Loss of smell is a common side effect of a Covid-19 infection, but it can also signal other problems.
Late last year, Pennsylvanian psychology student Kate Petrova attracted media attention with her slightly odd analysis. Having recorded Amazon customer ratings for candles between 2017 and late 2020, she'd noticed a dip in the average rating for scented, but not unscented, candles that started in early 2020.
You'll no doubt have worked out that this was something to do with Covid-19, which, on the way to killing more than 2.5 million people, has a notable side effect of reducing people's ability to smell. Petrova's analysis indicated a threefold post-Covid increase in complaints, such as that of Joe C: "I have bought French Vanilla Yankee Candles for years … Due to C-19, the store I get them from is closed. I ordered six candles through Amazon Services … I started two candles in my home and worked outside for three hours. I came back into the house and could barely smell anything. Something is wrong with these candles."
I've never bought a Yankee Candle, but I can tell you that just walking past a shop that sells such things is enough to overload my sinuses.
Reports of Covid-related anosmia (loss of smell) are sufficiently common that by the time epidemiologists got around to reviewing studies published on the subject before May 2020, 31 were available. Six reported the proportion of more than 2500 Covid patients who had experienced nasal side effects – up to two-thirds. Between one in five and one in three also reported ageusia or dysgeusia – a complete loss of taste or a distorted sense of taste.
Many of us will have experienced a temporary reduction or loss of smell as a result of allergies or colds. Both cause inflammation and a decrease in airflow to the olfactory receptors that are important for smell and taste. Some people – about a tenth of a percent – are born anosmic. Of course, it's hard to imagine what this is like if you've always been able to smell. And then there are the things we take for granted: is that a cake cooking or is the oven combusting dangerously?
Anosmia can also be a symptom of more than a common cold or not-so-common Covid. Up to 20 per cent of people who have a traumatic brain injury experience the condition, because of damage to the neural relay stations. The smell train cannot make it to the station.
In this case, the most common stations include the olfactory bulb that sits above our nasal cavity and the olfactory tract that connects it to other brain tissue. These don't look particularly large in humans, but, interestingly, they're a fairly large part of a rat brain.
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In the absence of Covid or a bop to the head, gradual loss of smell can also be a rare warning sign of a brain tumour. Tumours in different brain regions can affect smell through a variety of mechanisms – for example, by impinging on the relay stations or by disrupting how the brain pulls together sensory experiences into what we perceive as smell.
Back to candles. Apparently, the most highly rated Yankee Candle is Chocolate Layer Cake. That doesn't appeal to me. But Cafe Al Fresco – "The warm air of a bustling outdoor cafe kissed with sweet notes of cinnamon, coffee and a dash of caramel"– sounds more like me. Or something I'd like to drink, anyway.