Soil Filtration Stains are those dark lines running along the baseboards, in front of cold air return vents for the furnace, under long hanging drapes, around edges of heat vents, sofa skirts, around plastic chair mats and along dressers or other furniture that does not get moved very often. Soil filtration is caused by air (which contains various soils) being drawn through the nap of carpet or other fabrics. In the process of being drawn through the carpet fibers these airborne contaminants are filtered out as it passes through.
Soil filtration has three major soil components:
Larger soil particles; such as sand, clay, hair, fibers from paper & clothing as well as pollens, dead skin and mold spores.
Very fine particles; such as carbon, soot, smoke, pigments used in inks and fabric dyes; these soils stay airborne for long periods of time until finally being filtered out of the air and into your carpet fibers.
Oily soils; such as car emissions, smog, burnt items (like nicotine and candles), gases from cooking and (the largest contributor of all) our gas fired appliances.
All three of these soil types are filtered out by the carpet's nap however it's the very fine and the oily soil that causes the most problems. Over time the oily soils (gas vapors) dry out forming a resilient film which encases and bonds the very fine particles of soil within the carpet fibers. Once encased the very fine soils act like dye pigment and become fixed to carpet fibers by strong electrical forces.
Additionally airborne molecules composed of oxides of nitrogen or sulfur combine with water vapor (humidity) to form nitric or sulfuric acid. These diluted acids begin to effect dyes used in all fabrics causing a faded appearance which some call fume fading or atmospheric soiling. These acids also add to the overall carpet appearance and further complicate the removal of soil filtration stains.
Aside from open windows or forced air heating / cooling systems there are other air currents that allow soil filtration to appear at the wall edges, under doors and at duct vents. These other air currents are actually created by differences in air pressure; wind / cold / heat acting upon an outside wall of a house can create a vacuum effect inside wall cavities or on different levels of a house.
Soiled air is drawn (by pressure differences between rooms or levels) through the carpet fibers along the baseboard, around furniture and even underneath walls (filtering out the fine and oily soils as it goes). Pin holes in carpet backing created by the carpet installers' knee kicker are also susceptible to air currents. Pin-hole filtration lines are usually away from and perpendicular to the wall.
Doors that are always closed can also create a small vortex between the bottom of the door and the tips of the carpet fibers causing a filtration line to form under the door.
Warmer air on upper levels of a house will create thermal air currents as the warmer air expands; this expanding air volume must go somewhere, usually following the path of least resistance along wall edges, into cold air return vents and down the stair case all the while being filtered through the carpet fibers.
The homes' heating and cooling system is a major contributing factor by itself. No matter how efficient your furnace may be; hydrocarbons are being emitted into the ducts as the natural gas burns. These hydrocarbons are classified as oily soils when it comes to soil filtration stains. "IF" there was no such thing as dust and nothing else was being encased by the oily soils; our filtration lines would be a very light gold color and would hardly be visible in the carpet fibers.
Clogged Filters: In most new homes fresh air for the furnace is drawn in from an outside vent; this fresh air intake has a filter to stop dust and debris from entering the furnace system. When that filter becomes clogged with soil and debris the system can no longer draw the outside air it needs. Thus the furnace pulls air out of the basement rooms (like older furnaces) which creates a negative air system in the basement (vacuum effect) at the same time warm air is being forced up through the ducts which in turn creates a positive air system that increases air flow back to the basement.
This positive air flow (containing the soils previously mentioned) is now also being forced to pass through the carpet gulley, along the baseboards, through wall cavities, down stair cases, through the nap of the carpet located in front of the cold air return vents and any other cracks and crevices that may exist.
Day to day vacuuming of the wall edges (with a crevice tool) and timely professional cleaning will eliminate or drastically reduce the build up of soil filtration staining. Stop burning scented candles, smoke outside and try using the range hood fan when cooking.
Check all furnace filters for cleanliness and change them on a regular basis; you may also consider HEPA style filters that remove smaller particles. Installing additional filters inside the heat duct registers and cold air return vents will also keep the flow of soil air to a minimum.
However due to the complex chemistry and microscopic size of the particles that cause soil filtration stains it is very difficult (if not impossible) to remove this type of staining from carpet fibers once it has a foothold.