Ventilation systems are systems designed to provide fresh air to a building or room and to remove stale air. They are used to control indoor air quality by introducing outside air, controlling temperature, humidity, and air movement, and by removing contaminants such as dust, smoke, and odors. Ventilation systems can be either natural or mechanical. Natural ventilation systems rely on the natural movement of air to provide fresh air, while mechanical ventilation systems use fans and other equipment to move air.