With built-in heaters, vents became necessary for circulating the heat, and later the air conditioning, throughout the vehicle. One of the reasons is air quality. As the Government of Canada’s Web page on Ventilation and the Indoor Environment notes, ventilation, the movement of air through an enclosed space, is important for maintaining air quality. Just as in buildings, ventilation systems in vehicles help to keep the air fresh, either by circulating the air or by pulling in air from outside to renew the supply of oxygen.