What’s with those ghostly gray stains on the outside corners of the ceiling and walls? The stains outline the stud framing, and they even seem darker where there are nails in the drywall.
Most ghostly outlines like this are caused by soot in the air. Soot collects at framing, under doors, around pictures and on plastic appliances. Often the soot comes from burning candles, but it can also result from other combustion sources, like a gas fireplace.
Soot is very light; it floats easily and invisibly. Wherever the air slows or changes direction, soot can be deposited. This explains the stains at pictures and under doors, but why does soot outline the stud framing?
Often there is no insulation at the edges of studs on outside walls or ceilings. Instead, insulation is placed between the studs. As air slows around the cooler drywall at the uninsulated stud edge, soot gets deposited in a ghostly outline. The nails are also cooler than their surroundings, so you may see soot spots at the drywall nails or screw heads.
The quick fix: eliminate the source of soot, then clean and paint the walls. Before painting, use a good primer such as BIN to cover the remaining soot.