Mr. May, the author of ''My House Is Killing Me: The Home Guide for Families with Allergies and Asthma'' (Johns Hopkins University Press, 2001), said that while mold in a house is most often found on walls, floors, ceilings, carpets and fabrics exposed to moisture, one particularly troublesome hiding place is inside the ductwork and associated components of central forced-air heating and air-conditioning systems.
''This is the time of year my phone starts to ring off the hook,'' Mr. May said, explaining that as homeowners start up their central heating systems many find themselves suddenly coughing, wheezing and sneezing.
Mr. May said that while mold needs moisture, oxygen, a food source and a surface on which to grow, a mold spore in search of a home can come by those essentials relatively easily, even inside a central heating system.
That is because such a system constantly circulates mold spores found naturally in the air through parts of the system that often have dust on their surfaces.
Once a mold spore has embedded itself in that dust -- which provides the nutrients it needs -- all the spore needs is moisture. And that moisture, Mr. May said, can come from condensation produced by the air-conditioning coil, from a faulty humidifier attached to the system, or even from high levels of humidity in the air itself.
While it is possible for mold to grow in the ductwork of a central heating system, he said, it is more common to find it in the parts of the system that collect the most dust and have the greatest potential for being exposed to moisture: the air-conditioning coil and its fiberglass lining, and the cabinet that houses the blower fan.
''I've seen coil linings that were completely infiltrated by mold,'' Mr. May said, adding that when the heating system is turned on, the blower fan distributes mold spores throughout the house. ''Most people don't even know they have a problem until they start getting sick.''
In most cases, he said, the only way to determine with certainty whether mold is growing inside a central heating system is to gain access to the coil, its lining and the blower, and take a dust sample from the surface of the components.
That sample must then be examined under a microscope. ''I'm looking to see whether there's actively growing mold in the dust,'' Mr. May said.
He explained that he looks for actively growing mold because it is possible for mold spores from other areas of the house to get caught in the dust in the furnace.
''I'm trying to distinguish between mold that is being collected in the system and mold that is growing in it,'' he said.
The cost of an inspection can range from $200 to $1,000 or more, depending on scope and complexity.
If active mold is found in the heating system, Mr. May said, a homeowner should hire a professional remediation expert -- preferably one certified by the National Air Duct Cleaners Association, a trade group based in Washington -- to clean the furnace and the ductwork. Its Web site is www.nadca.com.
''You want to make sure the whole system is cleaned thoroughly,'' he said, explaining that in addition to removing any dust and mold in the blower cabinet and air-conditioning coil housing, a remediation expert should also clean the interior surfaces of the ductwork with a brush and a HEPA vacuum.
If there does not appear to be actively growing mold in the heating system, but members of a household have reason to suspect there is mold elsewhere in the house -- either because inactive spores were found in the heating system or because they are experiencing respiratory problems -- additional testing may be necessary.
Joshua Sarett, president of ALC Environmental, a testing and remediation firm in Manhattan, said that when his company inspects a house for a mold problem, the inspector takes air samples from both inside and outside.
''The outdoor reading gives you a baseline,'' he said, explaining that since mold spores are always in the air, it is necessary to look for a difference between the amount and type of mold spores inside and outside.
If high levels of hazardous mold spores are found in the house, Mr. Sarett said, the inspector will then make a visual inspection to determine where the mold is growing.
Ira Whitman, president of the Whitman Companies, an East Brunswick, N.J., company that specializes in environmental science, said that in most cases the most obvious indication of mold in a house is the presence of dark-colored spots on porous surfaces exposed to moisture.
If no obvious signs of mold are visible, Mr. Whitman said, it is possible that mold is growing inside a wall or ceiling.
''If you don't see the mold itself, look for signs of moisture,'' he said, explaining that leaks from apartments or appliances on the floor above can saturate the wood, wallboard and insulation in walls and ceilings, creating an ideal incubator.
Determining that mold exists in interior wall cavities, however, requires more work.
Damon Gersh, president of Maxons Restorations, a Manhattan company that specializes in restoring damaged property, said that to find mold behind walls it is often necessary to cut holes in wallboard or paneling to insert a small camera.
In most cases, he said, small amounts of visible mold on an exterior surface can be removed by scrubbing with a 5 or 10 percent solution of chlorine bleach in water.
Mold growing inside a wall, however, can be more difficult to remove. ''In some cases, we have to go in with respirators and Tyvek protective suits and take an entire room down to the studs,'' removing the interior surface of the walls, he said, adding that whenever a mold remediation project is completed, it is critical to make sure that the cause of the problem -- generally the infiltration of moisture into the house as a result of a leak from the roof or poor drainage -- is corrected.
The cost of such a cleanup can range from thousands to tens of thousands of dollars.
''You don't want to tear your house apart and put it back together,'' he said, ''only to have a new infestation six months down the road.''
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