The awkward, silent medical conditions that trigger foul body odour and bad breath... without you ever knowing: Here's how to check if YOU'RE one of millions of oblivious sufferers

It's an experience familiar to all of us – that awkward moment you find yourself stood next to someone with strong body odour (BO) or when a friend with bad breath leans in close for a chat.

It's easy to assume they just need to improve their personal hygiene but in fact there's often a sound medical explanation why some people are so malodorous. What's more there is a scientific reason they are often completely oblivious to their flower–wilting stench.

Experts say it's because when the brain is repeatedly exposed to the same familiar stimulus (in this case smell) it eventually stops registering it, a process known as habituation. And there is a special term for this process when it specifically relates to smell – olfactory fatigue.

'When an odour continues to be present, the information is no longer new and so the smell or olfactory system adapts,' Carl Philpott, a professor of rhinology and olfactology at Norwich Medical School told the Daily Mail.

This adaptation takes place within two olfactory bulbs located above the roof of the nose, which is where all the 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,' says Professor Philpott.

The reason our noses 'tune out' familiar smells, however pungent, is for self-protection, explains Simon Gane, a consultant rhinologist and ENT surgeon based at the Royal National Ear Nose and Throat and Eastman Dental Hospitals in London.

This, he says, allows the nose to be more sensitive to new or unfamiliar smells.

'Since these may be important for our self-protection – for example the smell of something burning – this makes sense from an evolutionary perspective,' Mr Gane told the Daily Mail.

'By tuning out familiar smells and instead registering those not experienced on a regular basis our nose helps determine what requires a response – for example, whether we need to run to safety or put out a fire.

'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.'

(This is also why people can't smell the aroma of their own home, even if to visitors it may be highly distinctive.)

The good news is that your sense of smell doesn't become less sensitive overall – it is solely less reactive to the specific smell or smells it is exposed to regularly – but it is reversible.

This was the finding of a study by researchers from the Technical University of Dresden who subjected a group of adults to two weeks of daily exposure to two distinctive chemicals – manzanate; which smells intensely fruity and crisp, and irone alpha which smells floral, woody and powdery. They found this temporarily reduced sensitivity to those smells.

However, after a week without exposure, participants recovered their original sensitivity according to the study results, published in the journal Chemical Senses in 2021.

Researchers also found that people did not become less sensitive to smells in general – only to the specific odour they had repeatedly encountered.

And, of course, desensitisation is not the only reason someone may be oblivious to their BO.

Around 3.3 million people have no sense of smell, a condition known as anosmia, but this can be down to a specific medical issue.

'Loss of smell is especially common with nasal polyps [soft, painless, noncancerous growths],' explains Professor Simon Lloyd, a consultant ENT surgeon at the Manchester Royal Infirmary and Salford Royal Hospital. 'Other conditions that can affect smell include hay fever, trauma to the nose through injury and tumours affecting the front part of the brain.'

Smell can also be compromised following an infection, such as Covid. However, people with anosmia are most likely aware they have a problem with their sense of smell, whereas those with olfactory fatigue do not.

When it comes to body odour, poor personal hygiene can of course be a factor – but there are lots of others.

One of the more common causes is excessive sweating. This might be due to fluctuating hormones, such as changes in oestrogen and progesterone during the menopause, or because of a medical condition known as hyperhidrosis, which can cause uncontrollable sweating.

Although sweat itself doesn't smell, bacteria found naturally on the skin break down the fluid released by sweat glands, particularly those found in the underarms and groin. In doing so, they produce compounds that cause the familiar smell of body odour.

Certain medications, including some antidepressants and antibiotics, have also been linked to body odour. This could be because they cause the body to sweat more so there is more fluid for bacteria to feed upon so providing greater scope for BO.

Some people's BO has a particular pungent lingering odour and that is often linked to their diet.

Eating sulphur-rich foods such as garlic and onions can lead to the release of these odour-producing compounds through sweat. Or wearing clothes made from materials such as polyester, or nylon can trap moisture and heat, creating an environment where bacteria can 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,' points out Mr Gane.

Habituation also explains why some people may not realise they have bad breath. Known as halitosis, the condition affects approximately 25 per cent of the UK population at any one time, according to NHS data.

'The most common cause of bad breath is the tongue, especially the back of it,' explains London-based dentist, Dr Alexa Balan. 'The tongue is not smooth, it has tiny papillae [bumps] 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.'

Gum disease or a drop in saliva production – perhaps through taking medication such as antihistamines which cause a dry mouth – can also play a part.

'Olfactory fatigue is a very real phenomenon in terms of bad breath,' adds Dr Azad Eyrumlu, dentist and CEO of Banning Dental Group in London.

'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. That's one of the reasons bad breath can go unnoticed, even when it's obvious to others.'

Awareness might also be reduced because bad breath may not come with obvious symptoms such as pain, explains Dr Soumaya Zinet, a cosmetic dentist and founder of the London dental clinic Invisible Braces 4u. The usual trigger is significant gum disease or bacterial build-up 'but someone can have these without feeling anything unusual, meaning they may not realise there is an issue'.

So other than asking someone you trust for an honest opinion, how can you tell if you have bad breath or body odour without realising it?

In terms of bad breath there are a few things you can try, says Dr Zinet.

'Lick the inside of your wrist, allow it to dry for a few seconds, then smell it. While not perfect, it can provide an indication of odour. Also check your tongue. A white or yellow coating, particularly towards the back of the tongue, can indicate bacterial build–up that may contribute to bad breath.'

Good oral hygiene, she adds, remains the foundation of preventing bad breath: Brushing twice daily, cleaning between teeth, cleaning the tongue, staying hydrated and attending regular dental examinations.

When it comes to BO, there are a few tricks that may help you detect a problem.

One hack is the so–called 'shirt test'. Simply leave a worn shirt in another room for around 20 minutes, then come back and smell the underarms. The idea is that this gives your nose a chance to take a fresh whiff of the smell.

This won't work when there is a medical reason for loss of smell, 'however, it could potentially work when body odour is linked to habituation,' adds Mr Gane.

Perhaps worth mentioning if a friend ever asks for an honest opinion about whether they have BO.