How does air pollution destroy our sense of smell?
PM2.5 levels were higher in neighborhoods where patients with anosmia lived, compared to healthy control group participants. Even when the effect of age, sex, ethnicity, body mass index, alcohol and tobacco use were taken into account, the same result was obtained: even a small increase in exposure to ambient PM2.5 may be associated with anosmia.
These findings have been repeated in studies in other regions of the world. For example, a recent study in Brescia in northern Italy showed that the more teenagers and young adults were exposed to nitrogen dioxide (another pollutant produced when fossil fuels are burned, especially from vehicle engines), the more sensitive their noses were to odors. Another annual study in São Paulo, Brazil, also found that people who lived in areas with higher particle pollution had a poorer sense of smell.
How does pollution destroy our sense of smell?
According to Ramanathan, there are two possible paths. One is that some pollution particles pass through the olfactory bulb and directly enter the brain and cause inflammation. “The olfactory nerves are in the brain, but there’s an opening at the base of the skull where small nerve fibers enter the nose,” says Ramanathan.
In 2016, a team of British researchers found the following metallic particles in human brain tissue that appeared to have passed through the olfactory bulb. Barbara Maher, a professor of environmental science at Lancaster University who led the study, said at the time that the particles were similar to those found in polluted air near busy streets. (Fireplaces and wood-burning stoves were also other possible sources).
Maher’s study shows that when tiny metal particles enter the brain, they can be toxic and cause oxidative brain damage that damages neural pathways; Although this is still theoretical.
Another possible mechanism, says Ramanathan, may not even require pollution particles to enter the brain. Particles of pollution by constantly hitting the olfactory bulb, cause inflammation and damage to the nerves and destroy them slowly. Think of this situation as coastal erosion, where sand and salt waves gradually erode the shoreline. Let’s say those air waves are full of pollution and the coastline is our nasal nerves.
Therefore, it is not surprising that anosmia mostly affects older people whose noses have been exposed to air pollution for a longer period of time. Interestingly, none of the Johns Hopkins patients lived in areas with excessive air pollution. Most of them lived in the green areas of Maryland, and none of them lived in highly polluted areas. This shows that even low levels of air pollution can cause problems in the long term.
A similar study was conducted separately by the Center for Aging Research at the Karolinska Institute in Stockholm. Postdoctoral researcher Ingrid Ekström was puzzled by findings from the early 2000s that showed more than 5.8 percent of adults in Sweden had anosmia and 19.1 percent had some form of smell disorder.
Knowing that the rate of anosmia is higher among the elderly, Ekstrom and colleagues designed a study using 3363 patients aged 60 years and older. Using sticks that gave off 16 common household odors, participants were scored based on the number of odors they could correctly identify.
As in the Baltimore study, participants’ home addresses were mapped based on urban air pollution, and here, too, a strong correlation was seen between higher pollution levels and poorer olfactory strength. “They had been exposed to pollution throughout their lives,” says Ekström. We don’t know exactly when their olfactory disorder started.” But he is confident that long-term exposure to pollution, even at low levels, has caused people’s olfactory disorders.
In 2021, the World Health Organization changed its health-based guidelines for the maximum annual average exposure to PM2.5 particles, changing it from 10 micrograms per cubic meter to 5 micrograms per cubic meter.
Stockholm, the capital of Sweden, is one of the few big cities in the world that is below the set level with an annual average of 4.2 micrograms per cubic meter. In contrast, Pakistan’s Islamabad has an annual average PM2.5 of 41.1 micrograms per cubic meter, while this average in Bloemfontein, South Africa is 42.3 micrograms per cubic meter.
This makes the Stockholm findings even more important: even Stockholm residents lose their sense of smell due to low pollution levels; How much worse can this problem be in areas where pollution levels are high?
It’s also a reminder of how local pollution can be, both outdoors and indoors. Cooking and heating methods may expose some households to higher levels of pollution than their neighbors.
Meanwhile, modern combustion methods from vehicle engines to new wood stoves can produce tiny nanoparticles that barely register in PM2.5 readings, but are small enough to enter our bloodstream and brain tissue directly.
Air pollution is the cause of a quarter of deaths from heart disease and stroke and almost half of deaths from lung diseases.
Maybe our sense of smell does not seem so alarming compared to the mentioned diseases. But Ramanathan and Ekström warn that the sense of smell and the problems caused by its lack are more important than they think.
Ekstrom works on dementia, and he says anosmia may be an early warning sign. “With dementia, and Alzheimer’s disease in particular, we assume that the progression of the disease begins decades before the first symptoms are observed,” Ekstrom says. Anosmia is one of the first symptoms. “When Alzheimer’s is diagnosed, approximately 90 percent of patients have anosmia,” Ekstrom says. The exact relationship between these two issues is unknown, but one of the theories that exists is that environmental toxins enter the central nervous system through the olfactory bulb and cause damage and start this cascade effect that may eventually lead to neurodegeneration.
For example, Maher Lancaster’s study showed that metal nanoparticles were directly related to the formation of senile plaques (damages that occur on the brain and are one of the neurological symptoms of Alzheimer’s disease and dementia). Despite such strong links, Ekström says, researchers have only recently focused on the role of the sense of smell in disease.