Soil filtration lines in carpet are distinct lines of discoloration caused by fine particle soils, referred to as micro occlusion soils in the air and oily vapors being drawn through the nap of carpet and other fabrics.

The source of filtration soil in a house is typically coal or wood burning stoves, candles, cigarette smoke, cooking oils, and carbon particles that escaped through the furnace filter.

Air circulates across the room, hitting the wall, ceiling and floor. Wherever the air hits the obstacle and changes direction it deposits soils.

Soil filtration lines occur through the seams of plywood and/or padding, at the bottom of baseboard moldings, in the crevices of steps, around spindles, and under doors. In other words, any crack or crevice that carries air, also carries contaminants that deposit on and discolor carpets.

The primary source of filtration soil is usually oils which initially appear gray but eventually oxidize and develop into a hard yellow film. In addition, soil filtration lines composed of oxides of nitrogen or sulfur dioxide can combine with ambient humidity over time to form diluted nitric or sulfuric acid that can permanently alter carpet dyes.

Minimize the possibility of soil filtration lines by preventing airflow through carpet and carpet edges by sealing cracks in the sub-floor, under baseboards and edges/crevices of stairs. Keeping air inside the home as clean as possible can be accomplished by reducing indoor air pollutants, such as cooking emissions, fireplace smoke, burning candles, cigarette smoke, and emissions from cleaning chemicals; and by the installation and regular replacement of high efficiency HVAC air filters.

Discoloration caused by soil filtration is not a carpet defect, but a situation in which dust and other airborne pollutants such as soot from burning candles or from poorly ventilated fireplaces accumulate on the carpet face fibers causing discoloration. The soiling condition can occur quickly, or it may develop over a period of months or years. The level of soiling is dependent upon the volume of airflow and the level of pollutants in the air.

   Filtration soiling is not a result of the quality of carpet selected. The condition will obviously appear more pronounced on lighter colorations than darker colorations.