Mold vs. Mildew: How to Tell and When to Worry

Find a dark spot in the bathroom or the basement and the first question is: is this just mildew, or is it mold? The difference matters, because mildew is usually a cleaning job, while mold can mean water is getting somewhere it shouldn't.

01What's the difference?

Both are fungi, and both grow where it's damp. In everyday terms:

Mildew usually:

• Sits flat on the surface.
• Looks powdery or downy, white, gray or light yellow.
• Shows up on shower grout, window sills, and damp fabric.
• Wipes off with a household cleaner.

Mold usually:

• Looks fuzzy, slimy or blotchy.
• Comes in green, black, blue or other dark colors.
• Grows into porous materials like drywall, wood, ceiling tiles and carpet.
• Comes back after cleaning if the moisture is still there.
• Smells musty.

02The simple test

Dab the spot with a few drops of diluted bleach. If it lightens quickly, it's more likely mildew. If it stays dark or comes right back, it's more likely mold. Either way, the real question is why it's damp.

03Why it matters

Mold needs moisture. Where there's mold, there's a water problem:

• A leak in a pipe, roof or window.
• Condensation from poor ventilation.
• High humidity in a basement.
• Water coming in through the foundation.

Some people, especially those with allergies, asthma or weak immune systems, can have health effects from mold. The EPA's advice: the key to mold control is moisture control.

04What you can clean yourself

The EPA's guidance for homes says you can generally clean up a small area yourself, less than about 10 square feet (roughly 3 feet by 3 feet):

• Fix the water problem first.
• Wear gloves, goggles and an N95 mask.
• Scrub hard surfaces with detergent and water, and dry completely.
• Throw out porous materials that are moldy (ceiling tiles, carpet, some drywall), since mold can be hard to remove completely from them.
• Don't paint or caulk over mold. It'll grow back underneath.
• Never mix bleach with ammonia or other cleaners.

05When to call a pro

• The moldy area is larger than about 10 square feet.
• It's inside walls, in the HVAC system, or keeps coming back.
• It was caused by sewage or contaminated water.
• Someone in the house has health concerns.

Look for a qualified mold remediation contractor, and find the moisture source with a plumber, roofer or waterproofer as needed.

06Prevent it

• Keep humidity down. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. A cheap hygrometer tells you where you are.
• Run bathroom fans during and after showers.
• Use a dehumidifier in the basement.
• Fix leaks fast and dry wet areas within 24 to 48 hours.
• Vent the dryer outside.
• Keep gutters and downspouts working so water stays away from the foundation.

If you find mold, the job isn't done until you know where the water came from. See how to find a hidden water leak.

Sources (1)

U.S. EPA: A Brief Guide to Mold, Moisture and Your Home.

Photo: Moldy Shower - Black Mold Growth in Bathroom (46195388355).jpg) by Tony Webster from Minneapolis, Minnesota, United States, CC BY-SA 2.0.

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