What is Filtration soiling?

Dark lines along walls, in doorways or in streaks on the floor where carpet cushion has not been taped are known as filter or filtration soil. When the natural airflow in a structure is polluted with fine, oily soils, the carpet filters this soil anywhere that there is a narrow opening for the air to pass through. This buildup requires extensive preconditioning, agitation, and dwell time to remove and in some cases may be permanent.