The air in our houses is full of dust particles, skin flakes etc. When the sun shines in through a window you can sometimes see it.
A light bulb in a table lamp sometimes makes the inside of the shade stain brown - particles in the room are lifted on a rising current of warm air. They stick to things they touch. The plastic lampholders particularly. You get a similar effect above your cooker but with airbourne grease and larger dust particles
Nothing wrong with your heaters - all you are missing is basic physics - smoking would make things worse - it does in my house and I get nagged for it.
It is also possible that the previous people did not decorate very often hence their brown walls.
Peccavi
June 2010