We have all been to someone’s house (maybe even our own) where you see those dark lines around the baseboard, and sometimes under doors that stay closed most of the time. They are rather unsightly, and can really be frustrating. The initial thought that goes through most people’s mind is that the homeowner just doesn’t vacuum enough. This may be true, but often it is not.
Those lines are what are known as soil filtration lines. Soil filtration is caused as the furnace turns on and creates a vacuum that draws air to the HVAC system while pulling dirt from the air and trapping it in the carpet. It most often occurs in the area described above because those are the areas that allow air to flow through them.
Depending on how long the problem has existed and the amount of dirt present, it can be cleaned. Unfortunately, no matter how often you clean them, they will most likely come back. Sometimes, the problem can be fixed by pulling the carpet up and putting some type of sealant (like silicone) between the sub floor and wall framing. Most times, however, you do not know the problem will exist until the walls are finished, which makes it much more difficult to repair.