What are the dark lines that form along the interior walls and stairs?
The dark lines that form along interior walls, under doors, and along stairs are called filtration lines. They are caused by forced-air, heating/ventilation systems. When air is forced into a room, existing air is forced out every crack or entry, passing through floor boards, under interior walls, or through the slots in the stairs. As the air exits, airborne dust and oils collect in the carpet, which acts as a filter. Unfortunately, filtration lines are very difficult to remove and return quickly if the cause is not remedied.
