Mold Resource Guide

Mold - Consumers panic over mold. Buyers are frantic about mold and sellers are frightened of it. Real estate brokers are intimidated by mold, and home inspectors are leery of it. People are moving out of their homes, and some are even having their houses demolished because of mold growth.

Outdoor mold is part of the natural world, but that doesn't mean we should live in its midst indoors. Here The Money Pit's indoor air quality expert, Jeffrey C. May, principal scientist of May Indoor Air Investigations LLC and co-author of The Mold Survival Guide: For Your Home and for Your Health (Johns Hopkins University Press, 2006), shares a few of the many questions people have asked him, and the answers to their questions. For information about other books he has written, see www.myhouseiskillingme.com.

What's the Big Deal?

Q: Why all the fuss? Haven't we been living with mold for centuries?

A: Well, yes, we have, but modern building materials like drywall are vulnerable to mold growth. Complicated heating and cooling systems, as well as air-tight buildings that aren't ventilated adequately, can make an indoor mold problem worse. Some kinds of molds cause allergy or asthma symptoms in those who are sensitized, so it's important to avoid the presence of mold growth indoors. If that's not reason enough, look at some of the dollar figures:

The cost of a single mold claim can range from $10,000 to $100,000 for the investigation, remediation, and additional living expenses, if the property owner has to move out.
In Texas in 2002, mold costs "surged to $2.3 billion."
Hilton's Kalia Tower in Hawaii had to close shortly after opening, due to mold contamination, the remediation cost over $50 million.

Q: Is mold growth more worrisome in some areas of the home than in others?

Mold growing on a nylon fiber on an almost new basement carpet.

A: Yes. I'm more worried about mold growing in a basement carpet, where air flows can carry spores and other byproducts of mold growth into the rooms above, than I am about mold growth in an unfinished attic, where air flows leak to the exterior (carrying the spores back to where they belong!).

What Are We Talking About?

Q: Will mold destroy the framing of my house?

A: Though the terms "mold" and "mildew" are generally used interchangeably, there are generally speaking two broadly different kinds of fungi:

Mushroom in the dirt of a basement wine cellar.
Fleshy macrofungi, which grow fruiting bodies that we call mushrooms or toadstools.
Mildew on a basement filing cabinet.
Powdery microfungi, or what laymen call mold or mildew, which produce microscopic spores along the surface of the fungal growth, causing spotty, light or dark patches.

Macrofungi can degrade cellulose (the main component in wood) and thus can cause structural damage in buildings, if the wood is wet enough and the fungal growth is extensive. On the other hand, with some exceptions, microfungi do not degrade structural cellulose (or they degrade it only at the surface) and thus do not pose structural threats to buildings; they do, however, produce spores that can impact health. Mildew growth indoors, therefore, is a potential health hazard to those who are sensitized (who react to the spores or fragments arising from a particular mold).

Q: What exactly is that dreaded "black toxic mold" that seems to be in the news a lot?

Black Stachybotrys chartarum mold on basement drywall.

A: Stachybotrys chartarum is often referred to as "toxic black mold," because it's black, and under certain conditions, this type of microfungus, along with others, such as species of Aspergillus and Fusarium molds, can produce mycotoxins which have an impact on human health. (Other kinds of molds can be black, too, including Cladosporium mold, the most common mold, and "toxic molds" produce mycotoxins only under certain conditions, and not all the time.) I'm not as worried about Stachybotrys chartarum as I am about species of Aspergillus mold, though, for several reasons.

Stachybotrys chartarum spores.

Stachybotrys chartarum requires wet conditions (steady leaks or a floor flood) in order to thrive, and thus is not as common as Aspergillus molds, which can grow in conditions of elevated relative humidity. And Aspergillus spores are smaller than Stachybotrys spores, and thus can remain airborne longer and are more respirable.

Worrying excessively about toxic molds takes the focus away from what I consider a greater threat: mold allergens that, in general, can affect those who are sensitized.

