As it applies in air conditioning and heating, mold is an undesirable growth within, on, or around HVAC equipment, usually associated with improper moisture or indoor humidity levels. Molds are undesirable because they can cause or increase health problems and odors. The spores of mold can become airborne. Overall the growth of mold can be retarded by proper equipment installation and maintenance. Mold spore growth can be discouraged by keeping proper indoor humidity levels.