Mold grows wherever moisture lingers on something it can feed on: drywall, wood, dust, fabric. Finding it is mostly a matter of finding the moisture, because where water collects, mold usually follows.
Mold shows up as fuzzy, slimy or speckled growth in black, green, white or other colors, often with a musty, earthy smell, usually where there's a leak, condensation or high humidity. If you can see or smell it, the EPA says testing usually isn't needed: clean it up and fix the moisture. For areas larger than about 10 square feet, the EPA suggests using its guidance or a professional.
Signs of mold
- Visible growth: spots or patches, fuzzy or slimy, in almost any color, often spreading from a corner, seam or stain.
- A musty smell, especially in closed rooms, basements and when the HVAC starts.
- Water stains, peeling paint or warped materials, signs of the moisture mold needs.
- Condensation on windows, pipes or ducts that keeps coming back.
Mold or mildew?
Mildew is a common name for mold, usually the flat growth that sits on a damp surface instead of digging into it. People tend to call the thin gray or white film on tile, grout and fabric "mildew," and the raised, fuzzy or slimy growth that spreads into drywall and wood "mold."
The EPA does not treat them differently. Whatever you call it, the response is the same: scrub it off hard surfaces, replace porous material it has grown into, and fix the moisture feeding it.
| Mildew (surface mold) | Other mold | |
|---|---|---|
| Look | Flat, powdery or downy | Raised, fuzzy, slimy or speckled |
| Color | Usually gray or white | Black, green, white, pink and more |
| Where | Damp tile, grout, fabric, showers | Drywall, wood, ceilings, ducts, dust |
| On hard surfaces | Wipes and scrubs off | Scrubs off if the surface is sound |
| If moisture stays | Returns in the same spot | Returns and can penetrate and stain |
What mold smells like
Mold smells musty, earthy or damp, the stale note of a closed basement or a gym bag left wet. The smell comes from gases the growth gives off, so it can be the first sign long before anything shows.
To trace an odor with no visible growth, follow your nose to where it is strongest, then check the damp and hidden spots nearby: under sinks, behind appliances, along baseboards and inside closets on exterior walls. A smell that rises when the air handler or furnace fan starts points at the HVAC system itself.
A musty or sour "dirty sock" smell in the first minutes of cooling is a known sign of growth on the evaporator coil. The coil stays wet while the system pulls humidity out of the air, and that dark, damp metal is an easy surface for mold and bacteria. Check whether duct cleaning is worth it when the smell rides along with the airflow.
What the colors mean
Mold grows in black, green, white, gray, pink, orange and brown. Color comes from the species and what the growth is feeding on, not from how risky it is.
You cannot tell "toxic mold" from harmless mold by looking. The EPA does not recommend identifying the species or color before cleanup, because the response is the same for all of it: remove it and fix the moisture. A dark patch is not automatically the "black mold" of headlines, and a pale one is not automatically safe.
Is it dangerous?
The EPA's position is practical: treat all indoor mold the same and remove it, because the risk cannot be read off the color or species. No federal limit defines a "safe" amount of mold, so sampling does not hand you a pass-or-fail number to act on.
"Black mold" and "toxic mold" are media labels, not a diagnosis you can make at home. Some molds can produce irritating or toxic compounds under some conditions, but you cannot confirm that by sight, and it does not change the plan: clean it up and dry out the source.
Mold and your health
The EPA links mold exposure to allergic reactions such as sneezing, a runny nose, red eyes and skin rash, and to asthma attacks in people who are both allergic and asthmatic. It can also irritate the eyes, skin, nose, throat and lungs in people who are not allergic. Reactions vary a lot from person to person; this is general information, not a diagnosis, so see a clinician about symptoms you think are tied to mold.
Where to look
- Bathrooms and kitchens: around tubs, showers, sinks and under cabinets.
- Basements and crawl spaces: walls, joists and anything stored against cold surfaces; see crawl space moisture barriers.
- Around leaks: roofs, windows, plumbing and appliance hoses.
