Hopefully someone out there has heard of this before... I have a customer who has light colored carpet in her home, and for some reason on the second floor there are black marks all along the walls. Two local carpet companies have said that it is caused by "an hvac system imbalance"; in other words, a positive or negative pressure on the house causing infiltration or the opposite. I find this hard to believe, because there are no marks on the carpet downstairs, and normal dust, be it from infiltration or other, is not black. Does anyone know what is happening here?