Identifying Mold

Q: My house smells musty, but I can't see mold anywhere. How can I clean it up if I can't find it?

A: Some mold growth is pretty obvious, if you know where to look: in closets that have exterior walls (walls that face the exterior of the building), and on the unfinished, wooden backs and bottoms of furniture that has been stored in damp spaces, for example. Sometimes, shining a flashlight sideways along a surface will reveal the fuzzy, often circular patterns of mold growth. Be careful, however, in your search not to move possessions around, and thus disturb any mold present and aerosolize (make airborne) spores. An air quality professional can help you find the sources of your musty odor.

Q: I think I have mold in my home. How can I have it tested? How do I know if the results are reliable?

A: There are several methods to test for mold, most of which are done by professionals: bulk sampling (removing some contaminated material), air sampling (taking samples of air with various instruments to find bioaerosol, or airborne particulate matter arising from living things, such as mold), and surface sampling (the removal of dust with a vacuum device, sticky tape or a swab). These samples are sent to a laboratory for analysis. Testing can be helpful if conducted correctly through, in my opinion, too many people, and that includes so-called "professional mold testers," don't understand the relationship between contaminants found in the air or dust, and the sources from which they came. And too many labs incorrectly analyze the samples, or write testing reports that are unintelligible to those who aren't scientifically trained.

If you decide to hire someone to test for mold in your home, do your homework: request information about the person's training and education, ask how the sampling will be done, seek references, and find out what information will be included in the report. In the end, though, the most important thing to do is to figure out why the mold is growing where it is, and to change the conditions so that, after you have the mold remediated, it will not return. This involves hiring someone who understands buildings before you test; a good place to start is with a home inspector who is a member of the American Society of Home Inspectors (ASHI).

Q: I decided to test for mold myself, so I bought a Petri-dish, home mold test kit. The lab results came back saying there was Penicillium mold in every room (the living room had eleven colonies) and a lot of Stachybotrys mold in the bedroom. What do you think I should do next?

A: The kind of Petri-dish (settle-plate) testing you refer to uses a small, flat dish containing a mold nutrient. The dish is supposed to be left open for a set period of time; then closed up, and allowed to stand for several days, either in your home or, for an additional cost, in the lab you send the dish to for analysis. Either you or the lab counts the colonies that grow. I don't trust the results of such tests, for a number of reasons. Some people leave the dish open too long, or have the test running with the windows open. And the recent rapid increase in the number of consultants investigating mold problems, and the proliferation of laboratories providing analyses (and perhaps using unqualified technicians to do so) have led to many misleading reports. So the first step to take is to have the testing done by professionals and the results analyzed by a qualified lab. If this second test confirms your original results, hire professionals to find and eradicate the sources of contamination (under containment conditions).

Q: There's a fuzzy white growth on my basement walls. Is that mold?

Efflorescence on a block foundation.

A: It may be mold, but it's more probably efflorescence: crystals of mineral deposits left behind when moisture migrates through a foundation wall. Efflorescence flakes will dissolve in vinegar or dry up into powder, which mold will not.

Q: There's green staining on the outside of my house. Could this be mold? And could it spread indoors?

A: If it's the color of plant leaves (chlorophyll green), what you describe is most likely algae or moss, and not a concern (except cosmetic, perhaps). However, if you have mushrooms growing on the exterior siding of your house, hire a building investigator pronto! The growth will not spread indoors.

Q: There are vertical, black stripes all over the exterior walls on the inside of my house. I'm sure it's mold. How do I get rid of it?

Soot on a wall above an electric baseboard convector.

A: What you describe is most likely soot, which deposits on the walls in stripes where framing studs are located, in a flame-like pattern above baseboard convectors, in dots where nails are located, or circular stains on ceilings above light fixtures.

Soot on a ceiling above a bathroom fixture.

