What's with this funky bathroom soot?

DEAR BARRY: Our laundry is located on the second floor, directly adjacent to the bathroom. Whenever I run the dryer, the bathroom becomes very humid if the door is shut. I've also noticed something like black soot on the bathroom walls. I wash it off, but it always comes back. What could be causing the humidity and the soot, and what can I do to resolve this? --Debbie

All rights reserved. This article may not be used or reproduced in any manner whatsoever, in part or in whole, without written permission of Inman News. Use of this article without permission is a violation of federal copyright law.