Ever open a door in your hallway and see a dark line on the carpet where the door was? This is the filter line. As mentioned on our allergen page, your carpet acts like an air filter for the entire house. When air is pulled under a door, dust is often deposited as the air is squeezed through the small space.
Filter lines can be kept to a minimum by vacuuming on a regular basis. It is also important to change your air conditioners filter more often if you see these lines appearing. If you so have some filter lines and want to remove them yourself you may try purchasing products specifically made for that purpose from carpet cleaning supply shops.