This is a common problem with steam heat as the radiator is at a high temperature often. This high heat creates a chimney effect of air moving over the surface. As a radiator is usually just a small portion of the wall, most of the air in the room must pass over it to be warmed, so all micronic particles in the air may settle in the area of the wall by the radiator.
Harold, are you saying that the radiators may be concentrating soot or dirt that is in the air and causing it to be deposited on the walls nearest the radiators?I have heard that folks in the habit of burning candles as a form of decoration may experiance this problemlso if your boiler is not vented properly and the flue gas is getting in the home you might experiance the same symptoms. Jim
In a sense, yes, Jim. The radiators heat the air creating air movement local to the radiator, so it carries air to the area of the radiator more concentratedly than to other locations. It doesn’t have to be soot, though the suggestion to check for abnormal amounts in the air is well taken.
fourthyear is correct about it being a common problem to any air moving device including baseboard, radiators and air registers.