Filtration lines are something we’re often asked to remove and clean up. It’s not always a parent to customers how and why they form. Filtration lines will form when air is forced to flow through fibers. And the fibers act as a filtering device to filter particle and chemical impurities out of the air. They often times show up on a lighter color carpet underneath doors that are often times kept closed. Another place where they show up is along the baseboard near walls where air is actually flowing through gaps or cracks along the bottom of the walls.