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 building. What happens is, when pressure change occurs within a room from either the effect of warm air rising or from mechanical pressures, 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.
Because soil filtration is a concentration of dirt particles, cleaning soil filtration areas can be very difficult.
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