Q: We are building a custom home and much of the wood used in framing it had a black fuzzy surface on it. I brought it to my contractor's attention and he said not to worry, that it's the way the wood comes from the mill. I believe about 80% of the wood used in our 3000-sq.-ft. home has this on it. I took a sample and sent it to a lab and it was determined to be cladosporium, penicillium and rhizopus/mucor. Should I be worried?

A: Based on what you have reported, the “black fuzzy stuff” appears to be mold growth. Wood from a mill can have fungal or mold growth often due to improper storing. Mold growth on the surface occurs if the wood is stored in wet conditions, which over long periods can result in decay that impacts structural integrity.

Typically wood is stored and treated to prevent such mold growth. After the wood is cut, it is usually kiln-dried to remove some of its moisture and stacked to assist in the drying process. Wood also has natural occurring antifungal chemicals; terpenes, the base of terpentine, is probably the most recognizable. These anti fungal chemicals inhibit the wood from decaying quickly under most conditions.

No building materials are sterile, so it is difficult to ascertain the level of the issue. It would seem that 80% of the lumber having visible growth is unusual and may indicate poor or extended storage in wet conditions. The mold types you listed are common outside types. They may cause allergic responses and in a few of the species (there are over 200 species in the genus Penicillium alone) there may be mold toxins.

Should you be worried? That depends on many factors. Will the wood be exposed to the inside of the house? Pathways between the mold and the building occupants are important in determining the mold’s impact on air quality. Is it in the attic or the bedroom? Although not desirable anywhere in the house, most of the time air rises in a building. The mold seen in an attic is not as great a threat as mold below a room or within the occupied spaces. Visible surface mold on wood is treatable with cleaning, sanding, disinfectants and encapsulants designed for treating wood.

The single most important factor in determining whether mold growth is hazardous to an individual is determining the individual’s preexisting health condition. Although no mold growth is desirable in a house, every breath we take is likely to contain mold spores. If you are allergic to mold or sensitive to mold irritants and the levels are high enough, reactions that can induce asthma attacks or other breathing problems can occur. If an individual has a compromised immune system, he/she may develop an opportunistic mold infection if the wrong mold species is present. However, this is a rare occurrence.

The chances are that if you are living in a wood frame house, there is some mold growth in the building. Most people coexist with these molds without symptoms as long as the growth is minor. I recommend that you do all that can be done to reduce the moldy materials being used and to treat any visible mold that is accessible. Either you or the contractor should reject lumber that is delivered with mold growth. The lumber should be kept as dry as possible on site and allowed to air dry before it is sealed into walls.

Thanks to Mark Goldman, who is a senior air quality scientist for Engineering and Fire Investigations (EFI), a firm that provides mold investigation and indoor air quality services nationwide. Mark can be reached at mark_goldman@efiglobal.com

QA archive >>

If you have a question for a mold expert, please send it to Brett Lyon, senior editor of construction.com at brett_lyon@mcgraw-hill.com.