Effect of particle concentration and semi-volatile organic compounds on the phenomenon of ‘black magic dust’ in dwellings

The spontaneous discoloration of indoor surfaces which occurs especially during the heating period has been intensively investigated in Germany since the beginning of the 1990s. On the basis of earlier studies and a questionnaire this phenomenon, referred to as ‘black dwellings’ or ‘black magic dust’ (BMD), was attributed to the presence of semi-volatile organic compounds (SVOCs) and their interaction with dust and particles. In a project funded by the German Federal Environment Agency an attempt was made to deliberately simulate this effect in suitable test chambers. To do so wall paints were used which had been doped with the plasticizers DEHP and DBP. They were applied in different quantities to appropriate wall surfaces in four room size stainless-steel chambers. In this way realistic air concentrations of these two compounds were obtained. An artificially arranged thermal bridge located above a radiator was intended to accelerate deposition of the black dust. Even when the particle concentration in the chamber was briefly increased, no discoloration could be detected. It therefore appears that a combination of dust, thermal bridges and elevated concentrations of plasticizer does not necessarily trigger the BMD phenomenon. With the aid of investigations into affected dwellings it was possible to identify different mechanisms. Strong sources of particles were identified in some apartments while in others the particle deposits were caused by convective air flows. On the basis of all results it can be concluded that the deposition of particles by thermophoresis, diffusiophoresis or sedimentation is responsible for the phenomenon.