The soiling is caused by the passage of air through ot across the carpet. Air carries microscopic particles of dirt and soot. As air passes over the carpet, these soil particles settle and become embedded in the carpet pile yarns. In areas where the air flows over the carpet more rapidly than normal, the carpet acts as a filter, extracting the soil particles from the air. The soil is very fine and can penetrate deeplyy into the yarns. Special techniques by a professional carpet cleaner are usually required to improve the appearance of soil filtration lines.
Unfortunately the discoloration cannot always be removed completely. The degree of removal depends on the amount and type of soil, length of time the soil has accumulated, amountof air flow, color of carpet and typeof fiber. The lines can be removed from most synthteic fibers. However in severe cases, especially on lightcolored carpets, traces may remain after cleaning. It is ususally very difficult to remove filtration soiling completely from wool carpets.