There are black lines forming on my carpet along the walls, under doors, around air vents, and along the edges of my stairs, how come?
You are likely having a problem known as filtration soiling.
What causes it?
Filtration soiling occurs because of air being forced into the room or space at a higher rate than it can escape from the same area, via the ventilation system. The air then seeks alternative escape routes, which usually end up being the gap between the carpet and the wall trim and under closed doors.
As the air is forced through these gaps, it passes through the carpet, which acts as a filter to the air. Any pollutants in the air are trapped by the carpet, and turn the carpet gray or black in that area. Common pollutants that can contribute to the problem include smoke from cigarettes and candles, cooking oils, fireplace ash, and dust.
This soiling does not occur as a result of insufficient vacuuming or improper vacuuming techniques (although consistently failing to vacuum the very edge of the room can help to amplify the problem). It is a problem that can occur in the cleanest of homes, and is not considered a reflection of house-keeping abilities.
How do you get rid of it?
Discoloration of the carpet caused by filtration soiling is very difficult to remove. The particles are extremely fine and thus end up firmly embedded in the fibers of the carpet. In some extreme cases, it may not be possible to fully remove all traces of the soiling.
For this type of soiling, it is best to let the professionals handle it. Contact your local carpet cleaning company and advise them of the situation, and ask for their advice. The cleaning method that is likely to be most successful is hot water extraction (commonly known as steam cleaning). A cleaning solution added to the water in the machine will help in the removal of the soiling.