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 home. What happens is, when pressure change occurs within a room from either the effect of warm air rising or from pressure created by your heater and air conditioner, air tries to flow either in or out of the room. The carpet along the wall edges serve as a filter as the air exits or enters under pressure.
Also, have you ever noticed a grayish or dark line at the doorway 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. This air will seek areas of lower pressure (the return air vents) through the largest available holes. With the door shut, the undercut of the door is generally the largest available hole. Therefore, as the air passes between the door undercut and the carpeting, the carpet acts like a filter taking out dirt and dust particles from the air. Over a period of time, the carpet will darken as the dirt accumulates.
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