For many people, covid-19 has given them their first glimpse of what it's like to lose their sense of smell.
Known as "anosmia", the loss of the sense of smell can have a huge impact on our overall health and quality of life.
But while a sudden respiratory infection can lead to a temporary loss of this important sense, your sense of smell may be gradually disappearing over the years due to something entirely different - polluted air.
Exposure to PM2.5 particles - tiny particles of air pollution, which are created mainly from fuel burning in vehicles, power plants and home fireplaces - has previously been linked to "sense of smell dysfunction", but usually only occurred within certain occupations or industries.
But new research has now begun to show the true scale, and potential damage, of the pollution we breathe in every day.
And her findings matter to all of us.
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On the underside of the human brain, just above the nasal cavity, there is an olfactory bulb.
This sensitive piece of tissue is covered with nerve endings and is key to the extremely diverse picture of the world that we get from the sense of smell.
It is also our first line of defense against viruses and pollutants that enter our brain.
But with constant exposure, those defenses slowly weaken - or break down.
"Our data show that there is a 1,6- to 1,7-fold increased risk of getting anosmia from persistent particulate pollution," says Murugapan Ramanathan Jr., a rhinologist at the Johns Hopkins School of Medicine in Baltimore.
He became one of the few experts in the field after he began to wonder if there was a connection between the large number of anosmia patients he encountered and the environmental conditions in which they lived.
The simple question he wanted to answer was this: do a disproportionate number of anosmia patients live in areas with higher PM2.5 pollution?
Until recently, among the few scientific studies on the subject was a 2006 Mexican study that used the strong odors of coffee and oranges to show that residents of Mexico City—which often suffer from air pollution—have, on average, a poorer sense of smell than people living in in rural areas of the country.
With the help of colleagues, including environmental epidemiologist Zhenyu Zhang, who mapped historical air pollution data in the Baltimore area, Ramanathan conducted a case-control study of 2.690 patients who attended Johns Hopkins Hospital over a four-year period. .
About 20 percent of them had anosmia, and most did not smoke.
Smoking is a habit known to affect the sense of smell.
And indeed, the level of PM2.5 particles found was "significantly higher" in neighborhoods where patients with anosmia lived compared to a control group of healthy patients.
Even after adjusting for age, sex, race/ethnicity, body mass index, alcohol or tobacco use, the findings were still the same: "Even small increases in exposure to ambient PM2.5 particles can be associated with anosmia." .
The findings were confirmed in other parts of the world in studies published this year.
A recent study conducted in Brescia, northern Italy, for example, found that the noses of teenagers and young adults became less sensitive to smells the more they were exposed to nitrogen dioxide, another pollutant produced by burning fossil fuels.
Another year-long study conducted in Sao Paulo, Brazil, also found that people who live in areas with higher levels of fine particulate matter pollution have an impaired sense of smell.
But how exactly does pollution impair our ability to smell?
According to Ramanathan there are two potential paths.
One is that some of the pollution particles pass through the olfactory bulb and reach the brain directly, causing inflammation.
"The olfactory nerves are in the brain, but they have holes in the base of the skull, where tiny fibers enter the nose that look crushed like pieces of the thinnest spaghetti," says Ramanathan.
"They are exposed."
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In 2016, a team of British researchers discovered tiny metal particles in human brain tissue that appeared to have passed through the olfactory bulb.
Barbara Marr, a professor of environmental science at Lancaster University in the UK who led the study, said earlier that the particles were "remarkably similar" to those found in air pollution near busy roads (home fireplaces and wood-burning stoves were another possible source). .
Barbara Marr's study suggests that these metal nanoparticles may, once in the brain, become toxic, contributing to oxidative brain damage that disrupts neural pathways, although this remains theoretical.
Another potential mechanism, Ramanathan says, may not even require pollution particles to reach the brain.
Hitting the olfactory bulb of maltenes daily, they cause inflammation and damage to the nerves directly, slowly wearing them down.
Think of it as coastal erosion, where sand and salt waves erode the shore; replace the waves with polluted air, and the shore with our nasal nerves.
In 2021, the World Health Organization changed the health guidelines for the maximum annual average exposure to PM2.5 particles, reducing it from 10 to five micrograms per cubic meter (µg/m3).
Stockholm, the capital of Sweden, is one of the few metropolises in the world that manages to stay below this level, with an annual average of 4,2 µg/m3.
In comparison, Islamabad, Pakistan has an annual average PM2.5 level of 41.1 µg/m3, while it is 42,3 µg/m3 in Bloemfontein, South Africa.
