A common fungus in water-damaged building materials, Aspergillus versicolor produces the mycotoxin carcinogen sterigmatocystin. Mycotoxins are chemicals (metabolites) produced by molds, which cause a toxic response in humans if ingested, inhaled, or in contact with skin. Mycotoxins can affect the immune system, nervous system, liver, kidneys, blood and blood clotting. Some mycotoxins are known to be carcinogens.

Research is being conducted to determine how fungal mycotoxins may contribute to vague health symptoms experienced by those who live in environments with a high fungal load. Many fungi produce mycotoxins, both inside spores and on their surfaces. Killing spores does not disable the toxicity of mycotoxins. In fact, mycotoxins can be present on or in spores, whether the spores are dead or alive. Stopping fungal growth does not stop adverse medical symptoms. Both spores and hyphae can be allergenic and/or toxic.