The availability of mold spores, a suitable medium such as bread, suitable temperatures and considerable moisture are the four basic requirements for the growth of bread mold. Bread mold spores range in size from 3-40 microns and are too small to be seen unaided.
These mold spores are practically everywhere, so providing the other three requirements of bread, suitable temperatures and moisture are met, the growth of bread mold is guaranteed over time. Mold can grow on almost all substances that contain organic hydrocarbons, such as bread. Skin oils, soap residues, wood, paper and organic fibers are other examples of organic compounds that can serve as food for mold. Mold can grow at room temperature, but many molds prefer a slightly elevated temperature. Too high a temperature can kill a mold colony. Given time, mold can even grow in low temperature environments, such as refrigerators. Moisture is the key ingredient in mold growth though, with various species of mold having different water requirements. Water activity is defined as the amount of equivalent relative humidity in the air that would keep a mold-growing medium from drying out. Most molds require a water activity of above 70% to grow properly. Maintaining a dry environment is the surest way to retard mold growth.