remediation.

Can I clean black mold with bleach?

On hard, non-porous surfaces, scrubbing with detergent and water and drying completely is usually enough, and is what the EPA recommends. If you use bleach, the CDC advises no more than 1 cup per gallon of water, ventilating well, and never mixing bleach with ammonia or other cleaners. Porous materials like drywall, ceiling tile and carpet that are moldy usually have to be removed, not cleaned.

Can I stay in my house with black mold?

For small areas, most healthy people can stay while it's cleaned up. For large areas, or if someone in the house has asthma, a weakened immune system or is getting sick, talk with a doctor and consider staying elsewhere during professional remediation, when the work area should be sealed off.

Black mold: what it is, health effects and what to do