Analysis: from cut grass to deodorants to smelly cheese, the olefactory cells in our nasal passages are always hard at work

By Sylvia Draper, TCD

What is smell?

Smell is our response to detecting air-borne volatile chemicals. At the back of your nose, between and just below your eyes, are nasal passages. They have a small patch of skin about the size of a five cent coin that is covered in fine filament-like cells or cilia (about five million of them). These are olefactory cells and they are coated in mucous. With each breath, you drag in air-borne chemicals that get picked-up and dissolved in the mucous and attach themselves to the cells. Our response is very instant because each cell is connected through a single synapse (one junction only between the nerve endings) directly to our brain.

Why are feelings wrapped up with smells?

Smells are processed in the same part of the brain (the limbic system) that handles memories and emotions. This means that memories and smells are intimately connected and we can recall where and when we experienced that smell before. Research shows that the recall of smells is still 65 percent accurate a year later, whereas our visual memory drops rapidly after three months. Smell therapy or smudging relies on tapping into positive connections. Some of these appear to be innate e.g. laboratory rats naturally respond fearfully to cat odour despite never encountering a cat.

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From RTÉ Lyric FM's Culture File, artist Becky Lyon on what the apocalyptic hellscape that is our future could smell like.

Can you predict whether a substance will have a pleasing or disgusting smell ?

The problem is that there are 1,000 identified types of receptor cells and one smelly molecule can bind to more than one at a time and in different ways. Sometimes there is clearly one way that dominates and you can relate it to one particular part of the molecule or the molecule’s shape.

Two compounds that differ in lots of respects, but have similar shapes are camphor and hexachloroethane. They smell the same which suggests that the detection process is lock-and-key based with similar-shaped keys (the molecules) unlocking the same smell response.

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From RTÉ Radio One's Mooney Goes Wild, Professor Emma Teeling from the Centre for Irish Bat Research, at UCD on bats’ sense of smell

Other molecules have similar active (functional parts) e.g. mercaptans which contain Sulfur smell like bad eggs and are deliberately added to natural gas to give it a smell. Amines which contain Nitrogen (with a lone pair of binding electrons) smell like fish.

Esters are easy to make from an acid and an alcohol, contain two Oxygen atoms – one in the carbon chain and one C=O pointing out of it. They tend to smell pleasant, but the smells are quite different.

We are 10 times more sensitive to putrid smells than musks or minty ones and 10,000 times less sensitive to pungent ones.

Below are some examples.

Formic acid (ants bites) and isobutanol – give you rum.

Salicyclic acid (wart remover) and methanol – give a wintergreen, deep-heat smell

Most esters are responsible for fruity odours like apple and pear – but the raspberry smell isn’t – it is a ketone i.e. only one O is present. So it is not as simple or straightforward as it seems.

Recent theories suggest that it is the location and distribution of electrons in the smell molecule that are important. It may be the vibrational frequency between connected atoms in the odour and their knock- on effect on the vibrations in the cell.

When your world smells of roses

How good is the human nose ?

The human nose can distinguish and identify 10,000 different odours. It even has chiral receptors which can pick up minor differences in the chemical structure of a compound. These are not obvious differences like one has a bit missing. Here the difference is a small as telling your right hand (four fingers and a thumb) from your left. We call them isomers.

Take as an example, two isomers called S- carvone and R-carvone (they are mirror images of each other). In structural terms, they have a hexagonal body with the same two arms sticking out, but one stands with its right foot out in front and the other its left. That is the only difference and yet S-carvone smells like spearmint and R-carvone like caraway seeds. Limonene is another example – one isomer smells of lemons and the other of turpentine!

Animals and fish have a much larger portion of their brain devoted to smell. Salmon migrate via smell elephants locate water by small over 100s of km. Nevertheless we can detect certain smells like ethyl mercaptans (an additive to natural gas to give it a smell) in one part in 2.8 billion parts of air. We are 10 times more sensitive to putrid smells which multiply activate the receptor cells than musks or minty ones and 10,000 times less sensitive to pungent ones.

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From RTÉ Lyric FM's Culture File, Dublin-based smell hunter Benoit Nicol  on his natural oils startup, The Nature of Things

Can you group substances according to their smell?

There is a lot of commercial and scientifically–related research in the cosmetic, deodorant and food industries. Trinity College Dublin chemistry graduate George Dodd isolated the key smell ingredients of whiskey and chocolate. Smells and the chemicals responsible are classed differently. They have floral and herbal categories in scents and citrus and caramel categories in food.

Scientists are determining the primary smell categories (like the primary colours) from which all other smells are made e.g. musks, mint, floral, camphors, ethereal, pungent and putrid. When a person is immune to one of these types they are said to be anosmic which is the smell equivalent of colour blindness.

Smells and the disguising of smells

Cut grass: it is a distress call from the damaged plant. There is a rapid release of traumatic acid to promote new growth and hexene and hexan, alcohols and aldehydes and Z-3-hexenylacetate (an ester). These ingredients are antiobacterial and protect the damaged plant from bacterial and fungal infections. They have a sharp, awakening odour. As grass-cuttings dry, cellular breakdown releases acetaldehyde which has a different smell.

Deodorants: sweat has no smell until it is broken down by the bacteria on the surface of the skin so deodorants contain alcohols and antimicrobials to destroy the bacteria that cause the smell.

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From RTÉ Lyric FM's Culture FIle, Xyrena's head perfumer Killian Wells discusses his range of punk perfumes 

Air-fresheners: these have a variety of working mechanisms. Charcoals absorb molecules reducing the chances of them reaching your nose, while cylindrical cyclodextrins have an hydrophobic interior which traps molecules and fragrances to mask smells and block the receptors.

Anti-perspirants: these contain aluminium and zinc salts which dissolve in sweat leaving a thin coating on the surface of the skin to block, clog or close the sweat glands. They are sometimes coated in microspheres of corn starch so that they are released at different times.

Professor Sylvia Draper is Professor in Inorganic Chemistry at the School of Chemistry at TCD. She a former Irish Research Council awardee.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