Beyond being simply allergenic or pathogenic organic spores, certain molds can produce mycotoxins.  Mycotoxins are chemicals produced by molds or fungi. Trichothecenes, Aflatoxins, Ochratoxins, Gliotoxin, Chaetoglobosins and Sterigmatocystin to mention a few. Where conditions are right, fungi proliferate into colonies and mycotoxin levels can become high. Mycotoxins vary greatly in their severity. Some fungi produce severe toxins only at specific levels of moisture, temperature or oxygen in the air. Some toxins are lethal, some cause identifiable diseases or health problems.  Some toxins effect the auto immune systems. Some toxins are carcinogenic. It's important to know if testing for mycotoxins is necessary and, if identified, how to remediate in an effective manner to provide a clean, healthy environment free of these toxins.