- HVAC systems: the cooling coil, drain pan and nearby ductwork, which stay damp when the system cools.
Finding hidden mold
Suspect hidden mold when the home smells musty but nothing is visible, or when you know there was a leak or flood and people report symptoms. The EPA notes mold often hides on the back of drywall and wallpaper, the top of ceiling tiles, and the underside of carpet and padding.
Look for what moisture leaves behind: brown or yellow staining, paint that bubbles, peels or cracks, and drywall, trim or flooring that warps or swells. Opening a wall or cavity can release a large amount of spores, so for anything beyond a small, obvious spot, use the EPA's detailed guidance or a professional.
Do you need a mold test?
If you can see or smell mold, the EPA says sampling usually isn't necessary, and no federal standards exist for acceptable amounts of mold. Testing can help when you suspect hidden mold, but the result needs interpreting by someone experienced. Either way, the fix is the same: remove the mold and the moisture behind it.
Use what you find, and how much of it, to decide who handles it:
| What you find | What the EPA suggests |
|---|---|
| You can see or smell it | Skip testing; clean it and fix the moisture |
| Growth under about 10 sq ft | Usually a do-it-yourself cleanup |
| Growth over about 10 sq ft | Use the EPA's detailed guidance or a professional |
| Sewage or flood water, or HVAC growth | Call a professional |
| Musty smell but nothing visible | Investigate hidden mold before testing |
Cleaning it up
- Small areas, under about 10 square feet, are usually a do-it-yourself job, following the EPA's guide: scrub hard surfaces with detergent and water, and dry them completely.
- Larger areas, or mold from contaminated water or in the HVAC system, call for the EPA's more detailed guidance or a professional.
- Porous materials such as drywall, ceiling tiles and carpet that have grown mold usually need replacing.
- Protect yourself with an N95 respirator, gloves and goggles.
Stopping it coming back
- Fix leaks and dry water-damaged materials within 24 to 48 hours, as the EPA advises.
- Keep indoor humidity below 60%, ideally between 30% and 50%; see the ideal indoor humidity level.
- Ventilate bathrooms and kitchens with exhaust fans.
- Reduce condensation by insulating cold surfaces such as pipes and exterior walls.
Frequently asked questions
How can I tell if it's mold or dirt?
Mold usually grows in patches that spread and return after cleaning, often with a musty smell, in damp spots. Dirt wipes off and doesn't regrow.
Do I need to test for mold?
Usually not, if you can see or smell it, according to the EPA. Clean it up and fix the moisture source.
Can I remove mold myself?
For areas under about 10 square feet, generally yes, following the EPA's guidance. Larger or HVAC-related problems call for a professional.
What humidity prevents mold?
Below 60%, and ideally 30% to 50%, according to the EPA.
What does black mold look like?
Black mold shows up as dark green to black patches, often slimy or spotty, on damp drywall, wood or grout. The color alone does not confirm the species or the hazard, which is why the EPA says to remove any mold the same way rather than identify it. Treat a dark patch as mold to clean and a moisture problem to fix.
What's the difference between mold and mildew?
Mildew is a common name for the flat, powdery surface growth, usually gray or white, on damp tile, grout and fabric. Mold is the broader term and includes the raised, fuzzy growth that can spread into drywall and wood. The EPA handles both the same way: remove the growth and fix the moisture.
Can mold make me sick?
The EPA links mold to allergic reactions like sneezing, runny nose, red eyes and rash, to asthma attacks in people who are allergic, and to irritation of the eyes, skin, nose, throat and lungs. Reactions vary widely between people. This is general information and not a diagnosis, so see a clinician about symptoms you suspect come from mold.
How do I find mold I can smell but can't see?
Follow the musty smell to where it is strongest, then check damp, hidden spots nearby: under sinks, behind appliances and inside closets on cold exterior walls. Look for staining, bubbling paint and warped materials that mark past moisture. If the smell rises when the HVAC fan runs, inspect the coil, drain pan and ducts.