Mold vs Mildew: What EPA Says and What to Do

U.S. Environmental Protection Agency (EPA) says molds are also called fungi or mildew. The word on a stain matters less than the moisture behind it, the size of the patch, and whether the surface is hard or porous.

The two words get used differently in everyday speech. EPA does not define mildew as a category of its own, so the difference people notice is about how the growth looks, not what kind of organism it is. Cleaning product labels that say "mold and mildew" should be read for the surfaces and steps they name.

Three things decide the next step, and EPA's guidance addresses each one: the moisture source, the size of the affected area, and whether the material underneath is hard or porous. A hard surface can be scrubbed and dried. Porous materials such as ceiling tiles and carpet may have to be thrown away once they become moldy.

EPA Says Molds Are Also Called Mildew

Its mold course defines molds as organisms found indoors and outdoors, and it says molds are "also called fungi or mildew." Molds are neither plants nor animals. They belong to the kingdom Fungi.

EPA's cleanup guidance is written for mold. Applying that guidance to what people call mildew is this site's reading, not an EPA statement.

For a reader with a dark spot on a bathroom ceiling, the order of work is the same either way: find the moisture, measure the patch, and then choose a cleanup path. The word on the stain does not change that order.

Moisture Decides Whether Mold or Mildew Grows

EPA says molds reproduce by producing microscopic spores, which are 2 to 100 microns in diameter and float through the air. Spores are found indoors and outdoors, and they cannot be eliminated from indoor environments. They will not grow if moisture is not present.

Growth needs moisture and food. EPA says moisture is the most important factor. Mold does not need much water to start: a little condensation in a bathroom or around a window sill can be enough. Mold can grow on virtually any organic substance.

EPA puts the principle in one line: "moisture control is the key to mold control." For a reader, that means finding the water source comes first.

Mold vs Mildew Is an Informal Split, Not an EPA Category

EPA does not define mildew as a separate organism, and it does not draw a line between mildew and mold. The split people use in everyday speech is informal. In that usage, mildew often means flat, powdery or surface-level growth, and mold often means thicker or deeper growth.

Color does not settle the question either. EPA says molds come in many colors, including white. The term "black mold" is not a species or a specific kind of mold, and neither is "toxic mold." The news media sometimes uses those terms for molds that may produce mycotoxins, or for Stachybotrys chartarum.

So a black patch is not automatically a different organism from a white one. The color names a shade, not a species.

Read the label for the surfaces it names and the steps it lists.

Looks and Smells Cannot Settle the Question Alone

No one can tell whether a mold is producing mycotoxins just by looking at it, according to EPA. A visible patch does not answer that question on its own, whether the word on the label is mold or mildew.

Smell gives a different signal. EPA says a moldy odor suggests mold is growing in the building and should be investigated. Strong smells come from microbial volatile organic compounds (mVOCs), according to EPA.

Health effects are another concern. EPA says inhaling or touching mold or mold spores may cause allergic reactions in sensitive individuals, dead or alive. It also says molds can cause asthma attacks in people who have asthma and are allergic to mold. Molds can irritate the eyes, skin, nose, throat and lungs of both mold-allergic and non-allergic people.

Dead mold still matters for cleanup. EPA says dead mold is allergenic and may cause allergic reactions, so killing mold is not enough. It must also be removed.

Cleanup Follows EPA's Mold Guidance

For hard surfaces, EPA says to scrub mold off with detergent and water and dry the surface completely. Absorbent or porous materials such as ceiling tiles and carpet may have to be thrown away if they become moldy. The EPA brief guide sets out those steps.

Size sets the path. EPA says that if the moldy area is less than about 10 square feet, roughly a 3 ft by 3 ft patch, in most cases you can handle the job yourself. A 3 ft by 3 ft patch is 9 square feet, which falls under that figure. If mold growth covers more than 10 square feet, or there has been a lot of water damage, EPA points to its Mold Remediation in Schools and Commercial Buildings guidance.

Timing matters too. EPA says it is important to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth.

Protect yourself while you clean. EPA says that to limit exposure you may want to wear an N-95 respirator, gloves and goggles without ventilation holes when cleaning moldy areas. EPA does not recommend chlorine bleach as a routine practice during mold cleanup. Never mix chlorine bleach with other cleaning solutions or with detergents that contain ammonia, because toxic fumes could be produced.

Do not paint or caulk moldy surfaces. EPA says to clean up the mold and dry the surfaces before painting. In a bathroom where spots keep reappearing, EPA says increasing ventilation, such as running a fan or opening a window, and cleaning more frequently will usually keep the growth to a minimum.

Side by Side, What EPA Says About Mold and Common Mildew Usage

This table sets the two words against each other. The appearance row is labeled as everyday usage because EPA does not define it.

Which Cleanup Path to Choose for Mold or Mildew

For most readers, the path depends on size and the water source, not the word. If the patch is less than about 10 square feet on a hard surface, EPA says in most cases you can handle the job yourself using its scrub, dry and ventilation steps. If the patch is larger or water damage is involved, EPA's remediation guidance is the next document to read.

This path does not serve a reader whose growth covers more than 10 square feet, or whose home has had a lot of water damage. For those cases, the EPA remediation guidance linked in the cleanup section is the document to follow. This page gives no health advice beyond EPA's published statements, so anyone with asthma or a known mold allergy needs guidance that goes further than this page.

The verdict would change if the patch grows past about 10 square feet, if the growth sits on porous material that EPA says may have to be thrown away, or if the growth returns after cleaning. EPA says that if you clean up mold but do not fix the water problem, the mold will most likely come back.

For mold vs mildew, measure the patch in square feet and find the moisture source before you pick up a brush.