Stop burning jar candles (some can emit a lot of soot, particularly after the candle has burned down about half way), and repaint. (You can test the stain by rubbing it with a little bleach on a paper towel; if the black color remains, it's soot. If the color goes away, it's mold.)

Avoiding Mold

Q: With homes being built tighter and tighter, are there ways to construct homes to reduce the risks of having a mold problem?

A: The tighter a home is, the more likely the build-up of moisture. Mildew on exterior and bathroom walls is common, and reducing the relative humidity indoors stops these problems. Indoor relative humidity is controlled with proper use of efficient kitchen and bathroom exhaust fans. And of course, dryers must be vented to the exterior. A heat-recovery ventilation (HRV) system can also be installed, to bring in fresh air and exhaust stale indoor air, but to avoid mold, these systems must be maintained scrupulously clean, and have pleated media filters with an efficiency rating of at least MERV 6.

Q: I have heard that drywall is a favorite mold food. Is there an alternative?

A: Drywall is like a baloney sandwich, where the paper on the two outer faces is the "bread," and the gypsum stuck in the middle is the baloney. Gypsum is a mineral, and mold cannot digest this "baloney," but the cellulose in the paper can be degraded by some molds. The drywall paper is like bread in another way; the paper contains up to 1% starch, another nutrient for mold. An alternative to traditional, paper-faced drywall is DensArmor® Plus (Georgia Pacific), a relatively new type of drywall in which the paper faces have been replaced by thin, fiberglass mats. Fiberglass, like gypsum, is a mineral, so there are no nutrients on which the mold can grow.

Q: What can I do to make sure I don't develop a mold problem in the future?

A: The most common places for mold growth are carpets in finished basements, and dusty, dirty liners (insulation) in heating and air-conditioning equipment. Using a dehumidifier and keeping the relative humidity in a basement at no more than 50% will keep mold from growing in carpeting, though it's also safer to use resilient or ceramic-tile flooring instead of carpeting. Using pleated media filters (with at least a MERV 6 rating) in heating/AC systems will minimize the chances of mold growth in these systems. Also, make sure that construction dust, especially sawdust, is prevented from entering the duct system during construction or renovation.

Another option is to install a whole-home dehumidifier. Unlike portable dehumidifiers which serve just a small area and must be emptied constantly, a whole home unit is permanently installed by a qualified HVAC technician into your heating and cooling system. It runs 24/7 to automatically reduce the amount of moisture in the air in your home. Aprilaire has been a leader in whole-home dehumidification. Their unit can remove as much as 90 pints of water a day from the air inside your home. Just remember that it is essential to clean the air filter on the humidifier several times a year; check at the same time that the cooling coil is spotlessly clean because mold can grow in the dust on the wet coil of any dirty dehumidifier. If you don't like washing the filter regularly, check with Aprilaire to see what throw-away, pleated media filter they recommend as a replacement for the washable filter.

Mold in a Mechanical System

Q: Our air conditioning smells like sweat socks. Could this be mold?

Black mold on the coil of a window air conditioner.

A: A smell like this usually signals the growth of bacteria or yeast, rather than the growth of mold (though all three microorganisms may be present and producing odor). If you have hot-air heat and/or central air conditioning (or a heat pump), it's important to keep the system as clean as you can, and have adequate filtration; use only disposable pleated media filters with a MERV rating of 6 or higher.

Q: Ever since we moved into our new house, our son has been constantly coughing. Now his doctor tells me he's asthmatic. He was fine before. Could the house be a problem?

A: You don't mention whether you have a family history of allergies and asthma, and I'm not a physician, so continue to discuss this situation with your doctor. That said, I don't recommend that people with mold sensitivities or allergies live in houses with hot-air heat, central air conditioning, and wall-to-wall carpeting. If your son's symptoms improve when he is out of the house for an extended time (for a family vacation, for example), I'd recommend that you have an air quality professional do some air and dust sampling for you, to see if you have sources of airborne allergens that may be affecting your son's health.

