Mold is a critical part of the natural order, helping to decompose organic material and return nutrients to the soil. But there's nothing natural about a mold garden flowering in the crawlspace of your house. Certain strains of mold -- namely stachybotrys chartarum (aka "black mold") and the aspergillis family of molds -- produce mycotoxins that can cause serious health problems if ingested [source: Nolo.com]. But even allergies to more common molds can lead to hay fever-like symptoms, dizziness and skin rashes [source: Department of Health and Senior Services].
Mold spores are carried by the wind and breed in moist environments. During the construction boom years, when materials were scarce and time was tight, builders sometimes resorted to using damp plywood, sealing it up behind drywall before it had a chance to dry. Unlike hardwoods such as cedar and oak, plywood and other processed construction materials contain tons of natural sugars, a feast for mold [source: Lstiburek].
Even in new homes, mold will grow quickly wherever moisture infiltrates and festers in sealed areas. If pipes aren't properly insulated, they can produce condensation, which collects between walls. If an attic doesn't have sufficient ventilation, even the tiniest leak can produce a forest of mold. If the heating and air conditioning ducts aren't properly sealed, they can carry mold spores throughout the house, spreading allergens to unsuspecting residents [source: Garber].
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