mVOCs (Microbial Volatile Organic Compounds) are substances naturally produced by molds as they grow and digest the surface they are attached to. They are produced during fungal growth as a waste byproduct, so they are primarily elevated when mold is in an active state. Their production can be dependent on the substrate and other environmental conditions as well. mVOCs have strong and unpleasant odors that many refer to as a “musty odor.”