Filtration soiling is a term used to describe a common problem which occurs in a number of homes.

Dark, grayish lines will begin to appear around baseboards, under doors, along edges of stairs and possibly in the center of the room.This soiling occurs more often in multi-level homes, but it can also occur in single level dwellings.

Filtration soiling can be very perplexing to cleaners and homeowners. The homeowner usually suspects that some type of carpet defect exists, but not in this case. The explanation is quiet simple. There are essentially two sources of soiling in the home: tracked-in soil, which enters the dwelling on the soles of shoes, and airborne soiling, which comes from a number of sources.

Tracked-in soil can become airborne when vacuuming is performed. Dust, smog, and other airborne pollutants can accumulate on the face of fiber carpet in areas with a concentrated airflow. These pollutants are carried with the flow of air through tiny cracks or other areas of concentrated airflow. Eventually, a residue will build up forming dark areas.

On multi-level dwellings these areas generally appear on stairs and at the edges of baseboards. These grayish lines may appear away from the walls where plywood-flooring materials have been joined. Generally, this concentrated airflow will force air from an upper level to a lower level of the home. Airborne particles will filter onto the face of the carpet and the accumulation of airborne particulates will occur.

On single level or multi-level dwellings, the accumulation may occur under interior doors. If a central heating, ventilation, and air condition (HVAC) system is utilized, accumulations may occur under a door that remains closed. If air flows into a room, it must have an exit source. If the door is closed, the exit air will be concentrated under the door and airborne particulate accumulation will occur.

The condition can occur quickly or it may develop over a period of months or years. The level of severity is dependant upon the volume of airflow and the level of pollutants in the sir. The condition will obviously appear more quickly on lighter carpet colors than darker tones.

Filtration soiling is not dependant on the quality of carpet selected and it does not indicate the existence of a manufacturer’s defect. The condition will appear on any carpet regardless of price, quality, style, fiber system, or construction. Recent innovations in soil and stain technology have reduced the effort previously needed to remove these contaminants. Complete removal of the contaminants is dependant on the type of pollutant. A professional cleaner should be consulted to perform the work.

Many of the pollutants may be oily in nature, thus attracting other dry soils. These soils may be auto emissions, residue from furniture polished, fine sand or clay particles, cooking oils, or a host of other soils. Filtration soiling rarely responds to normal cleaning procedures. Dry solvents, mineral spirits, citric or acidic solutions may be helpful. This should be followed by alkaline rinse or detergent solution.

Prevention is difficult to achieve. Solutions may be expensive as well as elusive. One possible remedy would be to seal the areas of airflow, but often another area will develop within the structure with an accelerated accumulation rate. Interior doors should remain open while the HVAC system is in operation.

Keeping the air inside the home as clean as possible is the only viable option. This can be accomplished by prohibiting smoking indoors, regular replacement or cleaning of HVAC filters, and the use of H.E.P.A. vacuum cleaner bags. Reducing indoor pollution is the key to reduction of filtration soiling.

Quoted from a
Technical Bulletin by The Carpet and Rug Institute (CRI)