The primary reason for this concentrated flow of air is pressure changes within a room. Pressure changes occur within a room from either the effect of warmer air rising or from your heater or air conditioner blowing
air into a room without a return vent. Because of the pressure change, air tries to flow either in or out of the room. For walls that are not sealed to the floor, air moves in and out of the room at these areas and that is why you will see it along walls and stairs. For these situations, unless the air leakage areas are addressed, the soil filtration will continue to occur.
room to the lower pressure 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. For these situations, you might consider leaving the door cracked so air can move around the door and not under the door.