What's the Difference Between Mold and Mildew?

mildew on books — These beautiful books are stored in one of Portugal's National Archives. Despite only being from the 1950s, the books are covered in mildew. Catarina Carvalho/Unsplash

Many people use the words mold and mildew interchangeably, but they are not the same thing. Yes, both are scientifically classified as fungi (spore-producing organisms), and there are some similarities between the two other than their scientific classification.

They are both filamentous fungi — multicellular fungi with branching tubular structures (hyphae) that form a mass of intertwining strands. They crave especially damp, moist conditions and both can grow on things in your home.

But mildew is usually easier to get rid of. And that goes to the heart of the differences between the mold and mildew.

What Is Mildew?

Mold and mildew spores are everywhere. By the time you see mold or mildew, you're looking at a colony of millions of spores.

Mildew is mold in its early stage and is typically fast growing. The spores are either water-, air- or insect-borne. Both mold and mildew like porous, organic material on which to grow: wood, paper, food, insulation, carpet or clothing — all the things in a normal home.

The biggest difference between mold and mildew is on the surface. That's a little play on words because mildew only grows on the surface of materials like your shower walls, windowsills and other places where moisture tends to be high. Mold, however, is a penetrating fungi (not a fun-guy at all) — that grows below the surface of whatever it's attached to.

So the powdery, fluffy spores you see on books and boxes in your basement? That's mildew. But good news! It's only on the surface.

If you find mildew, you can clean it off so the spores can't multiply or damage anything it's on. Then check the room's ventilation and maybe add a dehumidifier to the space. Bathrooms should have an exhaust fan to vent out warm, moist air. Check the seals around windows to prevent condensation.

What Is Mold?

molded house — Mold starts to grow within 24 hours of a home or structure flooded by water, and it can hide inside interior walls, causing structural damage. D_Townsend/Shutterstock

Mold is a different matter. It can be fuzzy or slimy. As we mentioned before, it grows on the surface of organic material and then penetrates it. Mold is usually white, blue, green, brown, gray or black.

Mold is only a problem when it grows indoors. Over time, the organic material that's covered with mold will become rotten. That's why mold often causes structural damage to homes and belongings, leading to mold remediation strategies and costly repairs.

Mold also can cause serious health problems. It produces allergens that can trigger severe allergic reactions, including hay-fever type symptoms — sneezing, runny nose, red eyes and skin rash.

Mold also can prompt asthma attacks in people who are allergic to it. Mold exposure also might cause irritation to eyes, nose, throat and lungs to people who aren't even allergic.

Doctors and scientists continue to research the effects of mold inhalation and in 2012 the National Institute for Occupational Safety and Health (NIOSH) published guidelines stating "studies have shown that exposures to building dampness and mold have been associated with respiratory symptoms, asthma, hypersensitivity pneumonitis, rhinosinusitis, bronchitis and respiratory infections. In 2019, NIOSH released updated guidance on how to best assess dampness in structures prevent health problems.

This type of exposure to mold would likely occur during cleanup of a home or building after a flood. The U.S. Centers of Disease Control and Prevention (CDC) says mold will start to grow within 24 to 48 hours in a flooded structure if it's not properly dried.

And unlike mildew, which you can easily clean yourself, the CDC says cleaning up toxic mold is very difficult and should be handled by qualified mold remediation professionals certified by the National Environmental Health Association (NEHA) or the American Industrial Hygiene Association (AIHA).