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For many people, being affected by Covid-19 has given them a sense of what it’s like to lose their sense of smell. Known as ‘anosmia’, loss of smell can significantly impair our overall well-being and quality of life.
The sense of smell is one of the richest and most comprehensive windows into the world around us. Its role is fundamental to our eating and our social interactions. It’s already helping us identify potential hazards.
A sudden respiratory infection like covid-19 can cause a temporary loss of this important sense, but our sense of smell could well have been gradually declining over the years for another reason: air pollution.
Exposure to PM2.5 – a collective name for small airborne pollution particles caused, in large part, by burning fuels in vehicles, power plants and homes – has already been linked to “olfactory disturbances‘, but usually only in industrial or professional settings.
But new research is beginning to reveal the true scale and the potential damage caused by the pollution we breathe in everyday life – and their findings are important to us all.
At the bottom of our brain, just above the nasal cavities, is the olfactory bulb. This delicate collection of tissue hairs with nerve endings is essential to the highly varied picture the world sends us through our sense of smell.
And olfactory bulb It is also our first line of defense against viruses and pollutants entering the brain. But with continued exposure, these defenses are slowly eroded or overwhelmed.
“Our evidence shows that there is a 1.6 to 1.7 times increase [do risco] developing bad breath with continuous particulate pollution,” says Murugappan Ramanathan Jr., a rhinologist at the Johns Hopkins School of Medicine in Baltimore, US.
Until recently, the little scientific research on this topic included a 2006 Mexican study that used strong orange and coffee odors to show a trend: residents of Mexico City (which often has problems with air pollution) have , average, weaker smell by people living in rural areas of the country.
Why;
How, exactly, does pollution destroy our ability to smell?
According to Ramanathan, there are two possible answers. One is that some of the pollution particles pass through the olfactory bulb and the reach the brain directlycausing inflammation.
“The olfactory nerves are in the brain, but they have little holes at the base of the skull where little fibers go to the nose,” says Ramanathan. “They are exposed.”
In 2016, a team of British researchers found tiny metal particles in human brain tissue that apparently passed through the olfactory bulb.
Environmental science professor Barbara Maher, from Lancaster University, UK, led the study. He said then that the particles were “surprisingly similar» in air pollution found near busy roads. Other possible sources were domestic fireplaces and wood stoves.
Maher’s study suggests that these nanoscale metal particles could, once they reach the brain, become toxic, contributing to oxidative brain damage that damages neural pathways, but that’s still a theory.
The other possible mechanism, according to Ramanathan, may not even require the pollution particles to reach the brain. Reaching the olfactory bulb almost every day, the particles cause inflammation and directly damages the nerves and slowly wears them down.
It’s no surprise, then, that bad breath disproportionately affects the elderly, as their noses are assaulted by air pollution for longer.
Consequences of loss of smell
Air pollution is known to cause a quarter of all deaths from heart disease and stroke and about half of all deaths from lung disease. So, by comparison, perhaps our sense of smell seems pretty low on the list of concerns.
But Ramanathan and lead researcher Ingrid Ekström warn that we have underestimated the importance of olfaction as a risk factor. Ekström’s research specialty is dementia. And bad breath can be an early warning sign.
“With the paranoia and especially Alzheimer’s disease, we think that the progression of the disease actually starts several decades before we can notice the first symptoms,” says Ekström.
And anosmia is one of the first symptoms. When Alzheimer’s is diagnosed, “almost 90% of patients suffer from anosmia,” according to Ekström.
The exact link is still unknown, but one theory is that “environmental toxins enter the central nervous system through the olfactory bulb and cause damage, triggering this cascade effect that eventually can lead to neurodegeneration“, he explains.
Maher Lancaster’s study, for example, concluded that metal nanoparticles are directly linked to formation of “senile plaques” — the brain lesions that are one of the neuropathological features of Alzheimer’s disease and other types of dementia.
But despite these strong links, Ekström argues that researchers have only recently “opened their eyes to the sense of smell” and its role in disease.
In several studies, loss of smell has been linked to an increased likelihood depression and anxiety. It is also known to be associated with obesity, weight loss, malnutrition and in cases of food poisoning.
The reasons are perhaps obvious – our noses play a key role in our experience of the world around us, influencing our ability to taste food and helping us avoid eating spoiled food.
People who have an impaired sense of smell may seek to consume foods with stronger aromas, which tend to be saltier and fattier. Already the total loss of smell can make people they lose the pleasure of eating and removing them from them, which causes weight loss—which is a problem, especially among the elderly.
Anosmia can also be an indicator of other wider health problems. Several studies, usually among smokers (for whom olfactory difficulties persist for up to 15 years after smoking cessation), have shown that olfactory disturbances are significant is associated with increased mortality among the elderly.
With all this, do we need to worry about the olfactory damage caused by the air pollution we are all exposed to? Clearly, the answer is “obviously yes.”
