Why what you smell may not be what I smell….

Have you ever wandered around a supermarket, an airport or just sat in a sunny park, when a fellow human passes you, leaving behind an astonishing scent-trail? A frying-pan in the face moment for you - yet they seem blissfully unaware of the assault on your senses. So, why is that? It may be because human smell perception (aka olfaction) simply varies, due to several interesting factors, not least, the olfactory receptors themselves.

We have Linda Buck and Richard Axel to thank for discovering them, not as you might think, in 1860 or even 1920, but 1991! Until the mid 1990s we really didn’t understand this smell stuff much at all. They were awarded the Nobel Prize in “Physiology or Medicine” for their three year quest up the human nose, delivering a clearer idea of what goes on when a little volatile molecule wanders up a nostril.

It turns out that the olfactory receptors are actually little proteins that sit on the surface of your olfactory receptor neurones. They spot different scent molecules in the air we breathe and then ping off messages to the brain that duly unscrambles them to understand how that last waft differs from the one before. The receptors are snuggled up next to the limbic system, which stores, sorts and replays our memories and emotions. So one seemingly innocent molecule can either transport you back to a nasty run-in with cough medicine or a heavenly day, sprawled out on freshly mown grass. This connection is known as the "olfactory pathway,” because scientists are good at finding things, but not thinking up poetic terms for the things that they find.

So every sniff is potentially an emotional roller-coaster; depending on your life so far. The smell of freshly brewed coffee could transport you to a romantic Parisian cafe…. or to the day your kitchen burned down. Having said that, there are some smells with more of a tendency than others to cause a relaxed or an alert state in most people. This isn’t a thumbs up from the limbic system, but the much more business-like Prefrontal Cortex. This is an area usually busy with decision-making and cognitive performance - fun stuff like that. Whilst we know that’s sort of what happens, there’s still no way of knowing whether you’ll get a shot of alertness with your rosemary smoothie unless you try it…I feel there may be another Nobel Prize, just waiting for someone to take apart the Prefrontal Cortex to examine its Smells filing system.

Back to the frying pan in the face moment in the park - you’ve just been engulfed in an eye-watering haze, so what is the wearer smelling? Probably not much, at least not much of the smell molecule group that just assaulted you. There are several contributing factors that could be having an effect…

We all vary in our ‘Threshold Sensitivity’, simply how much of a smell do you need before you can detect it? You may have a very low threshold and so you can smell most odours at a low concentration. But even if that is so, there will probably be anomalies in there, and you could be missing some altogether. As you age, or rather, as you become aged…it’s all a bit downhill: the concentration of a odour that you require to be able to detect it increases. Identifying it become more difficult, and distinguishing one from another becomes muddled….unless you practise! A well used, daily exercised, olfactory system of 80, can result in a more reliable olfactory information, than a youthful, but ignored group of receptors.

Then there is ‘Odour Intensity’ to consider. Some people can spot most odours the moment they’re airborne, but the ability does vary between individuals of any age. A gentle dab to one person can seem like drowning in a lake of eau de yuck to another. And the molecules themselves are not all made equal, as far as your olfactory receptors are concerned. Some hefty molecules may seamlessly float up and into your amygdala, barely raising a response from the odour check-points en route. Whilst a slip of a thing can set off all the trip-switches the moment it appears. The lady jogging behind the Wafter, could own a nose that misses both molecules altogether, yet a third option would leave her spluttering. And that third one might not even tickle your olfactory police, and so leave you uninterruptedly watching the sky, wondering if it’s worth taking out your phone for a dinosaur-shaped cloud.

Even if you and the lady jogger were to agree, you can both smell it, there is Odour Hedonics to consider… What might be ‘Snuggle Heaven’ to her could be ‘Cold Raindrops down the Spine’ to you - same as why your dog just seems prettier than all the other dogs….to you. 

Then there’s quite an interesting factor - the Switchy-off-and-on Sensation, as I think it should be called, but you may prefer the formal: Sensory Adaptation. This happens when you’ve been wearing a scent all day, your nose is bored, bored, bored. (It is easy to bore your nose). It likes new and exciting and when it doesn’t get anything, it loses interest. Whereas someone else, whose nose has become bored rigid with the scrambled eggs and herring they’ve been cooking all morning, will be just delighted to sniff you and your knock-em-sideways smell (your nose may not feel the same way about the new smell of fish and eggs coming its way). But the really interesting bit is that some scents can fool your easily bored appendage. The methyl ionones are well-known, pesky little volatile molecule examples. So, you sniff it, you get a violet scent, file that, trigger the relevant scent memories and continue your day. Except, 30 seconds later, you could sniff every inch of your wrist, scarf, hair and you would still get nothing - just air. You continue on you way, planning to return this very short-lived scent to the scent shop, when ‘Hello!’ It’s all about violets again, for about thirty seconds…..and then it’s gone again. This can liven up a dull day in the perfume making lab. It isn’t the only odour pixie out there, Galaxolide, for instance, is a mildish, sweetish, muskish sort of scent, in lots and lots of perfumes, but almost half the world are missing it’s olfactory delights entirely - because we don’t all have identical olfactory receptors, so for some, there’s nothing to receive it with…and certain genes are associated with specific receptors. Interesting eh?!

Well whilst that fascinating fact circulates, time for the really special-interest-nugget called the ‘Cross-Modal Interactions’! Which just means that your perception of an odour also depends on what you are looking at when you smell the smell, and what you are listening to and whether you’re happy or sad, hot or cold…..etc. It also changes when that limbic system of yours is in the driving seat too. Prolonged anxiety and depression can lead to hyposmia and even anosmia (no smell) although the latter is more likely to happen after a very nasty thwack on the receptors, or a virus with a talent for switching off smell and taste - the effect is usually temporarily. 

So, there’s your answer - we’re using different olfactory equipment, yours could be a younger model, I could be anxious, you might be cold, I might have strange associations with the scent of violet, which come and go, and I may spend my day scrambling eggs whilst my nose is yearning for open pastures. All of which could lead to you loving a perfume that just feels like summer flowers, but makes me think of wet dogs.