Take a long, deep breath. What do you smell? Chances are not much, seeing as most of us live in a sanitised world largely devoid of powerful odours.
You might think that’s a good thing – no one likes their body or home to have an overwhelming scent – but a lack of strong aromas might in fact be putting your health at risk.
That is according to neurobiologist Dr Michael Leon, a professor emeritus at the University of California, Irvine, who says that what we smell is vital for protecting our brain health – and much more.
He says: ‘Most people think we’re overstimulated in the modern world, but in the case of smell, we are under-stimulated.
‘Our brains evolved in a far smellier world than the one we live in today – strong odours were unavoidable. These days our sense of smell gets far less stimulation and as result we’re all olfactory deprived.’
The theory is that we spend too much time in air-conditioned offices and too little time in nature – and even when we do go for walks, human activity and climate change have significantly reduced plant diversity (and hence the novelty of smells) around us. Being ‘olfactory deprived’ is a problem because how much we smell is linked with how well we can smell, which itself is vital to our health. Dr Leon and his colleagues recently linked changes in our ability to smell to 139 conditions, including Alzheimer’s disease, anxiety, depression, sleep disorders and headaches.
While an inability to smell can sometimes be a symptom of a health issue – such as Covid, which can cause a temporary loss of function in tissue cells lining the nose – in other cases it seems increasingly likely that a loss of smell precedes a disease and may even contribute to its development.
Scientists are beginning to figure out why, and are examining whether boosting what we smell could bolster our olfactory abilities – and keep us healthier for longer.
A lack of strong aromas might be putting your health at risk, according to neurobiologist Dr Michael Leon
Scent receptors in the nose are linked with several brain regions involved in memory, emotion and language.
These pathways are partly why smells so easily trigger memories of our childhood or remind us of meaningful people in our lives. However, networks can degrade with age or disease, which may explain why a failing sense of smell can harm brain function.
Dr Leon believes olfactory deprivation contributes to this problem as the more we stimulate pathways in the brain, the stronger they become.
The theory is if our sense of smell is under-stimulated, these vital neural pathways degrade, potentially contributing to conditions such as dementia.
‘We’re working with deprived brains, so it’s no wonder that our memory goes south as we get older,’ Dr Leon says.
And it’s not only brain function that can be affected by smell. So too can inflammation – and therefore our immune system.
For a study in 2022, people were given various foul odours to sniff, which researchers found increased the concentration of a protein called tumour necrosis factor (TNF) in the participants’ saliva. This protein increases inflammation, helping the body fight infections.
In contrast, pleasant scents associated with positive memories have been shown in separate studies to decrease TNF and other chemicals associated with inflammation.
Chronic inflammation has been associated with multiple conditions, including heart disease and Alzheimer’s.
So could pleasant odours reduce the risk of some of these conditions?
To investigate, Dr Leon and his colleagues asked adults aged 60 to 85 years old to smell seven different pleasant aromas – rose, orange, eucalyptus, lemon, peppermint, rosemary and lavender – for two hours every night by using a diffuser over the course of six months.
The group exposed to these odours saw a startling 226 per cent improvement in memory tests compared with a group who went through the same routine but using very faint scents, according to the results published in Frontiers in Neuroscience in 2023.
Exactly how this process works to boost memory is not yet clear. But it may be that smelling lots of different odours over a lifetime strengthens and protects neural pathways in a manner similar to the concept of cognitive reserve – the idea that stimulating your brain builds up a ‘reserve’ that protects against the effects of age and damage.
People who speak a second language or who play a musical instrument have reduced risk of dementia, for example.
To counter the effect of olfactory deprivation we should seek out as many pleasant smells as we can, according to Dr Leon. ‘We call it “olfactory enrichment”,’ he says.
Forests might be a good place to start, as smelling pinene – the chemical released by many plants and trees – has several health benefits.
For instance, research published in the International Journal of Immunopathology and Pharmacology found pinene can have a calming effect by lowering levels of stress hormones adrenaline and cortisol as well as increasing levels of natural killer cells, which help the body fight infection and cancer.
And what you inhale while out on a walk might do even more than boost your smell intake. Scientists at the Royal Melbourne Institute of Technology in Australia and at the University of Newcastle recently proposed in the journal Advances in Nutrition that we can actually inhale beneficial nutrients (potentially including vitamins A and B12) from our environment: so-called ‘aeronutrients’.
In fact, a study of children in Ireland in 2016 found that those who lived near seaweed-rich beaches had higher concentrations of iodine (a nutrient released as a gas by seaweed and which is needed by the human body for metabolism) in their urine than children who lived elsewhere, despite similar diets.
‘These children showed almost three times higher iodine levels and lower deficiency rates than children living in seaweed-scarce coastal or more inland areas,’ says Dr Jean Debedat, a postdoctoral fellow, who researches personalised medicine and metabolic disease, at the University of California, Davis.
‘This study strongly suggests that breathing airborne iodine from natural environments can enhance [your] nutritional status,’ he adds.
Meanwhile, Dr Leon and his colleagues have developed Memory Air, a device that expels 40 odours twice a night as you’re sleeping.
Based on their and other experts’ research showing that being exposed to novel pleasant smells can boost cognition, they claim that Memory Air will help to prevent memory loss with minimal effort.
Set to launch by the end of the year, it will cost around $799 (£600) on top of a monthly subscription. While Dr Leon says the more pleasant smells we can inhale each day the better, Dr
Debedat has a word of warning: ‘We shouldn’t always trust our brain as it can play tricks on us. Smelling something pleasant doesn’t always mean it’s good; for example, indoor candles and incense, though lovely, can release harmful pollutants.
‘Always stick to the great outside if you are lucky enough to live close to it.’
