An air duct is a passageway used in heating, ventilation, and air conditioning (HVAC) systems to distribute air throughout a building. These ducts can be made from various materials, including metal, fiberglass, or flexible plastic, and are designed to carry conditioned air to different rooms. Properly functioning air ducts are essential for maintaining indoor air quality and comfort. They help regulate temperature and humidity levels while also filtering out dust and allergens. Regular maintenance, such as cleaning and sealing, can improve efficiency and prolong the lifespan of the HVAC system.