Most returns are how you discribed yours even if the Hvac unit is in the basement. and yes that brown stuff is just dust and if they are smokers it is even browner. If you connect into your return there is enough air move ment so I do not think mold would start, mold is more a fact of moisture, not humidity. I have a 120 in my living room and a 2x4' sump in my basement and I have a weather station in the house and my humidty does not get over 50% and that is when it is this time of year when I am not heating or running the AC, right now the house is closed up and it is 44% in the house and not running heat or AC.
