Black mold vs mildew: what's the difference, and does it matter?

Do this first: size it up instead of naming it

The question that decides what to do is not mold or mildew. It is how much, on what, and why it is wet.

1. Measure the patch. EPA says that if the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, in most cases you can handle the job yourself. ^[1]
2. Note what it is growing on: hard surfaces like tile, glass, sealed wood or metal, or porous ones like drywall, ceiling tile, carpet, cardboard and fabric.
3. Press the material around it. A practical check: soft, swollen or stained drywall around the spot suggests a leak or water damage, not just surface growth.
4. Find the moisture: shower steam, condensation on a cold wall or window, a plumbing leak, a roof leak or a damp basement.
5. Skip the mold test. EPA says that in most cases, if visible mold growth is present, sampling is unnecessary, and CDC does not recommend mold testing. ^{[1] [2]}
6. Take a dated photo so you can tell later whether it is spreading.

What is mildew, and what do people mean by black mold?

Mildew. EPA says mildew refers to certain kinds of mold or fungus, and that the term is often used generically for mold growth, usually with a flat growth habit. ^[3] University of Florida Extension puts it plainly: mildew is a type of mold. ^[4] University of Missouri Extension describes mildew as a thin, black or sometimes white growth produced by mold. ^[5] FEMA calls mildew and molds fungi that thrive anywhere there is a moist environment. ^[6]

Black mold. People usually mean a dark mold, often one they have read about online. CDC describes Stachybotrys chartarum as a greenish-black mold that grows on material with a high cellulose content, such as fiberboard, gypsum board and paper, and says constant moisture is required for its growth. ^[7] But many molds are dark. CDC says mold can look like spots of many different colors, ^[2] and EPA notes that molds come in many colors, including white and black. ^[1] So color alone does not tell you which mold you have.

The part that matters. CDC says it is not necessary to determine what type of mold you may have, and that all molds should be treated the same with respect to potential health risks and removal. ^[7] On health, CDC says damp and moldy environments may cause a variety of health effects, or none at all. People with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease may get lung infections from mold. ^[2] CDC adds that no test exists that proves an association between Stachybotrys and particular health symptoms, and that people with persistent symptoms should see their physician. ^[7]

Black mold vs mildew: common beliefs and what the agencies say

Table sources: EPA's mold and mildew page, ^[3] EPA's brief guide, ^[1] CDC's mold pages ^{[2] [7]} and EPA's mold cleanup page. ^[8]

Where mildew and other mold usually show up

The useful difference is where the growth sits and what it is feeding on. Thin growth on a hard, damp surface is usually a cleaning job. Growth inside a porous material that got wet from a leak is a moisture problem.

• Shower walls, tile, grout and windowsills. EPA says mildew often lives on shower walls, windowsills and other places where moisture levels are high. ^[3] University of Georgia Extension says it typically appears on bathroom tile and usually begins to grow when relative humidity stays at 60 percent with temperatures of 60 degrees Fahrenheit or higher. ^[9]
• Fabric, leather, paper and wood in closets and basements. Missouri Extension says the molds that cause mildew develop most often on cotton, linen, rayon, silk, wool, leather, wood and paper, and flourish where it is damp, warm, poorly lit or poorly ventilated, such as cellars, crawl spaces and clothing closets. ^[5]
• Drywall, ceiling tile and cardboard after a leak. CDC says Stachybotrys grows on high-cellulose materials such as gypsum board and paper when there is moisture from water damage, leaks, condensation or flooding. ^[7] EPA says porous materials like ceiling tiles and carpet may have to be thrown away if they become moldy. ^[1]
• Unaired spaces with a musty smell. EPA says molds can produce a strong musty odor in unaired places such as basements. ^[3]

Room by room help: spots over a shower are covered on the black spots on bathroom ceiling page, grout and caulk on the black mold in shower page, and damp basement framing on the mold on wood in basement page.

How to clean mildew or mold: one method for both

Because CDC treats all molds the same for removal, ^[7] the cleanup is the same whatever you call it. EPA's method for hard surfaces is to scrub mold off with detergent and water and dry completely. ^[8]

1. Fix or slow the moisture first, or EPA says the mold will most likely come back. ^[1]
2. Open a window or run the exhaust fan.
3. Put on protection. EPA says you may want to wear an N-95 respirator, and it recommends long gloves and goggles without vent holes. ^[1]
4. Scrub hard surfaces with detergent and water. ^[8]
5. Wipe with clean water and dry the surface completely.
6. Bag moldy porous items you cannot clean, such as cardboard or stained ceiling tile. ^[1]
7. Wait to paint or caulk until the surface is clean and dry. EPA says paint applied over moldy surfaces is likely to peel. ^[1]

Bleach? EPA says a biocide such as chlorine bleach is not recommended as a routine practice during mold cleanup. ^[10] CDC says that if you use it, use no more than 1 cup of household laundry bleach in 1 gallon of water, open windows and doors, and never mix bleach with ammonia or other cleaners. ^[2] Fabrics: Missouri Extension notes mildew leaves a musty odor and discolors fabrics, and can eat into them until they rot, and it advises removing mildew spots as soon as you discover them. ^[5]

When it is more than a cleaning job

• More than about 10 square feet. EPA points larger jobs to its remediation guide for schools and commercial buildings, which it says applies to other buildings too. ^[8]
• It keeps coming back. EPA says more ventilation and more frequent cleaning will usually prevent bathroom mold from recurring, or at least keep it to a minimum. ^[8] If it returns within days anyway, look for water you have not found.
• You smell mold but cannot see it. EPA says you may suspect hidden mold if a building smells moldy but you cannot see the source, and to consider hiring an experienced professional if you believe you have hidden mold. ^[1]
• The water was sewage or floodwater. EPA says to call in a professional who has experience cleaning buildings damaged by contaminated water. ^[1]
• The drywall is soft, swollen or stained. That points to water damage, not just surface growth, and EPA says porous materials may have to be thrown away if they become moldy. ^[1]

For budgeting, HomeAdvisor's June 2026 cost guide puts mold remediation at an average of $2,368, with a typical range of $1,223 to $3,757. ^[12] Washington's insurance commissioner says homeowner insurance usually does not cover mold unless it comes from water damage the policy covers. ^[13] Check your own policy.

How to keep mildew and mold from coming back

EPA says the way to control indoor mold growth is to control moisture. ^[14]

• Watch the humidity. EPA says to keep indoor humidity below 60 percent, ideally 30 to 50 percent. ^[1] CDC's line is no higher than 50 percent. ^[2]
• Run the bathroom fan. The Department of Energy's Building America Solution Center recommends leaving it on for one hour after a shower and venting it outdoors, not into the attic. ^[15]
• Keep things clean and dry. Missouri Extension says soil on dirty items can supply enough food for mildew to start growing, and advises drying soiled clothes before they go in the hamper. ^[5]
• Let closets breathe. Missouri Extension suggests opening closet doors or adding a fan, and hanging clothes loosely so air can circulate. ^[5]
• Run a dehumidifier in damp basements, with windows and doors closed while it runs, as Missouri Extension advises. ^[5]
• Dry condensation fast. EPA says to act quickly when moisture collects on windows, walls or pipes, because condensation can be a sign of high humidity. ^[1]

Common questions