Black mold in the shower

Is black mold in the shower toxic?

Its color doesn’t tell you that. The mold usually called “toxic black mold” is Stachybotrys chartarum, which the CDC describes as a greenish-black mold that grows on materials high in cellulose, such as gypsum board, fiberboard and paper, when they stay wet.¹ Tile grout and silicone caulk are not its usual home, and many molds are black.

More to the point, the CDC says all molds should be treated the same with respect to potential health risks and removal, and that it isn’t necessary to determine what type of mold is growing in your home.¹ California’s public health department says current consensus doesn’t justify labeling some molds as toxic molds that are especially hazardous to healthy people.²

That doesn’t make shower mold harmless to ignore. The CDC notes that people with asthma or mold allergies can react to mold exposure.⁴ If you have symptoms, talk to your doctor.

Why does mold grow in the shower?

Because it’s the wettest surface in the house. Mold needs moisture, and a shower gets soaked once or twice a day. The EPA puts it plainly: places that are often or always damp can be hard to keep completely free of mold.³

How do you clean mold off grout?

Scrub it with detergent and water and dry it. That’s the EPA’s method for mold on hard surfaces.³

1. Wear gloves and keep the fan running or a window open.
2. Scrub the grout lines with a stiff brush and a detergent solution.
3. Rinse, then dry the surface with a towel or squeegee.
4. Keep it dry for the rest of the day if you can.

The EPA doesn’t recommend bleach or other biocides as routine practice for mold cleanup, and warns never to mix bleach with cleaners that contain ammonia, which can produce toxic fumes.³ The CDC’s general advice is to clean bathrooms with mold-killing products.⁴ Either way, read the label and ventilate.

Some grout stays stained after the mold is gone. The EPA notes that mold can cause staining that cleaning won’t fully reverse.³ A stain that doesn’t come back fuzzy is cosmetic.

When does caulk need to be replaced?

When the black is under the surface of the caulk bead instead of on it. Caulk is a flexible seal, and once it peels slightly or loses adhesion, water and mold get behind it where a brush can’t reach. If the spots look sealed in after a good scrub, cleaning won’t fix them.

Replace it by cutting out the old bead completely, cleaning the joint, and letting it dry fully before applying new caulk. The EPA’s guidance is direct: don’t caulk over moldy surfaces.³ Caulk that keeps peeling or turning black within months also suggests the joint is moving or staying wet, which is worth looking at more closely.

How do you stop shower mold from coming back?

Ventilation. The EPA says that if shower mold seems to reappear, increasing ventilation by running a fan or opening a window and cleaning more often will usually prevent it from recurring, or at least keep it to a minimum.³ The CDC recommends bathroom exhaust fans that vent to the outside of the home.⁴

• Run the fan during the shower and for a while after
• Squeegee walls and glass after each use
• Leave the shower door or curtain open to dry
• Check that the fan actually moves air. Hold a square of tissue to the grille.

Some bathrooms have a window and no fan, or a fan that vents into the attic instead of outside. If yours vents into the attic, our guide to mold in the attic explains why that matters.

When is shower mold a sign of a bigger problem?

When the water is getting past the tile. Watch for loose or hollow-sounding tiles, grout that crumbles or feels soft, stains or bubbling paint on the wall behind the shower or the ceiling below it, and a musty smell that lingers after cleaning.

Surface mold on grout and caulk is a cleaning job, and you don’t need an inspector for it. If you see the signs above, or you aren’t sure whether you’re looking at mold, send us a photo. A mold inspection at $495 includes moisture readings on the walls around the shower, which show whether water is getting into them without opening anything up.