Mold in the Attic

Q: There's black mold in the attic of my three-year-old house. How can I tell if the builder used contaminated lumber?

Black mold on attic sheathing.

A: If you look at the black mold, you should be able to tell whether it was on the wood to begin with or grew after the house was built. If the rafters have black colonies on them, most likely, the wood was contaminated to begin with. In cold climates, if the sheathing is darkest on the north- or east-facing gable (worse near the outer, lower part of the sheathing), or if the mold appears only at the top of the rafters, mold may be growing due to excess moisture, most often caused by improperly vented bathroom exhausts and leaky attic accesses that allow warm, moist house air into the attic. But if you see oval- or other-shaped colonies that appear to be covered and bisected by rafters, the sheathing may have been moldy when installed.

Q: We just added a humidifier to our hot-air heating system, because the air in the house was so dry. Now I am noticing mold in the attic for the first time. Could the humidifier be causing mold to grow?

A: Yes. If there are any supply ducts in the attic, check to see if the ducts are leaking. which can lead to condensation and mold growth. In addition, be sure that there is adequate ventilation in the attic space, and that there are no openings through which house air can leak into the attic. However, the mold may have already been growing there, due to excess moisture from other sources: a bathroom venting up into the attic, or moist airflows entering the attic around recessed light fixtures in the ceilings below. If you already have ridge and soffit vents, do all you can to reduce or eliminate the sources of house air flows into the attic. As for the mold growth itself, small areas can be HEPA vacuumed (a HEPA vacuum cleaner has filtration that prevents the release of particulates in the exhaust and should, in my opinion, always be used for even everyday cleaning) and spray-painted to contain allergenic and irritating dust. Always wear at least a NIOSH-rated N95 mask or respirator when cleaning mold. See www.health.state.ny.us/environmental.

Mold in the Basement

Q: When my children romp around on the first floor of my house above the basement laundry, dust falls out of the exposed fiberglass insulation in the basement ceiling below. Is this something to worry about?

A: This could be a problem if your family has allergies and the dust is moldy or contains dander from a previous owner's pet. In addition, the fiberglass could be infested with mice or even with mold-eating mites: all sources of allergens that can find their way upstairs on air flows.

Q: We have a family room in our basement. We just discovered that our air conditioning unit down there has been leaking into the carpet. I've been dehumidifying the space, but it still smells musty. What do you recommend?

Aspergillus mold on a desk in a basement office that flooded.

A: First of all, if anyone in your family has allergies or asthma, I recommend that you avoid living in a house with a finished basement, which is particularly prone to mold growth. If you want to keep this finished basement family room, I'd recommend hiring a professional to remove the carpeting (under containment conditions) and then having a ceramic or resilient tile floor installed, with area rugs on top, if you want to have a softer floor covering. As for the carpet, anything fleecy or cushioned (and that includes upholstered furniture) that has remained damp for more than 24-48 hours should, in my opinion, be discarded.

Q: I live in an apartment building where each tenant has a storage space in the basement. The space next to mine smells really moldy. Is there anything I can do about this?

A: If your neighbor has laid cardboard boxes or other biodegradable materials directly on the floor or up against the wall, there is a strong chance that these possessions contain mold growth. Personal goods should be stored on plastic or metal shelving, away from the wall and up off the floor. There may also be a leak in that area of the basement. This isn't your storage area, but still, the air you breathe while in the basement is being affected; in addition, up to a third of the air in a small residential building can come from the basement, due to the stack effect (warm air rises). If I were you, I'd speak to your neighbor as well as to the building management about this problem, to see if you can get the space cleaned up (professionally, please!). Any leaks that are present should be repaired, and the basement should be dehumidified (with the relative humidity less than 50%) in the warmer months.

Q: Can mold in my basement spread throughout the house?

A: No. Mold needs moisture to grow, and unless there are leaks, basement mold doesn't mean that there is mold anywhere else in the house. The spores, however, from basement mold can travel throughout the house on air flows, but they are just part of house dust and cannot grow into colonies unless there is dampness present. That does not mean, however, that there is no reason for concern, because mold spores can cause symptoms in those who are sensitized.

