Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. The needed airflows include, for example, supply air, return air, and exhaust air. Ducts commonly also deliver ventilation air as part of the supply of air. The duct system prevents air contamination. Its job is to filter the exterior air as filtering removes dust, dirt, pollen, and other air contaminants so that you will have a healthy and comfortable atmosphere. If the air ducts are leaking, then these contaminants are also leaking into your house.