People with mold allergies may exhibit symptoms that include allergic reactions, asthma, and other respiratory issues. Mold can grow in air ducts if moisture is allowed to settle. When the heating or cooling system is turned on, the blasts of air can loosen the mold. The EPA suggests air duct cleaning of any visible signs of mold growth of the ducts or other components, but mold is often not in areas visible to the eye. Cooling and heating systems can also serve as a breeding ground for mold, due to standing water near air intakes and humidification systems.