These molecules enter the nose where they bind to hair-like extensions called cilia; the cilia in turn are attached to smell ‘receptors’, or nerve cells, at the top of the nasal passage.
If enough molecules reach a cell, it fires a signal down the olfactory nerves that run from the nose to the brain, where the smell is perceived.
As we get older we experience a ‘major decline’ in our ability to smell things. This process may be exacerbated by colds and pollution.
‘Every time we get a bad cold or get close to pollution, a toll is taken, causing damage to the epithelium (the area of the nasal cavity containing smell receptors), which can accumulate over time,’ says Richard Doty, professor of otorhinolaryngology and director of the Smell and Taste Centre at the University of Pennsylvania School of Medicine in the U.S..
People who have more colds may therefore be more likely to have a poor sense of smell later in life. Half of those aged 65 to 80 experience this ‘demonstrable loss’ of smell, says Professor Doty, while for those over the age of 80, three out of four people are affected in this way.
This in turn can have a dramatic effect on sense of taste – 75 per cent of what we perceive to be taste is actually smell, which is why we prefer stronger flavours as we get older.
Women’s sense of smell is best
Women tend to have a more acute sense of smell than men – one theory is that hormones play a part.
Indeed, women’s sense of smell may be most acute around ovulation, when oestrogen levels are highest.
‘These same fluctuations in smell cycle tend to occur in women on the Pill, too,’ says Professor Doty. Pregnant women also sometimes report a stronger sense of smell.
There may be an evolutionary reason for women being better able to detect smell – it could be protective, explains Tim Jacob, emeritus professor in smell and taste at Cardiff University.
‘When breast-feeding, for example, women need to be very selective about what they eat, and smell is an important part of that,’ he says.
One of the functions of our sense of smell is to alert us to dangers, such as poisonous gases or burning. However, we can adapt to unpleasant odours over time.
‘Take Gorgonzola cheese,’ says Professor Jacob. ‘It smells very bad and children hate it.
‘That bad smell is bacterial, but it’s not actually toxic or hazardous and we learn to overcome that first impression of disgust or aversion.
‘It takes days or years, depending on how often you are exposed to it.’
Similarly, after a few minutes of being exposed to a strong odour, such as fresh bread in a bakery, we tend not to notice it until we’ve left the room and come back.
Professor Doty explains: ‘The brain operates in terms of change and contrast – when there’s no contrast, you could say it becomes bored.’
Exercise boosts your receptors
Your sense of smell may be more acute when you exercise.
Exercise increases the air flow to the nose because you are breathing more, so more air flows over the smell receptors, explains Carl Philpott, honorary consultant ear, nose and throat surgeon at James Paget University Hospital in Norfolk.
‘Some of my patients say they get their smell back temporarily while exercising and then it goes away again.’
There may be other times throughout the day that your sense of smell improves – this is down to our ‘nasal cycle’, he says. ‘One side of the nose is always more congested – the blood vessels are more engorged – than the other in order to filter dust particles from the air and warm up the air you breathe in.
‘The other side will be relatively uncongested so you can breathe properly. Every few hours the process will swap sides so there will be a point, lasting a few minutes, where they are relatively equal.’
Your sense of smell may be slightly improved at this point.
Can’t smell? It could be diabetes
Anosmia, the inability to perceive odour, affects around 200,000 Britons, while around 5 per cent of the population is estimated to have problems with their sense of smell. As well as age-related decline, causes include viral infections or a blow to the head, which damage the olfactory nerves.
Damage to the brain through stroke may also cause problems – and loss of smell can be one of the first signs of neurodegenerative diseases such as Parkinson’s and Alzheimer’s, says Professor Doty. These diseases damage brain cells, which may be important to the sense of smell. However, those affected may be unaware of their loss of smell.
Diabetes can have an effect too. As Mark Vanderpump, honorary senior lecturer in diabetes and endocrinology at the Royal Free London NHS Foundation Trust, explains: ‘Nerve damage is a known long-term complication of diabetes.
‘It happens when the small blood vessels that supply the nerves get damaged in response to chronic high blood sugar.
‘Loss of sense of smell appears to be more common in the diabetic population, because of damage to the olfactory nerves.’
‘Losing your sense of smell can be devastating,’ adds Professor Jacob. ‘About 17 per cent of people become clinically depressed.’
And weight gain can become a problem.
‘Having lost pleasure from eating, some people lose weight, but some actually gain weight because they overcompensate for lack of flavour by eating too many sweet things – you still taste these.’
How smell affects your sex life
There is an important link between sex and smell, and some people with anosmia report a loss of libido.
‘Smell is a form of communication between partners – people respond to the smell of their partner,’ says Mr Philpott. ‘A lot of my patients who lose their sense of smell say they feel a lack of bond with their partners.’
Indeed, our sense of smell may help determine our choice of partners in the first place, adds Professor Jacob.
‘We’re drawn to people who have a different set of immune genes from us – an evolutionary mechanism to encourage us to pass different immune genes to our offspring,’ he explains. ‘And our own body odour is determined by our immune genes.’
Why cut grass smells so good
Smell is probably the most evocative of all the senses, triggering memories and affecting our mood.
According to a recent survey of 2,000 adults, freshly mown grass, pencil shavings and baby powder are the aromas most likely to send Britons harking back to the past.
‘Interestingly, sense of smell is subconscious,’ explains Professor Jacob. This is because it affects our limbic system, the primitive part of the brain where emotions are stored.
The power of smell was illustrated by a German study in 2008, which found that people exposed to the smell of roses during sleep had pleasant dreams, while those exposed to rotten egg odour had unpleasant ones.
‘Smell is the only sense which doesn’t switch off when we sleep,’ adds Professor Jacob.