Science explained: What is smell?

Freshly baked bread. Public toilets. Newly cut grass. Smells have an incredible influence over us, with some leaving us salivating whilst others make us wish we didn’t have noses at all. Catching a whiff of your dad’s post-Christmas Dinner fart seems both inevitable and unavoidable, but we rarely stop to consider the complex cocktail of chemicals swirling through the air into our nostrils, let alone the process that leads to us wrinkling our noses and kicking him off the sofa. With a whole 5% of our DNA dedicated to olfaction (the detection and translation of smells), it would seem it’s actually a pretty complicated process.

Smells start their journey at the source, with tiny molecules of the stinky substance drifting through the air, often carried by means of evaporation. Once these molecules reach our noses, they are filtered through rows of fine nasal hairs until they reach the olfactory epithelium – a tiny patch of skin responsible for the sensing of smells. This area is made up from hundreds of tiny receptors covered with mucus, which collect and absorb odour molecules as you breathe in. Molecules can also reach the olfactory epithelium through our mouths, taking a route via the back of the throat. Humans have around 450 different types of olfactory receptors, each capable of detecting several different types of odour molecules. Any individual smell could be one of a range of unique combinations of odour molecules meaning that, using our receptors and considering all the possible combinations, we can distinguish hundreds of thousands of different scents from one another. Once an odour molecule reaches a receptor cell, the two bind together, generating a signal which gets sent to a part of the brain called the olfactory bulb. This sorts through the different signals and sends them on to other parts of the brain. The collection and combination of these different signals leads to us piecing together exactly what it is we are smelling.

Sadly none of this will help you to escape from the stenches that make your eyes water, but perhaps next time the bin lorry drives by you’ll spare a thought for how clever your body is and it might just distract you!