You've seen all the headlines about dream houses lost to toxic mold, and your insurance company has already eliminated mold coverage from your homeowners' policy. BusinessWeek Senior Correspondent Larry Armstrong recently spoke with Jeffrey C. May, an air-quality expert and author of My House Is Killing Me! and The Mold Survival Guide (Johns Hopkins University Press), about how to determine if you have a mold problem and what you can do about it. Edited excerpts from their conversation follow. (Note: This is an extended, online-only version of the interview that appears in the June 21, 2004, issue of BusinessWeek.)
Q: What are the warning signs?
A: If you ever smell a musty smell, or if people tell you your clothing smells musty, you have a problem. Or if you're sick, and it's worse when you're at home. Chronic coughs, asthma, and sinus infections are often caused by mold.
I became asthmatic from two things: a portable air conditioner in my home office that was black with mold inside, and a moldy onion in my refrigerator drip tray. Every time the refrigerator cycled on, I'd be coughing and wheezing.
Q: Where is mold most likely to grow?
A: If you have central air conditioning, you're more likely to have a sick house. Then comes furnaces, because you have ductwork that often goes through cold, damp spaces. Moisture condenses inside and the ducts get moldy. And both have blowers that blow the allergens around, so you're always breathing it.
Q: What about basements?
A: I would say that over 90% of carpets in basements have mold or some other contamination -- they might have bacteria or yeasts or other organisms like mites. Not everybody is affected by it and, for the three-quarters of the people who don't have allergies, it's not a problem. But if you do, it can make you sick.
You'll commonly have mold in areas where the relative humidity is too high, and that's most often in finished basements or below-grade apartments, and in crawlspaces under the house. It's usually on cool outside walls and on furniture near the floor. Take a flashlight and shine it at a glancing angle across the wall and the mold almost lights up.
Q: So I can actually see mold?
A: It can be growing anywhere you have or have had leaks, so there'll be stains under plumbing or pipes or on ceilings. If you see a small patch of mildew under something that has been leaking a long time, there may be a lot of concealed mold that you have to be very careful about disturbing.
Q: What's the difference between mold and mildew?
A: They're basically the same thing.
Q: Can you prevent the problem?
A: Make sure there are no leaks or any kind of water intrusion from gutters or roofs or that sort of thing. And keep the relative humidity less than 50% in basements. Go out and buy a digital hygrometer -- a good one costs $30 or $40. If the relative humidity gets up to 70% or 80%, you know you're going to start getting mildew. Then you'll need to get a dehumidifier or use air conditioning to control it.
Q: What if you do everything you can do yourself to avoid mold and still have a problem?
A: The first step is to get an ASHI [American Society of Home Inspectors] home inspector who's knowledgeable about moisture. They don't charge an arm and a leg -- usually $300 or $400, or the going rate in your area -- and very often they can figure out what's causing the moisture problem and what the solutions might be.
In 80% of the cases, they can spot the problem with a visual inspection. And if not, they can usually recommend an industrial hygienist to take the next step, to do any testing that needs to be done. If you have to have someone come out to take samples, it will cost around $1,500. They can also recommend experts who can get rid of any mold you may have.
Q: Can you do any of this yourself?
A: You can take a sample from a surface with some Scotch tape and send it off to a lab to see if it's mold. And there are companies that will give you little bags so you can vacuum up the dust to see if you have a moldy carpet.
I wouldn't encourage people to buy the petri-dish kits that some hardware stores sell. They just take samples that settle out of the air and you have no idea where they come from: They can come from outside, from the carpet, from furniture, or whatever. They're a waste of money.
Q: Even on furniture?
A: Oh yes. There can be mold on antiques. People will purchase or inherit a piece of furniture that has been stored in someone's basement or a closed-up house for years. All of a sudden, their house will begin to smell. The finished surfaces may have been refinished, and the piece will look great. But the entire bottom and the unfinished parts of the back can be completely covered with mold.
The mold may never grow again, but the allergens will remain active for years. If they get disturbed, like when you open the bottom drawer of a dresser, they can still cause asthma and allergy symptoms.
Q: What's the most important thing a homeowner can do to prevent mold?
A: Keep the heating and air-conditioning equipment clean. Have the whole system -- the air conditioning coil, the blower, the ductwork -- cleaned, and then put in a good filter. Those inexpensive, throwaway filters are useless. You should only use pleated filters -- they're about $15 -- and avoid the washable types.
What you don't want to spend money on is an ultraviolet light. They'll charge you $500 or $800 and say that it will kill the mold. But if you have a contaminated air-conditioning system, it won't reduce the allergens at all.
The other thing is to keep your basement or crawlspace clean. Any mold there will travel up through the house. If you have a laundry in the basement, you'll bring the mold into the house on your clean clothes. The basement should be clean enough so you can eat off the floor.