Mold in between the Attic and Basement

Q: I keep bleaching mold off my bathroom walls, and it keeps growing back. How come?

A: There's too much moisture in the room. Thoroughly air the room out after you shower: always leave the door open and operate the ceiling exhaust fan (which should be vented to the exterior) for at least 20 minutes. In addition, in warm weather, leave a window open. A small, oscillating, tabletop fan can speed drying (but be sure to plug the fan into a GFI outlet) in all seasons. Hang damp towels to dry in another space.

Q: We stayed for a few days in a relative's house, and when we came back, our clothes smelled moldy. Can mold be carried from place to place on clothing?

A: Mold odors can be pervasive, even though the mold growth may be limited in scope. Despite the strong odor, however, there may not be any mold spores on your clothing. If, on the other hand, the mold growth in the house was disturbed (if you sat on a moldy couch or rug), your clothing may be carrying spores and even mold-eating mites. This isn't that different than getting pollen or pet dander on your clothing. You can wash or dry-clean your clothing. Figuring out the source of the moldy smell in your relative's house is another matter!

Q: How can I tell if my house is too damp, and thus conducive to mold growth?

A: Use a hygrometer to monitor the relative humidity (RH). Expect to spend at least $30 to $40 for a quality hygrometer. Measure in the middle of the room, as well as at cold, exterior walls. In the winter in cold climates, the RH should be around 35% and not go much over 40%. In the summer, the RH in the basement should not be higher than 50%. Some molds can grow at 70 to 85% RH.

Mold and Real Estate

Q: I am considering buying a house, but every time I visit the property, as soon as I get inside the front door I notice a distinct "moldy" smell. This goes away after a few minutes as I get used to it. Is it possible to fix this problem?

Mildew on wallpaper at an exterior wall.

A: If you or anyone in your family is sensitized to mold, I'd be very cautious about purchasing a house with a musty odor, since you don't know the potential extent of any mold growth present. (I'd also be cautious about buying a house in which the owner burns scented candles or has plug-in fragrance emitters, which may be masking a musty smell.) It's possible that the steps you could take to combat the mold would be very simple, but it's also possible that there could be a major concealed mold problem, and that any remediation required would be extensive and costly. The most prudent thing to do would be to try to determine the source of the mold odor before you purchase the house. This could mean making holes in the walls to investigate.

Cleaning Up Mold

Q: When can I clean up mold myself?

A: Small mold clean-up jobs (defined by the New York City Department of Health and Mental Hygiene, Bureau of Environmental and Occupational Disease Epidemiology, as "ten square feet or less" of mold growth(¹) can be tackled by occupants, as long as there is no one present who is sensitized to mold. Larger clean-up jobs should be handled by professionals. Before you tackle any mold problem, however, gather information (refer to the EPA publication, "A Brief Guide to Mold, Moisture and Your Home"), and see your physician.

Q: There seem to be a growing number of contractors who specialize in mold removal. How do I know which ones are qualified?

A: Ask for references from your insurance company and references from past clients, and seek information from professional training and certification organizations, such as the American Indoor Air Quality Council (www.iaqcouncil.org) and the Indoor Air Quality Association (www.iaqa.org). I also have a lot of information about professional remediation in Chapter 11 of my book The Mold Survival Guide: For Your Home and for Your Health (www.myhouseiskillingme.com). Specialized training is important in this kind of work, so find out what sort of education and certification a contractor has.

Q: Will my insurance company cover mold remediation work?

A: You'll have to refer to your policy or ask your insurance agent, but because mold remediation can be very expensive, more and more insurance companies are instituting coverage limits or charging extra for mold riders. One of my clients did have mold coverage in her insurance policy, but once the work was completed and paid for, her insurance company terminated her policy because she was now "high risk." Then her lender considered her mortgage in default because she didn't have insurance. So if the cost isn't too high, I'd be cautious about asking for insurance to pay for a mold clean-up, because it's a slippery slope.

