Have you ever noticed a grayish or dark line along the walls on your carpets? You probably do not want to know this, but it is not a defect in your carpet. It is called soil filtration and is a concentration of dirt that has collected on the carpet fibers
Soil Filtration is caused by positive and negative pressure within a room. What happens is, when pressure change occurs within a room from either the effect of warm air rising or your air conditioner or heater pumping air into a room, air tries to flow either in or out of the room. In the places with the dark lines, air is flowing in or out underneath the walls and the carpet along the wall edges serve as a filter. The carpet collects the dirt that is in the air and as a result the dark lines are created.
Have you ever noticed a grayish or dark line underneath the door of a room? Once again, soil filtration. It is quite common for dirt and dust to accumulate under doors when they are shut. The supply air vent blowing air into the room creates positive pressure in the closed room. The air in the room will want to leave the room through the easiest paths. With the door shut, the undercut of the door is generally the largest and easiest hole for the air to escape. Therefore, as the air passes between the door undercut and the carpeting, the carpet will act like an air filter and grab the dirt that is in the air. The result (like described above) is a dark line appearing just under the door. The best way to prevent doorway dirt filtration is to leave the door cracked so air escapes along the edges and not under the door.
Because soil filtration is a concentration of dirt particles that has occurred over time, cleaning it is extremely tough and will return after cleaning unless you can stop the air flow in these areas.
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