Those ugly black lines around the baseboards and under doorways.
If you have light colored carpet, you may have experienced dark around baseboards, under doors, and especially near the HVAC return. This soiling comes from air passing through the carpet as it is attempting to go through the edge of the carpet pile, microscopic particles of soil are 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 be achieved, although not in all cases.
To help prevent filtration soiling in the future, vacuum the edges of the carpeting on a regular basis, then wipe with a damp, white terry towel. Be careful around the tackstrip under the edge of the carpeting, you could nick your fingers!