Q: Once I get rid of mold, is it gone forever?

A: It's important to remember that both microfungi and macrofungi are living organisms. Mildew can grow within 24 to 48 hours whenever air, moisture (from leaks, spills, high relative humidity, and condensation), and food sources (including skin scales and insect body parts in dust, starch and cellulose in drywall, cardboard, and paper) are present. You can remove mildew growth through cleaning, but the mildew will likely reappear unless you change the conditions that were conducive to its growth in the first place. How do you do that? Control moisture (including relative humidity), and you control mold growth. Keep below-grade spaces consistently heated (to at least 60 degrees) in the cooler months and dehumidified (below 50% relative humidity) in the warmer months. Repair leaks as soon as they are discovered. Don't over-water plants. Avoid over-humidifying your home.

More Research

Q: Where can I find more information about mold?

A: There are a number of helpful publications as well as websites, many of which are listed on www.MoldUpdate.com. But here's a list of some to help get you started in your research.

Some Websites for Organizations:

Asthma and Allergy Foundation of America = www.aafa.org
Canadian Mortgage and Housing Corporation = www.cmhc-schl.gc.ca
Centers for Disease Control and Prevention (CDC), National Center for Environmental Health = www.cdc.gov
Children's Environmental Health Network = www.cehn.org/new
Indoor Air Quality Association (IAQA) = www.iaqa.org
Institution of Inspection, Cleaning, and Restoration Certification (IICRC) = www.iicrc.org
National Air Duct Cleaners Association = www.nadca.com
The New York City Department of Health, Bureau of Environmental and Occupational Disease Epidemiology = www.health.state.ny.us/environmental
Northeast Document Conservation Center = www.nedcc.org
Restoration Consultants = www.restcon.com
U.S. Environmental Protection Agency (EPA), National Center for Environmental Publications (NSCEP) = www.epa.gov/iaq

Some Publications:

"A Brief Guide to Mold, Moisture, and Your Home", EPA 402-K-02-003
"Clean-up Procedures for Mold in Houses", Canada Mortgage and Housing Corporation
"Flood Cleanup: Avoiding Indoor Air Quality Problems", EPA 402-F-93-005
"Guidelines for Assessment and Remediation of Fungi in Indoor Environments", New York City Department of Health and Mental Hygiene, Bureau of Environmental and Occupational Disease Epidemiology
"The Mold Survival Guide: For Your Home and for Your Health", Jeffrey C. May, Johns Hopkins University Press, 2004
"Policy Statement: Toxic Effects of Indoor Molds", Pediatrics (American Academy of Pediatrics), vol. 101, no. 4 (April, 1998), 712-14

About Jeffrey C. May

Over the last 18 years, Jeffrey C. May, a building consultant and Certified Indoor Air Quality Professional (CIAQP, Association of Energy Engineers), has investigated thousands of moisture, odor, mold, and indoor air quality problems in homes, offices, schools, libraries, churches, and even automobiles, and he has collected and analyzed over 20,000 air and dust samples. Jeff gives presentations on indoor air quality all over the country and is author of My House is Killing Me! The Home Guide for Families with Allergies and Asthma (2001) and My Office is Killing Me! The Sick Building Survival Guide (2006), as well as co-author of The Mold Survival Guide: For Your Home and for Your Health (2004), all published by Johns Hopkins University Press (you can find out more about Jeff and his books on www.mayindoorair.com and www.myhouseiskillingme.com). A former teacher and organic chemist (M.A. Harvard University), Jeff is principal scientist of May Indoor Air Investigations LLC in Cambridge, MA.

Footnotes

¹ Guidelines on Assessment and Remediation of Fungi in Indoor Environments (New York: New York City Department of Health and Mental Hygiene, Bureau of Environmental and Occupational Disease