Mold and mildew are not two different things so much as two ways people describe the same kind of organism. Mildew is a type of mold, and the word usually refers to mold that grows flat on the surface of damp materials: the gray film on a shower wall, the spots on a window sill, the smell on a damp towel. Other molds can grow into materials such as drywall, wood, and carpet, and those are harder to remove.
The difference matters mainly for one question: can you wipe it off, or does the material have to go? This guide explains how to tell, including how to tell black mold from mildew.
Mold vs. mildew at a glance
| Mildew | Other mold | |
|---|---|---|
| Appearance | Flat, powdery or downy | Raised, fuzzy, or slimy |
| Color | Usually white, gray, or yellowish; can darken to black spots | Green, black, brown, white, and other colors |
| Where it grows | Hard or smooth surfaces: tile, grout, caulk, glass, window sills, fabric | Porous materials: drywall, wood, ceiling tiles, insulation, carpet |
| Depth | Stays on the surface | Can grow into the material |
| Cleanup | Detergent and water usually removes it | Porous materials often must be removed |
| Warning signs | Returns in humid rooms | Spreading patches, water stains, soft or warped materials, a strong musty smell |
Garden "powdery mildew" and "downy mildew" on plants are different organisms that attack living plants. They are not a household mold problem.
How to tell them apart at home
Look at three things:
- The surface. Growth that sits on tile, glass, or sealed surfaces and wipes away is behaving like mildew. Growth that stains drywall, wood, or ceiling tiles, or comes back through fresh paint, is growing into the material.
- The texture. Flat and powdery points to mildew. Fuzzy, slimy, or raised points to other molds.
- The surroundings. Water stains, bubbling paint, soft drywall, or a smell that stays after cleaning suggest a moisture problem beyond everyday bathroom humidity.
If you are unsure, clean a small area with detergent and water. Surface mildew comes off and the surface underneath looks normal. A stain that stays, or growth that returns within days, suggests the mold is in the material or the moisture source is still active.
Black mold vs. mildew
Dark spots do not automatically mean black mold. The black spots on shower grout, caulk, and tile are usually surface mold, often called mildew, feeding on soap residue and daily moisture. Stachybotrys, the mold usually called black mold, grows on cellulose-based materials such as drywall paper, ceiling tiles, and wood that stay wet for a long time.
So location is the clue. Dark growth on tile and grout is most likely mildew. Greenish-black growth on the drywall behind a leaking toilet, on a ceiling under a roof leak, or low on walls after a flood is more likely a mold growing into the material. Only a lab can identify the species, and the CDC says identifying it is generally not necessary because all molds are removed the same way. See what black mold looks like.
Does mildew turn into mold?
Mildew is already mold, and it does not change into a different species. But the moisture that feeds mildew can let growth spread from hard surfaces to porous ones: into caulk, grout, painted drywall, and fabric. Once that happens, cleaning the surface is no longer enough. Stained caulk has to be replaced, and moldy drywall usually has to be cut out.
Are mold and mildew equally harmful?
All molds, including mildew, produce allergens and irritants. The EPA says mold exposure can cause allergic reactions such as sneezing, runny nose, red eyes, and skin rash, can trigger asthma attacks in people with asthma who are allergic to mold, and can irritate the eyes, skin, nose, throat, and lungs. Mildew is usually less of a concern because it tends to cover small areas and is easy to remove, not because it is a harmless kind of mold. If anyone in the home has asthma or allergies, clean it promptly and keep humidity down.
What mold and mildew smell like
Both produce a musty, earthy smell. A faint smell in a bathroom that clears once the room dries is common. A strong or persistent smell, especially in a room with no visible growth, usually means more growth somewhere hidden: behind drywall, under flooring, or in the air conditioning system. See signs of mold in your house.
How to clean mildew
The EPA's approach for small areas works for mildew:
- Wear gloves and eye protection, and ventilate the room.
- Scrub hard surfaces with detergent and water.
- Dry the surface completely.
- Replace caulk or sealant that stays stained.
- Do not paint or caulk over growth; clean and dry first.
The EPA does not recommend biocides such as bleach as a routine part of cleanup. If you use any cleaner, never mix bleach with ammonia or other products.
When it is more than mildew
Treat it as a bigger mold problem if:
- The growth is on drywall, wood, ceiling tiles, or carpet rather than hard surfaces
- It covers more than about 10 square feet
- The wall behind tile feels soft or sounds hollow
- It returns within days or weeks of cleaning
- There is a strong musty smell with no visible source
In those cases, see DIY vs. professional mold removal and what a mold inspection involves.
Preventing both
- Keep indoor humidity below 60 percent, ideally 30 to 50 percent, as the EPA recommends
- Run bathroom and kitchen exhaust fans, and keep them running for a while after showers
- Fix leaks promptly and dry wet materials within 24 to 48 hours
- Keep the air conditioning drain line clear and the drain pan clean
- Hang towels and wet laundry to dry instead of leaving them in piles
Next step
If you own your home and the growth goes beyond surface mildew, such as mold on drywall, spreading patches, or a smell you cannot trace, call the number on this page to be connected with an independent local mold inspection company.