If you have light colored carpet, you may have experienced these dark lines that appear on the carpet. This soiling is called filtration soiling because it comes from air passing through the carpet as it is attempting to get through the crack between the carpet and the baseboard or under a closed door. As the air passes through the edge of the carpet pile, this microscopic soiling is deposited there (hence the term filtration soiling). This occurs over a significant period of time and is not noticed until a build up has occurred.
This type of soiling is difficult to remove because the soil particles are so fine that they penetrate deep into the carpet pile, sometimes into the backing of the carpet. Can it be removed? Sometimes. With special products, time and effort, reasonable results can sometimes be achieved. To help prevent filtration soiling, vacuum and wipe the edges of the carpeting on a regular basis.