3 Smell (olfaction)

The human body could in theory detect thousands of different smells but not all molecules which reach your nasal passages can be detected using your sense of smell. To be smelled, a molecule must have certain characteristics. Consequently, your study of the sense of smell (or the process of ‘olfaction’) begins with the properties of the molecules that give rise to an odour. 

Odour molecules, or ‘odorants’, have a number of chemical properties which allow them to be both detected and identified by the olfactory system (the system which enables you to smell a particular substance). These are:

• size
• volatility
• chemical composition.

While you will consider them in turn, it is important to recognise that the properties are all related to each other.