Appliance of Science: How are smells made?

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

Appliance of Science: How are smells made?

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

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