Basic requirements for mold growth
When dealing with flood emergencies, it's important to keep in mind that there are five basic requirements for mold to grow:
Organic Food Source -- Of course, myriad building components and materials serve the purpose here, especially those made of cellulose (paper, wood).
Moisture -- No moisture, no mold. But even high humidity (water activity) on the surface of organic materials or even inorganic materials with a biofilm can be sufficient to change dormant mold spores in the air we are breathing right now into active fungus growth. Under the right conditions, mold growth and amplification is logarythmic; i.e., it multiplies rapidly. Of the 3000 species of mold, 40 or so are considered toxigenic.
Temperature -- The ideal range is 68-86 degrees F, with around 78 degrees F (25 degrees C) being optimum for lab cultures. By way of comparison, most water damaged structures start out at 75-80 degrees F.
Stagnant air -- Molds simply don't like rapid moving air for two reasons: It makes them difficult to concentrate for rapid growth, and rapid air movement causes evaporative cooling.
Time -- Molds don't just overwhelm a structure overnight. Typically, they take three to four days to get a foothold. Amplification to a problem stage may take a few days longer. Of far greater importance is the amplification of bacteria from normal household soils, which degrades the condition of a clean water release changing it to Category 2 ("gray") or even Category 3 ("black") water in two to four days (ref. IICRC S500).
This is the reason we use the quantity of airmovers specified above. With that level of air movement, we deny mold two of the conditions it needs for rapid growth and amplification -- stagnant air an optimum temperature. Again, rapid air movement not only stirs up the air considerably, but it also caused evaporative cooling of materials.