We've lived in our house for almost nine years, and have had a recurring problem in our central hallway. At one end, in which a few steps lead up to the master bedroom, dark (almost black) soot-looking stains have continuously formed on the ceiling and upper walls. On a couple of occasions I have partially (but not completely) removed these using a household cleaning agent and water. Then they form again! We've considered that this may be mold, except it doesn't look like any I'm familiar with. It looks like dirt or soot. We can't imagine where this may be coming from. Directly above is the entrance to our attic (a small removable panel) but no stains are forming on that, and it seems to be pretty well sealed. The only other explanation we can think of is we sleep with a floor fan turned on (mostly for white noise) and this might be blowing dust or dirt out into the hallway, which rises to the ceiling. Yet this doesn't seem like an adequate explanation either. Any ideas?
Sounds like soot to me.
Soot deposition occurs where there is convective or other air flow that deposits the particles by collision.
It would only be mold if there were a source of moisture.
I have seen soot patterns as a result of missing ceiling and/or wall insulation. These create colder surfaces where there is increased air flow and hence greater soot deposition.
To avoid soot in the air, don't burn jar candles.
May Indoor Air Investigations