What are filtration lines?
These are the lines running along walls, edges of steps, etc.. What is happening is that the carpet is literally filtering the air in the house, just like your furnace filter. For example when the furnace comes on, it creates a strong vacuum in the house (doors slamming), this pulls air thru any crevice it can find, where there is a space between boards. They can be very hard to get out. Approximately 50% of these stains will not come out with a regular cleaning, the others have to be done with what I call the hand and knees operation.
