Our sense of smell is one of the largest windows we have to experience the richness of the world around us.
It plays a vital role in everything we taste, in our social interactions, and can even help us detect potential threats.
But an invisible hazard in the air we breathe could be eroding our olfactory powers.
Many people were able to experience what it is like to lose their sense of smell when infected with Covid-19.
Loss of smell is known as anosmia and can have a substantial effect on our general well-being and quality of life.
But while the loss of this important sense due to a sudden respiratory infection is often temporary, there may be another factor that has been gradually eroding our sense of smell for years: air pollution.
pollution and smell
Exposure to PM2.5 – a collective name used to describe the small particles of air pollution emitted by combustion from cars, power stations and our homes – has previously been linked to the development of “olfactory dysfunction,” but typically only in occupational or industrial settings.
Now a new study is beginning to show the true scale of the pollution we breathe, and the damage it can cause.
And the results are up to all of us.
Below our brain – just above our nasal cavities – is the olfactory bulb. This sensitive piece of tissue is packed with nerve endings and is essential to the incredibly varied olfactory image we get from our noses.
It is also our first line of defense against viruses and contaminants that could enter the brain. However, with repeated exposure to external agents, these defenses slowly wear down or break down.
“Our data shows that there is a 1.6- to 1.7-fold increase (in the risk of) developing anosmia due to sustained particulate contamination,” says Murugappan Ramanathan Jr, a rhinologist at Johns Hopkins School of Medicine, Baltimore. .
Dr. Ramanathan has become one of the few experts in this field after he began to wonder if there was a relationship between the large number of patients he was seeing with anosmia and the environmental conditions in which they lived.
The simple question I wanted to answer was this: were a disproportionate number of anosmia patients living in areas with higher PM2.5 pollution?
discovering the culprit
Until recently, the scant scientific research on this topic included a 2006 Mexican study that used strong odors (of coffee and orange) to show that Mexico City residents – who constantly battle air pollution air- tended to have on average a poorer sense of smell than people living in rural areas of the country.
With the help of colleagues – including environmental epidemiologist Zhenyu Zhang, who mapped historical data on air pollution in the Baltimore area – Ramanathan was able to develop a case-control study of data with a group of 2,690 patients who they had attended Johns Hopkins hospital for a period of 4 years.
About 20% had anosmia and most did not smoke, a habit known to impair the sense of smell.
Sure enough, PM2.5 levels were found to be “significantly higher” in the neighborhoods where the anosmia patients lived compared to healthy control participants.
Even when adjusting for age, gender, race or ethnicity, body mass index, alcohol use, or tobacco use, the findings were the same: “Even small increases in ambient PM2.5 exposures may be associated with anosmia.”
Even more striking is that none of the Johns Hopkins patients lived in areas with excessively high air pollution: many lived in leafy areas of Maryland, and none came from pollution hotspots.
This suggests that even low levels of air pollution could cause problems over a long enough period.
But how exactly is pollution destroying our ability to smell?
metal particles
According to Ramanathan, there are two potential pathways. One is that some of the pollution particles pass through the olfactory bulb and directly into the brain, causing inflammation.
“The olfactory nerves are in the brain, but they have little holes at the base of the skull where little fibers go into the nose, [parecen] almost like little pieces of angel hair pasta,” says Ramanathan. “They are exposed.”
In 2016, a team of British researchers found tiny metal particles in human brain tissue that appeared to have passed through the olfactory bulb.
Barbara Maher, a professor of environmental sciences at Lancaster University in the United Kingdom who led the study, said at the time that the particles were “strikingly similar” to those found in air pollution along busy roads (she cited smokestacks household and wood stoves as another possible source).
Maher’s study suggests that these nanoscale metal particles could, once in the brain, become toxic and contribute to oxidative brain damage that damages neural pathways. However, it still remains a theory.
A matter of wear
The other potential mechanism, Ramanathan says, may not even require the pollution particles to reach the brain.
By hitting the olfactory bulb almost daily, they cause inflammation and direct damage to the nerves, slowly wearing them down.
Something similar to what happens with coastal erosion, in which sandy and salty waves devour the coast – replacing the waves with polluted air and the coast with our nasal nerves.
Modern combustion methods can create nanoparticles so fine that they are small enough to directly enter our bloodstream and brain tissue.
Unsurprisingly, anosmia disproportionately affects older people, whose noses have been attacked by air pollution longer.
The price of not smelling
Air pollution is known to cause a quarter of all deaths from heart disease and stroke, and nearly half of all deaths from lung disease.
By comparison, perhaps, our sense of smell seems to be at the bottom of the list of concerns.
But both Ramanathan and postdoctoral researcher Ingrid Ekström, from the Karolinska Institutet’s Center for Aging Research in Stockholm, warn that we are underestimating the importance of smell.
Ekström was puzzled by findings from the early 2000s showing that more than 5.8% of adults in Sweden had anosmia and 19.1% had some form of olfactory dysfunction.
She points out that anosmia can be an early warning sign for dementia, which is her specialty.
“With dementia and especially Alzheimer’s disease, we assume that the progression of the disease actually starts several decades before we can see the first symptoms,” says Ekström.
Anosmia is one of those early symptoms. When Alzheimer’s is diagnosed, “nearly 90% of patients have anosmia,” he explains.
Despite such strong links, Ekström argues that only now have researchers “opened their eyes to the sense of smell” and its role in disease.
Quality of life
Several studies have linked loss of smell with increased odds of developing depression and anxiety, and it is known to play a role in obesity or weight loss, malnutrition, and cases of food poisoning.
The reasons are obvious at best: our noses play a key role in our experience of the world around us, affecting our ability to taste food and helping us avoid food that has gone bad.
A poor sense of smell can mean that patients are likely to reach for stronger flavored foods, often salty and greasy ones.
Conversely, a total loss of smell can cause people to lose the pleasure of eating, which ultimately reduces body weight, a particular problem among the elderly.
Ramanathan says he has seen many patients who “can’t taste food, can’t smell wine, those things that gave them pleasure in life.”
Death predictor?
Anosmia could also be an indicator of other, broader health problems.
Numerous studies, generally of smokers -for whom impaired smell persists This, even 15 years after quitting, have shown that olfactory dysfunction is significantly associated with higher mortality among older adults.
One study in particular even hypothesized that anosmia could be used as an indicator of a greater chance of dying in a five-year period, from any cause, among older adults.
In a study of 3,005 US adults ages 57 to 85, it was found that those with anosmia were four times more likely to die within five years than their peers.
The researchers concluded that the impaired sense of smell could be an “indicator” of the accumulation of toxins from the environment or slower cell regeneration.
So should we care that air pollution, to which we are all exposed, impairs our sense of smell and causes anosmia?
Clearly, the answer lies somewhere between “yes” and “hell yes.”
Solutions
For Ramanathan, traffic pollution and waste incinerators are the top local pollution concerns in Baltimore.
“Air quality is important,” he says. “I think we need strict regulations and control.”
Ekström admits that tackling air pollution isn’t simple.
World events can also cause unexpected changes in behavior: Ekström mentions anecdotally that winter wood burning has been on the rise in Stockholm as concerned residents turn away from Russian gas.
But even the low-level air pollution we’re exposed to every day “needs to be taken more seriously,” he says.
And what is more, “definitely impaired smell as well.”
It may interest you:
* Recovered from COVID: they feel the smells of food but distorted
* Women are able to tell if a man is single with their nose
* The reason why some people do not recover their sense of smell after having suffered Covid
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