A UNSW researcher explains how our perceptions of smell differ and why products like perfume or deodorant donโt always make us smell nicer.
We all perceive smells differently and thatโs why one personโs pleasant perfume can be anotherโs nauseating pong, a UNSW researcher explains.
Dr James Hayes, of UNSW Engineering, said there were a range of factors which influenced how people detected odour and these could change over time.
His background in psychology has informed his research on the impact of arduous odours on communities, among other topics, and his work for the UNSW Odour Laboratory.
Dr Hayes said a humanโs ability to process odour differed to other mammals, such as dogs or rodents, which had large portions of their brains devoted to detecting odour.
โMany parts of the human brain have been co-opted to regulate other things, for example, emotions,โ he said.
โSo, even though these areas of the brain are used for emotions, because they are still related to odour you find that a lot of odours emotionally affect people quite strongly.
โWe also have olfactory memories which are typically the earliest memories that we form, as well as being the longest lasting; for example, particular smells might remind you of your childhood.โ
How humans perceive smell
Dr Hayes said humans perceived scent via olfactory receptor neurons (ORNs), located in the nasal cavity, but ORNs could change in effectiveness according to a personโs age, health or environment.
โIf anything disturbs how those ORNs sense odours, then you will have changes in your perception of smell,โ he said.
โAge is certainly a factor: ORNs constantly recover and regenerate as they break down from exposure to chemicals, but age slows down that process a fair bit.
โI did a study which showed that smoking also changes your perception of odour.
โSome mental health conditions can even influence a personโs sense of smell.โ
Dr Hayes recalled the time he suffered temporary anosmia, meaning he lost his sense of smell. A related condition is hyposmia: a reduced smell sensitivity.
โThere are certain odours that will completely burn off your ORNs if they are in high enough concentrations. This happened to me some years ago in a lab,โ he said.
โThere were a whole bunch of chemicals for odour testing that had been forgotten about for about 10 years and had expired. Most of the glasses had cracked.
โI helped to move the box of chemicals and my sense of smell became very strange โ all I could smell was something between wood, glue and ball bearings โ and then my sense of smell disappeared for about three weeks.
โMy neurons were completely burnt off, but they grew back.โ
Dr Hayes said traumatic injury and genetics could also influence the ability of ORNs to regenerate.
Gender and culture
Dr Hayes said several other surprising factors also played a role in our ability to detect odour, including gender and cultural background.
โPeopleโs sense of smell deteriorates as they age, but it seems to happen faster for men,โ he said.
โWomen, overall, have a better ability to detect odour โ they are able to smell more odours than men, and also perceive those odours more strongly than men.
โBut generally, peopleโs ability to perceive odour starts to worsen about age 60, although there is a great deal of variation between individuals.โ
Dr Hayes said cultural differences also affected a personโs sense of smell.
โWhile itโs difficult to separate cultural differences from genetics, a large study which presented a range of odours to different cultures around the world, showed that cultures which had little experience with particular odours were less likely to be able to detect them,โ he said.
โFor example, the study revealed that people in Japan had a difficult time detecting liquorice, for instance, and all the aniseed flavours.โ
Human instinct
Despite many factors affecting our ability to perceive odour, however, humans had an evolutionary understanding of whether particular smells represented a safe or potentially dangerous situation, Dr Hayes said.
โThere are some things in nature which are very good markers for humans to avoid, for example, things that emit sulfur are typically decaying or rotten โ they have a lot of bacteria in them โ so, we have a natural impulse to avoid those smells we understand are bad,โ he said.
โAmmonia is another one which has a very distinct smell and when itโs in sufficiently high concentrations, it can be fatal โ so, we have learned to stay away from such sources of odour, because in the long run that will save us.
โOn the other hand, pleasant-smelling things like flowers suggest freshness and people give flowers to each other, which relates to the emotional part of our brain in a positive way.โ
Nonetheless, Dr Hayes said there were certain gases and chemicals which smelt pleasant or had no scent, which could still represent a fatal source of odour to humans: for example, carbon monoxide.
โSo, humansโ instinctual understanding of which odours are safe or unsafe is imprecise, but in terms of our ability to detect rancid odours like sulfurs, we are very good at detecting them and about as good as the best analytical equipment available,โ he said.
Why people smell
Having explained the factors behind humansโ ability to detect odour, Dr Hayes turned to the question of why people have body odour.
He said hygiene, diet and environment were major factors, excluding medical conditions which could cause unpleasant body odour, such as liver disease, diabetes or kidney issues.
โThe human body produces a lot of different oils from various glands, such as sebaceous oils and other oils which make the skin smooth, greasy or whatever it might be,โ Dr Hayes said.
โThese oils are odourless, but we have bacteria on our bodies that process and digest these oils and that’s where body odour comes from.
โEnvironment also plays a role; for example, if youโre out in the woods you probably smell earthy, or if you’re in an industrial area, you probably smell smoky.โ
Dr Hayes said diet also caused body odour.
โThe classic example is garlic โ your body processes it into allyl methyl sulphide (AMS), which is then exuded as an unpleasant odour through your pores,โ he said.
โSo, as the saying goes, you are what you eat; almost anything you eat will have some impact on your odour.
โBut chances are, if itโs a common food amongst people you associate with, youโre less likely to notice the body odour from it, if at all.โ
The strawberry conundrum
Dr Hayes said masking oneโs body odour with scented personal care products, such as perfume or deodorant, would not necessarily have the desired effect of making you smell better.
โIโve examined this in my research: itโs something I call the โstrawberry problemโ. Compare strawberries and lemons. The smell of lemon is obvious and comes predominately from one chemical, limonene,โ he said.
โStrawberries, on the other hand, have about two dozen chemicals that are responsible for the smell of strawberry, but your body does not tell you that you are smelling different chemicals, it just says you are detecting strawberry.
โThis is unusual, because having one chemical in lemons, or multiple chemicals in strawberries, both result in the same outcome: you are smelling a particular piece of fruit.
โAnd, we can smell lemons and strawberries individually at the same time, because of complex neural processing in our brains.โ
Dr Hayes said the same principle applied to mixing scented products like deodorant with body odour.
โDeodorant could be made of multiple chemicals and you detect it as one particular scent, even if itโs overwhelming your ORNs,โ he said.
โYour brain, however, is still capable of putting the deodorant scent to one side and also noticing body odour at the same time.
โWhen I was in school after sport, everyone drowned themselves in body spray in an effort to completely overwhelm everyone elseโs ORNs and kind of hide their body odour, but it does not really work that way.โ
Dr Hayes said another problem with scented products was people attributing certain scents to negative situations.
โYou could have regular encounters with someone with bad body odour who is always using a particular deodorant,โ he said.
โUsing classical conditioning from psychology, you will associate that particular scent with a bad experience because you also smell that personโs body odour โ it happens all the time.โ
In conclusion, Dr Hayes said he doubted the effectiveness of using scented products to combat body odour unrelated to medical conditions.
โIn theory, it could be possible to find some sort of chemical to treat and remove body odour, and many companies may well claim their products do,โ he said.
โThings like deodorant might help with mild body odour, but the best thing you can do is practise good hygiene and watch your diet and environment.โ
