Appliance of Science: How are smells made?

Smells are memory makers.

This week’s question comes in from Rob in Co Clare; he would like to know how smells are created and how they travel.

You have probably heard that humans have a poor sense of smell compared to other animals and that it is less important than some of our other senses, but that’s not true at all; smell is a fascinating sense, it triggers our memories and emotions and is the first sense we use at birth. Let’s start with the basics.

How are smells made?

A smell is created when a substance releases molecules (particles) into the air. For us to detect the smell, those molecules need to enter our nose.

The more volatile the substance is (the more easily it gives off molecules), the stronger its smell. Non-volatile objects, such as steel, don’t have much, if any, smell at all.

How do smells travel through the air?

Smells travel through the air by a process called diffusion; air particles, and the odour particles within the air, move freely in all directions. They are constantly moving and eventually they spread out through the air around them.

How do we detect smells?

Once the odour particles enter our nostrils they are detected by olfactory receptors near the back of our nose. These receptors then send signals to an area of the brain called the olfactory bulb and the chemical composition of the mix of odours it determined. What is really interesting is that the smell detection pathway within the brain also connects to the amygdala and hippocampus, areas connected with emotion and memory. Smells can literally change how we feel or help us make connections and lay down memories.

Some people have no sense of smell, or they lose it during their lifetime. This is called anosmia. When we lose our sense of smell it can have a big impact on our psychological wellbeing.

The genetics of smell

A large portion of the human genome is taken up with the olfactory receptor gene family. That is quite an incredible fact. For a sense that has often been dismissed, we have actually vested a lot of our genetic code in it. Not so surprising when we consider that it is usually our first line of defence.

One study suggests that humans can detect and discern one trillion different odours.

Smelling with our whole body

Olfactory receptors were once thought to be localised to a small area within the nose, but in recent years they have been detected all over the body; from muscles, kidneys, liver, lungs and even blood vessels. Why do we need to smell things inside our bodies? Perhaps these receptors have a basic function to play in many parts of the body.

Olfactory receptors are sensitive chemical receptors; they detect changes in chemical concentrations. It is likely that they act in a similar manner in other parts of the body, detecting the presence of certain chemicals, or the changes in their concentrations. Olfactory receptors have even been found in human sperm. It is thought that they actually detect chemicals produced by the egg, and swim towards this ‘smell’. Who knew that smell had such an important role to play in the inception of life itself!

Naomi is a science communicator and mother to three inquisitive children. She can be found at

Email questions to


Dónal Clancy is a musician from An Rinn in Co Waterford. He will perform the music of his late father, Liam Clancy, in a special online solo performance on Thursday at 7pm as part of this year's Clonmel Junction Festival.Question of Taste: Dónal Clancy

BETWEEN 1973 and early 1975, John Lennon split with Yoko Ono, took up with his assistant May Pang and embarked on a period of intense creativity and outrageous behaviour. Lennon later described this time as his “lost weekend”.Rufus Wainwright has returned a new man

Stan O’Sullivan tells Ellie O’Byrne about the genre-busting album from 2007 that probably doesn’t get the recognition it deservesB-Side the Leeside: 'Louder & Clearer' with Stanley Super 800

In recent times one of the most recurring and troubling conversations I have with teenagers, in therapy, is around their use of marijuana. Often parents seek out therapy because they have noticed a dramatic shift in their child’s behaviour.Richard Hogan: Beware of making light of your teen's marijuana use

More From The Irish Examiner