(also known as "Draught Marking" "Fogging" or "Dust Marks")

Appearing as dark lines or soiled areas on carpets around the edges of rooms, under doorways, in closets, at the bottom of baseboards, surrounding heat ducts or floor outlets, on staircases and around railings and, in extreme cases, "across the middle of a room in the configuration of the floorboards," (1) this unsightly and annoying problem is often most visible on lighter-colored carpets and is more commonly found in newer homes.  But regardless of its location, the reason for filtration soiling is always the same:  "the accumulation of airborne soil -- dust, smog, tobacco smoke and other pollutants -- where airflow is concentrated and directed over or through a carpet's pile.  The carpet 'filters' out these pollutants and gradually becomes soiled and dark."  "Filtration soil consists primarily of extremely fine particles -- much smaller than soil from other sources." (2)

What is Airflow?

Air is never truly stagnant; it is constantly in motion, whether due to "heating and air conditioning systems, thermal expansion and contraction of air, or natural convection currents in the structure." (2)  In older homes, drafts were common.  The air moving back and forth through gaps in the walls discouraged the microscopic particles from falling to the floor and accumulating on the carpet. Unfortunately, the energy crisis of the 70's brought about the indoor air crisis of the 90's and dragged it right on into the new millinium. Now studies show that the air inside a home may be five times more polluted than the air outside!  With today's airtight, energy-efficient homes, the majority of airflow is between rooms, under interior walls, around closed doors and through heat ducts.  It can also be caused by wind blowing through windows that regularly remain open.
Based on my research, the answer is "YES -- to a point."  In other words, eliminating where the air tends to move through or over the carpet is the only way to prevent the carpet from acting as a filter.  But the process maynot be an easy one.  The carpet must be disengaged from the walls and stairways, then any and all existing gaps along baseboards, walls, "holes" in carpet from tack strips or installation must be seals, possibly with silicone caulk or polyurethane foam insulation.  However if any gaps remain, the moving air will find them and, instead of being eliminated, the filtration soiling problem will simply be relocated.  Another preventative measure is frequent vacuuming -- especially along the baseboards with a crevice tool.  Frequently changing furnace filters and cleaning the air ducts will help to reduce the probable return of filtration soiling.  Another trick is to leave interior doors open even when a room is not in use. This will allow air to move more freely between rooms instead of forcing it under the doors or across the top of carpet.  Of course, regular professional carpet cleaning never hurts, either.

Can Existing Filtration Soil Be Removed From A Carpet?

In many instances, the dark lines can either be removed or, at the very least, lightened. A few of the determining factors are:
What You Should Know About "Filtration Soiling"

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** Type of carpet fiber: For instance, because it is a natural fiber, filtration soiling on wool carpet must be treated differently than filtration soiling on synthetic carpets.  Dye types can also be an influencing factor on how filtration soiling is treated (i.e. temporary vegetable dye vs. permanent manmade dye).  Berber, with its tightly wound fiber construction, can be another "problem child" when it comes to removing filtration soiling.

** Composition of the particulates:  Oily particles, such as those from candles, cooking oils, lamp oils, oil furnaces, etc. are more stubborn to remove than simple dust.  Airborne road grime, which may contain petroleum products from cars and blacktop, are usually a challenge.

** Color of the carpet:  Light colored carpet tends to show filtration soiling faster and more often than dark colors -- just as spots & spills will be more visible on a white shirt than on a darker-colored or patterned shirt.

** Length of time that the particles have been building up in the carpet:  The longer the filtration soiling is allowed to sit on the carpet, the greater the probability that it will penetrate any open dye sites on the carpet's filaments.  Once that happens, the chance of eliminating the filtration soiling from the carpet is greatly reduced

** Budget:  Treatment of filtration soiling is both a technical and a very labor-intensive process.  The amount of labor and time involved will directly impact the final cost.

Keep in mind, however, that if the preventative measures listed above are not taken, then the problem will return.  And it can be in as little time as a couple of months. 

The term "sick building syndrome" has become part of the American vocabulary as a way to describe the disease that modern architecture has created.  In other words, what is often viewed as a defect in the carpet or an incompetent carpet cleaner is actually the result of air-tight, energy efficient building methods.

We hope that this addresses all of the questions you may ever have about filtration soiling.

(1)  DesignCare:  Technical Information "Draught Marking"
(2)  1988 - 2007 Floorcovering Forensics Company and Ray Morgan /
What are some of the sources of filtration soiling?

From The Outside
auto exhaust
jet / train emissions
factory discharge
back yard burning
mowing grass
stirring up dust

Can Filtration Soiling Be Prevented?
? ? ? ?
From Inside Your Home
candle soot (from oily / scented candles)
tobacco smoke
cooking (especially from oils or grease)
wood or pellet burning stoves
dirty furnace filters
dust accumulated in heat ducts
pet dander
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