Would you believe ‘dust,’ airborne particles? This black line or dark line is called a “soil filtration line.” The difference in air temperature inside and outside the closet causes air to flow beneath the door almost continually. And air contains many contaminants, such as dust, soot from vehicle exhaust, industrial gases, and oils from cooking vapors. As the air passes under the door, the carpet is acting as an air filter, collecting the contaminants in the air. You will see evidence of the same kind of soiling in other areas of your home: 1) Under the doors to rooms that are normally kept shut, such as spare bedrooms; 2) around floor vents; and 3) along some walls at the crevice between the baseboards and the carpet. Under floorlength drapes, sometimes the shape of the pleats is inscribed on the carpet by soot in the air. What we’re talking about is “air pollution” inside the home. It settles on floors, furniture, drapes, shelves, pictures, knickknacks, … everything! It’s unhealthy and should be periodically cleaned away.