If you have a light colour carpet, you may have experienced these dark lines that appear on the carpet.
This soiling is called filtration soiling (draught marks) because it comes from air passing through the carpet and as it is attempting to get through the crack between the carpet and the baseboard or under a closed door microscopic soiling is deposited there.
This occurs over a 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 they penetrate deep into the carpet pile, sometimes into the backing of the carpet.
To help prevent filtration soiling, vacuum and wipe the edges of the carpet on a regular basis. Preventing airflow through carpet and carpet edges by sealing cracks in the subfloor, as well as under baseboards and edges of stairs may reduce filtration soiling problems from re occurring.
Although the complete removal of contaminants from this type of soiling can be complicated, depending on the type of contaminant materials present a professional carpet cleaning company may be able to reduce these marks but complete removal cannot be guaranteed.