This probably makes the Stockholm findings even more relevant, if even Stockholmers' sense of smell is impaired by low levels of pollution, how much worse will it be in regions with higher levels?
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It's also a reminder of how extremely localized pollution can be, both externally and internally.
People's cooking methods and heating choices may expose them to higher levels of pollution than their neighbors.
Meanwhile, modern combustion methods from vehicle engines to the latest "eco" wood stoves can create nanoparticles so tiny they barely register on PM2.5 readings, but can enter our bloodstream and brain tissue directly.
Air pollution is known to cause a quarter of all deaths from heart disease and stroke, and almost half of all deaths from lung disease.
Compared to that, perhaps our sense of smell is much lower on the priority list of concerns.
But both Ramanathan and Ekstrom warn that we underestimate the importance of our sense of smell at our peril.
The specialty of Ekstrom's research is dementia.
And anosmia could be an early warning sign.
"With dementia, and especially with Alzheimer's disease, we assume that the disease begins to progress several decades before we notice the first symptoms," says Ekstrom.
Anosmia is one of the first symptoms.
By the time Alzheimer's is diagnosed, "almost 90 percent of patients already have anosmia," Ekstrom says.
What the exact link is remains unknown, but one theory is that "environmental toxins enter the central nervous system via the olfactory bulb and cause damage, setting off a cascading effect that can eventually lead to neural degeneration."
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A study by Barbara Marr of Lancaster, for example, showed that metal nanoparticles were directly linked to the formation of "plaques of senility" - lesions on the brain that are one of the neuropathological hallmarks of Alzheimer's disease and dementia.
Despite such strong connections, Ekstrom claims that only recently have scientists "opened their eyes to the sense of smell" and its role in disease.
Loss of the sense of smell has been linked to an increased likelihood of depression and anxiety in various studies, and is known to play a role in obesity, weight loss, malnutrition and cases of food poisoning.
The reasons may be obvious - our noses play a key role in experiencing the world around us, influencing our ability to taste food and helping us avoid food that has gone bad.
A poor sense of smell can mean that those who suffer from it will seek out stronger tasting foods, which are very often salty and fatty.
In contrast, a complete loss of the sense of smell can turn people away from food and cause them to stop enjoying it, eventually causing them to become malnourished - a particular problem among the elderly.
Ramanathan met many patients who "can't taste food, can't smell wine, things that gave them pleasure in life."
He recalls one patient who was a professional sommelier, for whom getting anosmia was both a personal and professional disaster.
Smell and taste are also linked to memory.
"People don't remember what the pastry they ate in France looked like, but they remember how the bakery they bought it smelled," says Ramanathan.
When we smell a scent again, it can transport us right back to that moment in that bakery.
This, again, raises the question, which, admittedly, still needs to be thoroughly studied, whether the reverse situation also applies and whether the fact that we can no longer smell the smell spoils our ability to create new memories in the same way.
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Anosmia can be an indicator of other, broader health problems.
Numerous studies, usually conducted on smokers whose impaired sense of smell persists even 15 years after quitting, have shown that olfactory dysfunction is significantly associated with increased mortality among the elderly.
One particular study even hypothesized that anosmia could be used as a predictor of greater likelihood of death from any cause among the elderly over a five-year period.
In a study of 3.005 American adults aged 57 to 85, those with anosmia were four times more likely to die five years later than their peers.
The scientists concluded that the deterioration of the sense of smell could be a "harbinger" of the accumulation of toxins from the environment or slowed cell regeneration.
So, should we care that the air pollution we are all exposed to impairs our sense of smell and causes anomia?
The answer obviously lies somewhere between "yes" and "but what".
Ramanathan, for whom traffic pollution and waste incinerators are among the top local pollutants in Baltimore, says "air quality is important."
"I think we need strict regulation and control," he says.
Many people may not even be aware of the pollution they are exposed to, so they rely on politicians to regulate it to protect the population in the surrounding areas.
"It's one of many pollution-related conditions," adds Ramanathan.
“But this is a special question, isn't it? If you have COPD [chronic obstructive pulmonary disease], you can probably still enjoy a glass of wine. But not with this one."
Ekstrom says the fight against air pollution is not simple.
World events can also cause unexpected changes in behavior.
Ekström cites the anecdotal example that winter wood burning in Stockholm is on the rise as concerned residents wean themselves off Russian gas.
But even the low-level air pollution we're exposed to every day "has to be taken more seriously," she says.
Moreover, "an impaired sense of smell would also have to be taken more seriously".
*Tim Smedley is the author of the book Cleaning the Air: The Beginning and the End of Pollution, published by Bloomsbury.com.
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