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Filtration Soiling Carpet Cleaning Services in Australia

Dark, grey or brown lines under doors, around walls, underneath draperies and along the edges of stairs are symptoms of an aggravating problem called filtration soiling. This article explains the cause of this problem and suggests effective methods of prevention and correction. The removal of filtration soiling will not be successful with normal carpet cleaning procedures. The removal of filtration soiling is a separate process which can take up to 40 minutes per room. Filtration soil removal is a time consuming and sometimes expensive exercise and you need to consider the cost of filtration soil removal to carpet replacement.

What is Filtration Soiling?

Filtration soiling is 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. The most common area you see filtration soil is around and close to walls. Filtration soil is more evident on white or cream colored carpet.

In addition to the areas shown in the accompanying photographs, filtration soiling can occur anywhere air flows over or through the pile, including places where the carpet’s back has been punctured or torn by installation tools, along seams that abut baseboards, around railings on stairs, and other areas. You may even find it around the holes created in carpet’s backings by the pins of tackless strip. In these cases, the path taken by the air differs, but the cause of the problem is the same.


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Filtration soiling may appear over a period of only weeks, or it may take months or even years to become visible. The severity of the soiling will be proportional to the volume of airflow and the relative dirtiness of the air. Its colour will vary according to the type of contaminants present. Auto emissions are grey to black; tobacco smoke is tan to brown (and easier to remove by cleaning).

It is most visible on carpet of lighter colours, such as off-whites, light greys, beiges and pastels.

Is the Carpet Defective?

No. Filtration soiling is not the result of any defect in the carpet or its components, nor is it an indication of low quality. It can appear on any carpet regardless of price, style, colour, construction or face fiber. Since it is caused by airborne pollutants being deposited on the carpet after it is installed, filtration soil is not affected by anything done when a carpet or its pile fibers are manufactured. While fluorochemical soil retardants (e.g., Scotchgard, Teflon, etc.) may make it somewhat easier to remove, no fiber modification or protective finish can prevent filtration soiling from occurring.

Can it Be Prevented?

Leaving inside doors open as much as possible will help prevent filtration soil from accumulating in doorways. In other areas the airflow itself must be stopped. Along walls and staircases, the carpet will need to be disengaged and the gaps through which air flows must be completely sealed off. (Silicone caulking and polyurethane foam insulation are ideal for this purpose.) However, if any gaps remain, air will find them. Sealing off some gaps may simply force air to find another route—possibly through the carpet in another area.

What Causes the Airflow?

In most cases, the airflow is created by heating and air conditioning systems, thermal expansion and contraction of air, or natural convection currents in the structure. It also may be caused by wind blowing through windows that regularly remain open.

Can It Be Corrected?

Usually filtration soil is at least partially correctable by a professional cleaner; however, it rarely responds completely to routine cleaning. Filtration soil consists primarily of extremely fine particles—much smaller than soil from other sources—which can be very difficult to remove. It is thought that because of their small size, the particles are held stubbornly to the fiber’s and are not easily removed in the normal carpet cleaning process. Some components of filtration soil—auto emissions, for example—are oily in nature and have an affinity for synthetic carpet fibers. These factors, combined with the fact that the soil typically remains on the fibers for months or years, make filtration soil highly resistant to ordinary cleaning. Natural fibers like wool cannot tolerate the chemically aggressive cleaning techniques usually required to remove filtration soil.

The largest case (by dollar value) I ever worked on was a lawsuit against a construction company, who was blamed for severe airflow problems, which caused extreme filtration soiling. As (bad) luck would have it, nearly all of the units had white or beige carpet with nylon or polyester pile. By conducting extensive cleanability testing, I came up with a four-step procedure which successfully removed 90-100% of even the most severely affected areas:

  1. Pre-spray the affected areas with Ramsey’s Pre Oil Break diluted 1:1 with hot water. Agitate the pile gently with a dry, white terry–cloth towel. (A cotton towel has a lot more surface area than bristled brushes.) Allow to dwell for 10 to 15 minutes.
  2. Thoroughly extract the pile with a 1% solution of ProChem’s Emulsifier Plus extraction detergent. (Note: A defoamer will be needed in the extractor’s recovery tank when extracting Pre Oil Break.)
  3. Pre-spray the affected pile again using ProChem’s Ultrapac Renovate diluted 16:1 with hot water. Agitate the pile gently with a dry, white terry–cloth towel. Allow to dwell for 10 to 15 minutes.
  4. Thoroughly extract the pile again.

Repeat these steps until it stops responding or, more likely, it’s all gone. Note that these agents may damage paint and wood finishes, and some weakening of the carpet’s lamination may occur, at least while the carpet is wet. (That’s nothing a little latex won’t fix.) As you can see filtration soil removal is a time consuming and sometimes expensive exercise and you need to consider the cost of removal to carpet replacement.

If you’re going to the trouble of disengaging the carpet to seal off areas where air flows, you might not have to worry about cleaning at all. You can take advantage of the reinstallation to restretch the carpet (using a power stretcher, of course) and physically trim off the soiled edges. If the airflow is completely stopped, then the problem is permanently solved.