Terry Kennaway, of Waikanae, asks :-

Five years ago I bought a house which had been lived in for years by a heavy smoker. Nothing I do has completely got rid of the smell and I am curious to know how there can continue to be smell output when there is no smell input. How do smell molecules behave? If I know that, maybe I can defeat the blighters.

Alan Happer, an organic chemist at the University of Canterbury, responded.

If I knew the answer I would immediately contact the insurers of houses that have been used as Labs for making Methamphetamine.They have a similar problem. So do the Organic labs in Chemistry departments.

Most organic compounds have a detectable smell, but are not sufficiently volatile for us to detect them. Organic liquids tend to be more volatile, but if they are, any smell tends to swept away by air currents. Also many do not have a strong odour, and this may even be pleasant. Tobacco smoke consists of very small droplets of a not very non-volatile organic liquid of mixed composition. Some of its constituents have an unpleasant characteristic smell. The droplets collect on any surface they contact. They remain there until they slowly evaporate over time.

The problem is the nature of the surface. In the case of metal or glass there is no problem. They can be removed from these by normal cleaning. However if the surface is organic in nature, as many are, before the droplets have sufficient time to evaporate they penetrate into it. (Organic droplets are attracted to surfaces of organic nature.) In a typical house, such surfaces would include those of fabrics (especially curtains and carpets), plastics, wood, paint and wallpaper. Of these materials, they can be easily removed from the curtains by washing or drycleaning, and carpets by cleaning (or replacement). All of the others could in theory be cleaned, but in practice the droplets will have penetrated into the surface, and as a result are much more slowly lost to the air. Unfortunately this means their smell is going to hang around much longer.

I think the problem is usually tackled by covering the surface (repainting wood surfaces or repapering wallpaper), but I don't know how effective it is. Any unpainted wood is usually varnished or polished with wax. This would prevent absorption of the droplets by the wood, but unfortunately they would be absorbed into the coating, which is also organic.