With nearly 48 years in the cleaning business, I've probably seen everything that can happen to a carpet. In 1972, I investigated a problem with an acoustical wall for Indiana University. A beige carpet on the wall in a lecture hall had dark spots about four inches high and two inches wide, equally spaced and a few inches apart. It looked as if the entire wall had large, dark measles. When the wall was cleaned the spots returned in just a few weeks.
A very porous cinder block used in the construction of the partitions proved to be the culprit. The room breathed through this porous block and the carpet acted as a filter retaining all of the air borne debris. The core of the block, which were stacked in neat rows, acted like little chimneys carrying the air to the opening above the drop-ceiling. The tops of the cores were sealed to prevent the air flow and the problem was solved.
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