The amount of moisture required for mold development is seldom addressed in the microbiological literature. In the laboratory molds are cultured in media with a high moisture content, but the precise level is seldom mentioned in their reports. The covered petrie dish creates a microclimate where the mold can flourish undisturbed. With regard to the growth of mold outside the laboratory, sources do indicate that the hygoscopic nature of materials affects the growth of mold. Materials which absorb and hold moisture from the air require lower levels of ambient relative humidity than do less hygroscopic materials. Thus, in a non-laboratory environment, the mold has at its disposal two sources of moisture, the air surrounding the item and the moisture held by the item itself.