What is filtration soil? and what can be done about it?

Washington Post article that featured us

Recently we were contacted by columnist Jeanne Huber from the Washington Post in reference to filter soil.  Jeanne told me that readers send in questions and pictures of problems they have in and around the house. Most recently a lady sent in a photo of her white carpet that had ugly black stains around the base molding, under doors, and at the edges of the stair treads. Filter soil is more common than one might think. As a busy carpet cleaner here in Fredericksburg Virginia we see a lot of it.

Unfortunately these black lines don’t come out with the standard carpet pre-spray and hot water extraction that many carpet cleaners use. There are techniques, chemicals and special detail tools that carpet cleaners can use.  In most cases this eyesore can be brought to an acceptable level.  We can usually achieve an 80% improvement or better. There are some homes that are too far gone to fix. Replacement with a darker colored carpet is recommended for these.  Houses that have very severe cases where the black has made it’s way further into the rooms and around items that have sat on the floor for a long time. Items such as big desks, box springs with no bed frame, or boxes stored on the floor in closets.

Read the Washington Post article here

What is filtration soil and what causes it?

The simple answer to this question is pollution within the home. The carpet actually acts as a filter, trapping very fine particles that circulate through the home. These microscopic oily particles are most commonly the bi product of anything that burns, such as candles, fireplaces, furnaces, stoves, etc. These polutants are then circulated throughout the home by the homes heating and cooling system. When carpet is installed there is padding under it, except at the last couple of inches along the wall where the tack strip gets installed to hold the carpet in place. This allows air to flow down the wall cavities and pulled through the carpet edges to make it’s way back to the air return. If you have ever taken a light switch cover off for a painting project you can place your hand close and actually feel the air flowing through the wall cavity.

What can be done about filtration soil?

Unfortunately filtration soil can be very difficult to both prevent and remove once it has set in. Here are some things you can do to help.

  1. When choosing carpet consider a darker color as white and other lighter colors will show this gray/gray filter soil more.
  2. Cut back on the candle burning and fireplace usage.
  3. Keep up with the furnace filters, we do ours quarterly and set a reminder. Unfortunately, the filters with the higher MERV ratings that are capable of catching the finer particles are not recommended by hvac manufacturers as they can put too much strain on the system.
  4. Keep doors to unoccupied rooms open. When the bedroom doors are kept closed the air return in the hall will suck the air out of the room from under the door, leaving a black line.
  5. Frequent vacuuming, utilizing the crevice tool along the edges.
  6. When possible, keep items off of the floor such as cardboard boxes, and other flat items. Box springs should always be on a bed frame, never directly on the floor.
  7. Annual professional carpet cleaning.

Help from a professional carpet cleaner

Once filtration soil sets in the only hope will be a professional carpet cleaning company that is experienced in filter soil removal. They will have the right products and tools to get the job done. Because soil filtration removal is a difficult and time consuming process it comes with a price tag. Many folks aren’t willing to go for the cost, especially knowing that it will likely return.  Many carpet cleaners will often tell customers that it simply will not come out and it is permanently stained. This is because it is such a hassle. It is always worse behind furniture that never sees a vacuum. So all the furniture needs to be moved, cleaned under and replaced to their original place with furniture blocks. Most of the filter soil jobs we do are for people that need to sell their house and just don’t want to replace the carpet.

We first got into the carpet cleaning business back in 2006. Quickly we realized that we needed to ask every caller if they noticed any black lines. Under doors or around the perimeter of rooms. Nothing is worse than showing up to a carpet cleaning job and realizing there is a filter soil issue. We then have to explain to the customer, whom will likely have thought it was just dust and you’re trying to pull the ol bait and switch.  So educating folks on this subject before arrival is key!

Do all homes have filter soil?

Not every home has these filter soil issues. Some have it in just some rooms or along some walls, but not every wall. Some have it along every wall.  I have been trying to take note over the years to find patterns such as age of home? interior wall or exterior wall? but I have not really pinned it to any of the above, it just seems some walls are more air tight than others.


I took this picture on a job as I found it interesting that during the original installation of the carpet they used a knee kicker, which is a standard carpet installation tool. This knee kicker pierced holes in the carpet backing allowing air to flow through and filter soil to set in.


This is the picture that was in the Washington Post article. It appears to be a a stair riser with severe filter soil. This most likely took many years to set in and as you can see this was a white carpet, which I suggest avoiding for future installations.


This picture is a good example of the typical/average filter soil we see.