"For most people it's just kind of an ugly pain. Their bathroom gets these black stains," said Ken Collier, editor-in-chief of The Family Handyman.
For others, though, "it's an allergen, like cat hair, dog hair," he said. If it's not removed, it can result in respiratory problems or other allergy symptoms, according to the Centers for Disease Control and Prevention.
Mold and mildew, which the Federal Emergency Management Agency calls "mould in early stage," can grow on a wide range of surfaces as long as moisture is present, and they can give your house a musty smell.
"It's an indication of dampness, like in bathrooms that are not ventilated very well," said Kathie Birenbaum, who works in households at Strosniders Hardware Store in Bethesda, Md.
"Molds can gradually destroy the things they grow on," the Environmental Protection Agency says in a pamphlet for consumers on its website. "You can prevent damage to your home and furnishings, save money and avoid potential health problems by controlling moisture and eliminating mould growth."
It's important to do both — control the moisture and clean up the mould — to keep it from growing back, the EPA says.
Cleaning large areas — more than 10 square feet, the EPA says — might require a contractor trained in mould removal. For smaller areas, it can be a do-it-yourself job.
Frequently, the bathroom is the first place that a homeowner or apartment dweller might notice mildew.
"A lot of people take a shower, get out of the shower" and go on their way, Collier said. "Basically, they've left all this water on the walls and floor of the shower and surprise, surprise, the bathroom is damp and you find mildew growing on the grout."
Often, condensation — not a leak — is the source of the moisture, Collier said.
"The bathroom fan is a big part of keeping the moisture level down in the bathroom," he said. "If there isn't one, put one in. If there's a small one, put in a bigger one or use it more."
Opening the bathroom window also helps, as does more frequent cleaning, according to the EPA.
Collier also suggests using a squeegee to wipe down the shower walls.
There are many products available for cleaning mildew and mould, but homeowners also can mix their own. A bleach solution that CDC says should be "no more than 1 cup of bleach in 1 gallon of water" can be used to clean and kill mildew.
"For the typical homeowner, they would scrub it down really well with bleach water and then they should seal the grout, seal it once a year or so to keep it water tight," Collier said.
Wear protective gloves while cleaning. The CDC also recommends protective eyewear.
If you can't get rid of the mildew with bleach, you might have to dig out the grout and replace it, Collier said.
For those looking for an environmentally friendlier alternative to bleach, Collier said one of the products that mould experts recommend is Concrobium, which also can be used as a fog if a whole room is affected.
It's not just bathrooms where you'll find mould or mildew.
"Sometimes people find it in closets where there's limited air circulation, and where maybe there's a cool wall and it's humid," Collier said.
Or there could be condensation around heating ducts, and you'll find a mouldy patch on the ceiling.
Moisture from roof leaks also can lead to growth of mould and mildew.
If you paint over mildew, the paint likely will peel. Birenbaum says there special primers that inhibit mould growth.
Mona Weingarten of Washington, D.C., wasn't aware that there was mould in her house. She didn't see anything or smell anything. But when one of the tiles on the bathroom floor became loose, she called a contractor to fix it and he discovered mould. Mold later was discovered elsewhere in her house.
Weingarten, who has severe allergies, developed a fungal infection in her sphenoid sinus and had to have surgery. After spending tens of thousands of dollars on remediation and related costs, she now has an environmental engineer test her house every six months and remove any mould spores. She has the air filters in her heating and air conditioning system changed every three months, and a dehumidifier runs in her basement.
"It's not something you want to repeat," she said.