On your radio program, I heard a woman talking about a dark discoloration at the perimeter areas of her carpeted floor. The discussion turned to the possibility of mildew, but there is another likelihood that you didn’t cover. It is referred to as air-filtration soiling, and is becoming more and more common as consumers select lighter and lighter colors to offset the smaller rooms in modern floor plans. Newdirt-resistant carpet fibers also add to the problem. The dirt remains on the surface and is more visible.
Air-filtration has to do with air movement from warmer to cooler areas (room to room and inside to outside). As the air moves, the carpet pile yarn acts as a filter, filtering out dust and soot from the air creating a black discoloration.