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Our noses are worthy of a bigger fuss than they perhaps receive. While they can be a handy place to rest your specs, or even decorate with a spot of jewellery, there’s still a lot of science going on inside that bump in the centre of our faces.

Up inside our nostrils, there are around 400 different types of olfactory receptors (the things which detect the chemical compounds that make up different smells), analysing each whiff which wafts up there, then sending the information to our brain. Once there, it decides if that particular smell is pleasant, repulsive, or simply neutral.

Somebody who knows a lot about that process is Professor Krishna Persaud from the University of Manchester. An expert in biochemistry, he has been studying the effect smells have on humans for more than 30 years. He spoke to ����ý Bitesize about the process which enables humans to differentiate between different smells, as well as the odours which can arouse different reactions in us.

What happens between our nose and our brain?

The 400 different olfactory receptors mentioned earlier aren’t very selective on their own. They have to work together to produce the tapestry of smells we experience in our lives.

“Each sensor can react to a number of different types of chemicals,” Professor Persaud said.

“The way we perceive smell is really due to the pattern of information that’s generated by all the different sensors being interpreted by the brain in terms of a particular kind of smell. One smell is not one chemical, it’s very often a mix of many, many different chemicals.”

When it comes to the impact a smell can have on our mood, that’s as much down to our previous experiences as the moment our nose begins to register it.

Christmas, incense and chocolate

“When you are born,” explained Professor Persaud, “you are more or less like a blank slate.”

“You associate odours with your experience. The smell of your mother, the smell of milk, these are all pleasant experiences.”

Another (usually) pleasant experience is Christmas, especially childhood ones. And if your household included a real tree, with the unmistakable smell of pine needles, it could do you the world of good during Decembers in later life.

Professor Persaud continued: “You might associate the smell of pine cones and the Christmas tree with a really nice occasion. It’s really reassuring. It changes your mood. You feel happier and cosier.”

The same can also be said of incense sticks, which are sometimes burned during yoga sessions to relax people. Producing woody smells, particularly sandalwood, Professor Persaud says these can be associated with other smells in our home environment, such as the wood in furniture or the polish used to clean them, calming us as we smell it due to its associations with a safe environment.

Another smell that can change your mood for the better is, perhaps unsurprisingly, chocolate. Professor Persaud said: “Chocolate has chemical compounds present in it that make us feel good. It releases endorphins in the brain so just the smell can make you feel better.”

Acclimatising to odours

We may not realise it but we’re surrounded by smells all the time. Thanks to the relationship between our nose and brain, however, we’re not aware of all of them.

Professor Persaud explained: “You can saturate your brain in terms of the stimuli that you’re sending to it. If you have a smell that’s present in the environment, you adapt to it very quickly and stop perceiving it.

“Say you go into your kitchen when you get home. You open the door and get a blast of cooking smells. You re-enter two minutes later and you can’t smell a thing. What your sense of smell is hardwired to do is react to the changes in your environment rather than a constant environment.

“It’s an adaptation phenomenon that happens both at the receptor level of the nose as well as in the brain. Your brain will shut down to certain stimuli and let others through.

“This is very important. Think of a little mouse. It needs to react rapidly if a predator comes along, such as a cat. If an environment is already saturated with the background smell of a cat, the mouse perhaps won’t care. But if a cat comes in and gets closer, the mouse will care because that odour [of the cat] is changing.”

Why new car smell and coffee makes us feel special

Few things say extreme retail therapy like that first trip in a brand new car, complete with that ‘fresh out the factory’ smell which is both unmistakable and difficult to describe at the same time. Is it plastic? The upholstery? The tyres?

Whatever the cause, it’s something the industry is keen to retain. Professor Persaud said: “It’s a very important thing for car manufacturers. It’s very often, deliberately, induced into the car to make people perceive a sense of luxury so a smell of leather, for example, is one that complements a new vehicle.

“Car manufacturers are very conscious of bad smells so they will do a lot of tests on new cars before sending it out into the market to make sure the smell is satisfactory to the customer.”

But we can still sense luxury in a product, even if the smell is slightly different each time. A good example of this is coffee.

“The smell of coffee is several hundred chemicals altogether in your nose,” said Professor Persaud.

“You will say ‘it’s coffee’ but then that particular mixture you’re sensing for one brand of coffee will be slightly different for another. A high quality espresso will be very different compared to a brand of instant coffee. You will still smell coffee but a completely different kind because the composition of those chemicals will be different.”

How you react to those smells depends on your preference for a particular brand. Context is everything. While some may enjoy the smell of espresso coffee, others will hate it as they associate it with, for example, a bitter taste. As with other smells, your reaction is based on a previous experience, so if you sipped a particularly upmarket brand of espresso in a sunny Italian square during a dream holiday, smelling it again years later will hold a positive memory.

The smell not everyone can smell

In his 30 years of study, there is one smell in particular that fascinates Professor Persaud above all others. Mainly because fewer than two-thirds of us are capable of smelling it in the first place. It’s called androstenone and it’s a pheromone found in pigs.

He said: “About 40 per cent of the population cannot smell it and of the people who can, they find it to be a stale, urine-like odour. It’s very off-putting. It’s fascinating because if you can perceive it, it’s detectable in 0.02 parts in every billion of this compound. People who can’t smell it, can’t smell it at all but for those who can, it’s unbearable.

“People who can’t smell it are not expressing their receptors for that compound.”

And for those who can, it’s unlikely to do much to boost their mood.