Do these dark marks look familiar? These marks are sometimes called ghost stains, or ghosting, because they often appear at wall-ceiling junctures, on carpet under doors & around picture frames on walls. Contributors include burning candles, smoking, gas fireplace logs, even auto exhaust from attached garage.
Stains at wall-ceiling junctures usually mean a combination of attraction, impaction, and gravity are occurring. The first signs are usually seen on exterior walls, along ceiling joists, studs in walls, and around nail heads which have cooler surfaces.
In order for ghosting to occur, there must be a driving force to push particles against a surface. Driving forces include electrostatic forces and moisture (attraction), forced air (impaction), and gravity. The location of the particle deposition will often give a hint as to which of the three forces may be causing the problem.
The Connecticut Department of Public Health has an informative article about Attraction, Impaction and Gravity. Read more