The soiling is caused by the passage of air through or across the carpet.  Air carries microscopic particles or dirt, dust 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 deeply 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, amount of air flow, color of carpet and type of fiber.  The lines can be removed from most synthetic fibers.  However, in severe cases, especially on light colored carpets, traces may remain after cleaning.  It is usually very difficult to remove filtration soiling completely from wool or olefin carpets.