Putrescine originates in rotting flesh, and is quite literally, the smell of death. It contains two amine groups, one at either end of the molecule, and is one of the breakdown products of some of the amino-acids found in animals, including humans. Although the molecule is a poisonous solid, as flesh decays the vapour pressure of the putrescine it contains becomes sufficiently large to allow its disgusting odour to be detected. It is usually accompanied by cadaverine, a poisonous syrupy liquid with an equally disgusting smell.
Putrescine and cadaverine also contribute towards the smells of some living processes. Since they are both poisonous, the body normally excretes them in whatever way is quickest and most convenient. For example, the smell of semen and urine are 'enriched' by the presence of these molecules, as is the odour of bad breath. A related molecule, hexamethylenediamine is one of the monomers used in making Nylon-6,6. However we cannot smell this in nylon because the amine groups are chemically bonded to their neighbours in the polymer backbone.
