An emerging public issue in Denmark is passive smoking in residential environments where non-smokers are exposed to harmful smoke from their neighbours. There are various ways that smoke infiltrates one flat from another. The air infiltration rate between two flats in a multi-storey building depends on the construction, tightness and age of the building. Earlier results from this project showed that, in the most critical cases, the transfer of ultrafine particles was about 9% when the source flat was located below the receiving flat. The purpose of the present study was to identify the ways in which smoke infiltrates from one flat to another and also to examine technical solutions for preventing or reducing infiltration of ultrafine particles from the source flat to the receiving flat. One of the technical solutions examined was sealing of the floor in the receiving flat. The study was carried out in the field in a multi-storey building and cardboard and plastic foil of polyethylene were used for sealing the entire wooden floor in the receiving flat. Another technical solution examined was a novel air circulating ductwork. The efficiency of the novel air circulating ductwork was examined by investigating the removal of ultrafine particles from a lit cigarette in a laboratory environment. The results showed that in the case where the receiving flat was sealed, the concentration of particles in the receiving flat was non-correlated with (or independent of) the emission of particles in the source flat. The test of the air circulating ductwork showed that the removal efficiency ranged from approx. 30% to 60% after 10 minutes, i.e. when the cigarette had burned out.