Soil filtration lines are dark soiled areas that develop gradually on carpet. They are most common around the edges of a room next to the wall, under floor-length draperies, and under doors. However, they can develop anywhere there is an air space, such as between floorboards or gaps in the subflooring. Also known as soil lines, smog lines, and perimeter soiling (when they occur around walls), the problem is usually more obvious near heating ducts, electric floor outlets, and gas valves.
Bedroom doors that are closed at night, especially when windows are left open, are likely to develop these lines. The soiling is caused by the passage of air through or across the carpet. Air carries microscopic particles of 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 exceptionally 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 discolouration cannot always be removed completely.
The degree of removal depends on several factors, including the amount and type of soil, the length of time the soil has accumulated, the amount of air flow, the colour of the carpet, and the type of fibre. The lines can often be removed from most synthetic fibres. However, in severe cases, especially on light-coloured carpets, traces may remain after cleaning. It is usually exceedingly difficult to remove filtration soiling completely from wool or olefin carpets.
