Millions of people are completely oblivious to their own body odour or bad breath because the human brain automatically stops registering familiar smells.
Experts say this process, known as olfactory fatigue or habituation, explains why a person's own flower-wilting stench can go unnoticed even when it is obvious to others.
"When an odour continues to be present, the information is no longer new and so the smell or olfactory system adapts," said Carl Philpott, a professor of rhinology and olfactology at Norwich Medical School.
This adaptation happens in two olfactory bulbs located above the roof of the nose, where smell nerves connect and signals are processed.
"Inside the bulbs are interconnecting nerves that perform what's known as 'down-regulation', this means they block the same signal being sent on to the brain unless the information is new," Philpott said.
Simon Gane, a consultant rhinologist and ear, nose and throat surgeon at the Royal National Ear Nose and Throat and Eastman Dental Hospitals in London, said the nose tunes out pungent familiar smells for self-protection.
This allows the nose to be more sensitive to unfamiliar smells, which makes sense from an evolutionary perspective because it helps determine if a response is required, such as running to safety from a fire, he said.
"The challenge with body odour is that you can never truly escape your own smell. Your own scent is constantly right under your nose, so you quickly become accustomed to it," Gane said.
He said this also explains why people cannot smell the highly distinctive aroma of their own home, even when it is obvious to visitors.
Temporary desensitisation
The sense of smell only becomes less reactive to specific smells it is exposed to regularly, and the process is temporary and reversible.
A 2021 study by the Technical University of Dresden published in the journal Chemical Senses found adults who were exposed to two distinctive chemicals daily for two weeks experienced a drop in sensitivity to those specific odours.
The participants were exposed to manzanate, which smells intensely fruity and crisp, and irone alpha, which smells floral, woody and powdery.
They recovered their original sensitivity after a week without exposure, and the researchers found people did not become less sensitive to smells in general.
Desensitisation is not the only reason people cannot smell their own body odour. About 3.3 million people have anosmia, a complete lack of a sense of smell.
Simon Lloyd, a consultant ear, nose and throat surgeon at the Manchester Royal Infirmary and Salford Royal Hospital, said loss of smell is especially common with nasal polyps, which are soft, painless, noncancerous growths.
Other conditions that affect smell include hay fever, head trauma, tumours affecting the front of the brain, and infections such as Covid.
People with anosmia are mostly aware they have a problem with their sense of smell, whereas those with olfactory fatigue are not.
Causes of body odour
Body odour can be caused by poor personal hygiene, but excessive sweating due to fluctuating hormones, such as changes in oestrogen and progesterone during the menopause, or a medical condition called hyperhidrosis is also a common factor.
Sweat itself does not smell, but bacteria naturally found on the skin break down fluid released by the sweat glands, particularly in the groin and underarms, producing the compounds that cause body odour.
Medications including some antidepressants and antibiotics can cause the body to sweat more, providing more fluid for bacteria to feed upon.
Eating sulphur-rich foods such as onions and garlic can lead to the release of odour-producing compounds through sweat. Wearing synthetic materials such as nylon and polyester can trap heat and moisture, creating an environment where bacteria thrive.
"Whatever the reason, if you have an ongoing problem with BO, your nose just becomes even more accustomed and you won't realise how ripe you may be," Gane said.
Bad breath and olfactory fatigue
Habituation also means people often do not realise they have bad breath, or halitosis, which affects approximately 25 per cent of the UK population according to National Health Service data.
Alexa Balan, a London-based dentist, said the back of the tongue is the most common cause of bad breath.
"The tongue is not smooth, it has tiny papillae with deep fissures between them where bacteria, food particles, dead cells and mucus can collect. This creates the perfect environment for bacteria that produce unpleasant smells," Balan said.
Gum disease or a drop in saliva production, which can be caused by a dry mouth from taking antihistamines, can also play a part.
Azad Eyrumlu, dentist and chief executive of the Banning Dental Group in London, said olfactory fatigue is a very real phenomenon in terms of bad breath.
"Our brains are remarkably good at filtering out smells that are constantly present, which means people can become unaware of their own breath over time," Eyrumlu said.
Soumaya Zinet, a cosmetic dentist and founder of the London dental clinic Invisible Braces 4u, said bad breath can go unnoticed because it may not come with obvious symptoms like pain, and a person might have bacterial build-up or significant gum disease without feeling anything unusual.
Zinet suggested people check their tongue for a white or yellow coating, particularly towards the back, which can indicate bacterial build-up.
She said people can also lick the inside of their wrist, allow it to dry for a few seconds, and then smell it to get an indication of any odour.
Good oral hygiene remains the foundation of preventing bad breath, Zinet said, including brushing twice daily, cleaning between teeth, cleaning the tongue, staying hydrated and attending regular dental examinations.
To detect body odour, Gane said people can leave a worn shirt in another room for 20 minutes before smelling the underarms, which gives the nose a chance to take a fresh whiff of the smell.
This will not work when there is a medical reason for loss of smell, but it could work when body odour is linked to habituation, he said.
