Mold vs mildew: what's the difference and why it matters

What is the difference between mold and mildew?

Mildew is a kind of mold, not a separate organism. EPA’s definition: “Mildew refers to certain kinds of mold or fungus. The term mildew is often used generically to refer to mold growth, usually with a flat growth habit.” EPA adds that mildew often lives on shower walls, windowsills and other places where moisture is high.

Mold is the broader term. EPA says molds include all species of microscopic fungi that grow as multicellular filaments called hyphae. They can grow on almost any organic material, including clothing, leather, paper, and the ceilings, walls and floors of homes with moisture problems. Mold spreads by tiny spores that EPA says float through indoor and outdoor air all the time and start growing when they land on a damp spot.

So when people ask “mold or mildew?”, they are usually asking a practical question: is this a surface problem I can wipe off, or has it grown into the material? That depends far more on where it is growing than on what it is called. CDC’s guidance supports this approach. It says you do not need to know the type of mold; if you see or smell it, remove it and fix the moisture. For identification, health and cleanup topics, start at our mold hub.

How can you tell mildew from mold?

There is no lab-free test that separates the two, because mildew is a type of mold. What you can judge is how the growth looks and what it sits on, which tells you how hard it will be to remove. The table uses EPA’s descriptions.

If the spots are pink or orange slime rather than flat specks, see our pink mold guide. If they are white and crusty on concrete, see white mold vs efflorescence.

Is mildew dangerous?

Mildew carries the same kinds of health effects as other indoor molds, because it is mold. EPA says molds produce allergens and irritants. Breathing or touching mold or spores may cause allergic reactions in sensitive people, such as sneezing, runny nose, red eyes and skin rash. Molds can trigger asthma attacks in people with asthma who are allergic to mold, and can irritate the eyes, skin, nose, throat and lungs of allergic and non-allergic people alike.

CDC says exposure to damp and moldy environments “may cause a variety of health effects, or none at all.” People with asthma or mold allergy may react more severely, and people with weakened immune systems or chronic lung disease can develop lung infections from mold. CDC’s Stachybotrys fact sheet says all molds should be treated the same with respect to health risks and removal.

What changes the risk in practice is how much growth there is and who is exposed. A few spots of shower mildew are a far smaller exposure than a large patch on a basement wall. But “it’s only mildew” is not a reason to leave it. Our mold symptoms guide lays out what the evidence does and does not support, and our black mold guide explains why color does not signal danger.

What does mildew smell like, and what causes it?

The musty smell people call “mildew smell” comes from chemicals mold gives off as it grows. EPA calls them microbial volatile organic compounds, or mVOCs. Because they are volatile, they are released into the air quickly, and because they often have strong or unpleasant odors, they produce the “moldy odor” or musty smell associated with mold growth.

EPA notes that in unaired places like basements, molds can produce a strong musty odor. The smell is useful information: EPA says a moldy odor suggests mold is growing in the building and should be investigated. A persistent musty smell with no visible growth often points to mold in a hidden spot, such as behind drywall or wallpaper, under carpet, or around leaking pipes inside a wall, all of which EPA lists as common hidden-mold locations. Our guide to signs of mold in a house covers how to look.

On health effects, EPA says the effects of breathing mVOCs are “largely unknown.” Exposure has been linked to symptoms such as headaches, nasal irritation, dizziness, fatigue and nausea, but EPA says more research is needed to know whether everyday home exposures cause any health effects. Air fresheners mask the smell without changing the moisture problem.

Why do mold and mildew grow where they do?

Both need water. EPA says there are many types of mold, “and none of them will grow without water or moisture.” Spores are always present indoors, so the deciding factor is whether a surface stays damp.

That is why mildew is a bathroom regular. Showers create steam, tile and grout stay wet, and soap scum gives mold something to grow on. Windowsills collect condensation when warm indoor air meets cold glass. EPA’s brief guide shows condensation on the inside of a windowpane and says condensation can be a sign of high humidity.

Deeper mold growth usually follows a bigger or longer-lasting water source: a roof or plumbing leak, a flood, a wet crawl space, or high humidity in a closed-up room. EPA’s key rule is timing. If wet or damp materials are dried within 24 to 48 hours after a leak or spill, in most cases mold will not grow. CDC recommends keeping indoor humidity no higher than 50 percent, and EPA says below 60 percent, ideally 30 to 50 percent. Our mold prevention guide covers ventilation, dehumidifiers and humidity targets in more detail.

How do you clean mildew and surface mold?

On hard, nonporous surfaces like tile, tubs, glass, sealed countertops and painted trim, EPA’s method is simple: scrub the mold off with detergent and water, then dry completely. Fix the source of moisture, or it will return.

EPA does not recommend using bleach or other biocides as a routine practice during mold cleanup. EPA says it is usually not possible or desirable to sterilize an area, and that dead mold may still cause allergic reactions, so it must be removed, not just killed. If you choose to use bleach on a hard surface, CDC says use no more than 1 cup of household bleach in 1 gallon of water, open windows and doors, and never mix bleach with ammonia or other cleaners because it produces a poisonous gas.

Wear protection. CDC recommends at least a NIOSH Approved N95 respirator, non-latex gloves and goggles without vent holes. For bathrooms specifically, EPA says that if shower mold keeps reappearing, increasing ventilation (running a fan or opening a window) and cleaning more often will usually prevent it or keep it to a minimum. Our bathroom mold guide covers caulk and grout, and what actually kills mold compares cleaners.

When is it more than mildew?

It is time to treat the problem as a mold job, not routine cleaning, in a few situations.

It is in a porous material. EPA says absorbent materials such as ceiling tiles and carpet may have to be thrown away if they become moldy, because mold can grow into the crevices and be difficult or impossible to remove completely. The same applies to drywall that has stayed wet; see our guide to mold on drywall.

It is big. EPA says that if the moldy area is less than about 10 square feet, roughly a 3 ft by 3 ft patch, you can usually handle it yourself. Larger areas, or areas with a lot of water damage, call for professional help that follows EPA’s remediation guidance or professional standards.

It keeps coming back. Recurring spots on walls or ceilings, as opposed to a shower, suggest a leak, condensation inside the wall, or a humidity problem that cleaning will not fix.

It involves HVAC or contaminated water. EPA says not to run an HVAC system you suspect is contaminated, and to bring in a professional when sewage or other contaminated water caused the damage.

For what professional work costs, see our mold remediation cost guide, and for how to vet a company, our guide to choosing a mold remediation company.