An air-vent line is a type of plumbing or piping system used in buildings to allow for the ventilation of air from enclosed spaces. It is typically composed of a series of pipes or ducts that connect to air vents or grilles throughout the building. The air-vent line is designed to allow for the exchange of stale or contaminated air with fresh outdoor air, which helps to maintain healthy indoor air quality and prevent the buildup of harmful pollutants or gases. This type of system is commonly found in heating, ventilation, and air conditioning (HVAC) systems, and it may also be used in plumbing systems to prevent the buildup of sewer gases or other noxious fumes.