Loss of smell is a hallmark of Covid-19, prolonged air pollution exposure can double the risk

Covid patients who experienced nasal congestion more likely to regain their sense of smell sooner.

Exposure to PM2.5 is associated with a numerous heath conditions including cardiovascular disease, and lung cancer. Now, a new study says that long-term exposure to air pollution can also cause loss of smell, a hallmark of Covid-19.

Loss of smell is one of the common symptoms of Covid-19 and it is often accompanied by loss of taste. In fact, this could be one of the earliest signs of the deadly viral disease. According to a study published in the Journal of Internal Medicine earlier this year, about 86 per cent of COVID-19 patients experience partial or complete loss of the sense of smell. Majority of those (almost 55 percent) who had complete loss of smell, called anosmia, had a mild form of the disease, and the symptom lasted up to 22 days. But it is not just Covid-19 infection that can rob your ability to smell and taste. Long-term exposure to air pollution can cause similar olfactory disturbances, said researchers from the Johns Hopkins University in the US.

They found that prolonged exposure to high levels of particulate matter (PM) 2.5 -- a mixture of solid particles and liquid droplets found in the air -- increases the risk of losing one's smell by nearly twice (a 1.6- to 1.7-fold increase). One possible reason may be that the olfactory nerve, which contains the sensory nerve fibres relating to the sense of smell, is located directly in the path of inhaled PM2.5 materials, the researchers suggested in their paper that appeared in the JAMA Network Open.

Who are vulnerable to smell loss due to air pollution?

Older people are especially vulnerable to this risk of losing smell due to air pollution. said lead author Murugappan Ramanathan, rhinologist and associate professor of otolaryngology -- head and neck surgery at the University's School of Medicine.

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Though long-term exposure to PM2.5 represents a common risk factor for the loss of sense of smell, it is potentially modifiable if sources of PM2.5 components are better controlled, he added.

PM2.5 had been indicted as a likely culprit for anosmia in previous studies too. In addition, exposure to PM2.5 is associated with higher risk of cardiovascular disease, lung cancer, decline in cognitive thinking ability, chronic obstructive pulmonary disease, asthma and premature death.

How COVID-19 causes loss of smell explained

In an article published in Science Advances last year, an international team of researchers led by neuroscientists at Harvard Medical School had explained the mechanism behind loss of smell in patients with COVID-19.

They found that olfactory support cells, not neurons that detect and transmit the sense of smell to the brain, are vulnerable to infection by SARS-CoV-2, the virus that causes COVID-19. The sensory neurons do not express the gene that encodes the ACE2 receptor protein, which coronavirus uses to enter human cells. But the protein is expressed in cells that provide metabolic and structural support to olfactory sensory neurons, they said.

Based on these findings, the research team concluded that the novel coronavirus changes the sense of smell in COVID-19 patients not by directly infecting neurons but by affecting the function of supporting cells. This implies that SARS-CoV-2 infection is unlikely to lead to permanent loss of smell in most cases.

Study author Sandeep Robert Datta, associate professor of neurobiology in the Blavatnik Institute at HMS, noted that anosmia is also associated with a variety of mental and social health issues, particularly depression and anxiety.

With inputs from agencies

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