i was told by someone recently it has to do with the radiators needing bled out every now and again because the dirty water inside them is coming through
Yes, this is typical of airborne dirt and pollution including dust (sorry). It is very small particles that you cannot see being carried up in the warm air and sticking to the wall. You will find it very difficult to clean off. If the walls are painted they soak into the paint. If you have smokers in the house it will be worse as the smoke particles will be drifting about in the room.
If you put a narrow shelf above the radiator it will disrupt the airstream and reduce staining.
Similar staining can appear above hot light fittings.
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