Black mold in the house: what it is and what to do

What people mean by black mold

Many common indoor molds are dark green, grey or black. The name is often used for one species, Stachybotrys chartarum, which grows on materials that have stayed very wet for a long time, such as soaked drywall or ceiling tile. But you cannot identify a species by looking at it, and several harmless-looking molds are just as unwelcome indoors.

That is why Health Canada’s guidance does not depend on the type. Its advice is to control dampness and clean up any mold, regardless of species. Identifying the mold does not change what needs to be done.

Symptoms people report

Health Canada links indoor mold and dampness with:

• Irritation of the eyes, nose and throat
• Coughing and phlegm build-up
• Wheezing and shortness of breath
• Worsening of asthma symptoms
• Allergic reactions

People respond differently. Infants and children, older adults, and people with asthma, allergies or weakened immune systems are more likely to be affected. If you have symptoms you think are related to mold, see a health professional. A restoration company cannot diagnose them.

Where dark mold usually shows up

• Bathrooms, on caulking, grout and ceilings where steam collects
• Basements, on the lower part of walls, behind furniture and on stored cardboard
• Window frames and sills, where condensation runs down in winter
• Under sinks, around a slow leak
• Attics, on the underside of the roof sheathing
• Behind wallpaper or panelling, after a leak

Mold needs moisture. Wherever you find it, something there has been damp.

What to do

Small amounts. Health Canada describes mold as a small amount when there are three or fewer patches with a total area under one square metre. That can generally be cleaned by the homeowner: wear rubber gloves, eye protection and a well-fitting N95 respirator, and wash the surface with water and dish detergent. Dry it thoroughly.

Larger amounts. For a patch larger than three square metres, mold that returns after cleaning, or mold hidden inside walls or ceilings, Health Canada advises hiring a professional. Removal is done inside a contained area so that spores are not spread through the house.

In every case. Find and fix the moisture: a leak, condensation, poor ventilation or high humidity. Without that, the mold returns.

What not to do

• Do not dry-scrub, sand or scrape it. That releases spores.
• Do not point a fan at it.
• Do not paint over it.
• Do not rely on bleach for drywall or wood. It lightens the stain without removing growth inside porous material.

When it is urgent

Mold is rarely an overnight emergency, but act promptly if there is active water, if the growth is spreading quickly, if it is in a bedroom or living space, or if anyone in the home is in a higher-risk group.

Common myths

“It is only dangerous if it is the toxic kind.” Health Canada does not divide indoor mold into safe and unsafe types. Its advice is the same for all of them: remove it and fix the dampness.

“If I cannot see it, it is not there.” Mold grows on the hidden side of materials: the back of drywall, under carpet, inside cabinets. A persistent musty smell is evidence even when the room looks clean.

“Bleach kills it for good.” Bleach may lighten the surface. It does not reach into drywall or wood, and without a moisture fix the growth returns.

“A dehumidifier will get rid of it.” A dehumidifier prevents new growth by keeping the air dry. It does nothing to mold that is already established.

“New houses do not get mold.” Tightly sealed new homes hold humidity, and any house can have a leak. Age is not protection.

If you rent

Tell your landlord in writing as soon as you find mold, with photos and the date. The landlord is responsible for the building and for repairing the cause. Keep a copy of what you send.

More: mold removal and remediation.