Our sense of smell is one of our most rich and diverse windows into the world around us, influencing what we taste, how we interact with others, and even helping us detect potential dangers. However, a threat in the air we breathe may be eroding our olfactory abilities.


A bout of Covid-19 provided many people with their first taste (or lack thereof) of what it is like to lose their sense of smell. Loss of smell, also known as “anosmia,” can have a significant impact on our overall wellbeing and quality of life. While a sudden respiratory infection may cause a temporary loss of this important sense, your sense of smell may have been eroding for years due to something else – air pollution.

Exposure to PM2.5, the collective name for small airborne pollution particles caused primarily by the combustion of fuels in vehicles, power plants, and our homes, has previously been linked to “olfactory dysfunction,” but only in occupational or industrial settings. However, new research is beginning to reveal the true extent of – and potential damage caused by – the pollution we breathe in every day. And their findings are relevant to all of us.


The olfactory bulb is located on the underside of our brains, just above our nasal cavities. This sensitive tissue is densely packed with nerve endings and is essential for the vastly different picture of the world we get from our sense of smell. It also serves as our first line of defence against viruses and pollutants that enter the brain. However, with repeated exposure, these defences gradually wear down – or are breached.

“Our data show a 1.6 to 1.7-fold increase [risk of] developing anosmia with sustained particulate pollution,” says Murugappan Ramanathan Jr, a rhinologist at Johns Hopkins School of Medicine in Baltimore. That’s a 160% to 170% increase in percentage terms. He became one of the few experts in this field after he began to wonder if there was a connection between the large number of anosmia patients he was seeing and the environmental conditions in which they lived.


He wanted to know if a disproportionate number of anosmia patients lived in areas with higher levels of PM2.5 pollution. Until recently, the only scientific research on this topic was a 2006 Mexican study. which used strong coffee and orange odours to demonstrate that residents of Mexico City, which frequently suffers from air pollution, had a poorer sense of smell than people living in rural areas of the country.

Ramanathan set up a case-control study of data from 2,690 patients who had attended Johns Hopkins Hospital over a four-year period with the help of colleagues, including environmental epidemiologist Zhenyu Zhang, who created a map of historic air pollution data in the Baltimore area. Around 20% had anosmia, and the majority did not smoke, which is known to impair the sense of smell.

Indeed, PM2.5 levels were found to be “significantly higher” in anosmia patients’ neighbourhoods compared to healthy control participants. Even when adjusted for age, sex, race/ethnicity, body mass index, alcohol or tobacco use, the findings came up the same: “Even small increases in ambient PM2.5 exposure may be associated with anosmia”.


The finding has been confirmed in studies published this year in other parts of the world. One recent study in Brescia, northern Italy, found that the more nitrogen dioxide – another pollutant produced when fossil fuels are burned, particularly from vehicle engines – teenagers and young adults were exposed to, the less sensitive their noses became to smells. Another year of research in So Paulo, Brazil It was also discovered that people who lived in areas with higher levels of particulate pollution had a poor sense of smell.

But how exactly is pollution impairing our ability to smell?


There are two possible paths, according to Ramanathan. One possibility is that some pollution particles pass through the olfactory bulb and enter the brain, causing inflammation. “Olfactory nerves are in the brain, but they have little holes at the base of the skull where little fibres go into the nose, [like] little pieces of angel hair pasta,” Ramanathan explains. “They’ve been exposed.”

A group of British scientists discovered tiny metal particles in human brain tissue that appeared to have passed through the olfactory bulb in 2016. The particles were “strikingly similar” to those found in airborne pollution near busy roads, according to Barbara Maher, a professor of environmental science at Lancaster University in the United Kingdom who led the study (domestic fireplaces and log stoves were another possible source). Maher’s research suggests that once in the brain, these nanoscale metal particles could become toxic, contributing to oxidative brain damage that damages neural pathways, though this is still a theory.

According to Ramanathan, the other potential mechanism may not even require pollution particles to enter the brain. They cause inflammation and damage to the nerves directly by hitting the olfactory bulb almost every day, gradually wearing them away. Consider it similar to coastal erosion, where sandy, salty waves eat away at the shoreline; replace the waves with polluted air, and the shoreline 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


As a result, anosmia disproportionately affects older people, whose noses have been assaulted by air pollution for a longer period of time. Surprisingly, none of the Johns Hopkins patients lived in areas with excessively high levels of air pollution; many of them lived in leafy areas of Maryland, and none were from pollution hotspots. It implies that even low levels of air pollution can cause problems over time.

A similar study was recently conducted by the Aging Research Center at the Karolinska Institute in Stockholm. Ingrid Ekström, a postdoctoral researcher, was perplexed by findings from the early 2000s that revealed more than 5.8% of adults in Sweden had anosmia, and 19.1% had some form of olfactory dysfunction. Knowing that the prevalence of anosmia was higher in the elderly, Ekström and colleagues devised a study that included 3,363 patients aged 60 and up. Participants were given a score based on the number of correctly identified smells using strongly scented “sniffing sticks” of 16 common household smells. In the same way that the Baltimore study did, the participants’ addresses were mapped and analysed based on municipal air pollution readings. And, as in Baltimore, there was a strong correlation between higher levels of pollution and poorer smelling ability.

“They have been exposed to pollution their entire lives,” Ekström says. “We don’t know when their olfactory impairments began to improve.” However, she is “confident” that long-term exposure to pollution, even at low levels, was the cause.

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