What’s the deal with COVID-19 and our sense of smell?

“Turn your students loose on smell,” encourages NCSE Executive Director Ann Reid in this look at how COVID-19 affects this underappreciated sense.

person smelling flowers

On January 28, 2021, The New York Times Magazine cover story, by Brooke Jarvis, was entitled: “What Can COVID-19 Teach Us About the Mysteries of Smell?”

Reader, I loved this story so much, for so many reasons. Because there was cool biology, because it highlighted an area of science that nobody thought was very important, because it shone a light on an aspect of the human experience that we largely ignore, and, especially, because I think your students will really love learning more about their senses of smell and coming up with cool experiments that they can do at home.

And if you’re a nerd like me, who loves finding out that there are words for things that I didn’t know needed describing (my personal favorite: crepuscular — look it up), there are plenty of great ones in this story. Like anosmia: total loss of smell; hyposmia: partial loss of smell; parosmia: distortion in how things smell; and phantosmia: smelling things that aren’t there.

Here’s an experiment anybody who lives with others can do. Ask each person in the house to wear the same t-shirt for two days. Put the t-shirts in a box. One at a time, each family member takes turns holding the t-shirts up to the nose of another blindfolded family member and asking them to identify whose shirt they’re smelling. Before they start the experiment, ask everyone whether they think they will be able to distinguish the shirts by smell. Aside from the “oh my god my little brother smells so bad this will be easy” comments, I suspect many students will think it’s not possible. But I suspect they’re wrong.

Because it turns out that humans have much better senses of smell than we think. Take this experiment: blindfolded students could follow a scent trail in the grass, with two-thirds of the students able to follow a 10-meter trail in under 10 minutes. Or this totally mind-blowing story of a woman who can smell Parkinson’s Disease. Mothers can recognize their own newborns by smell. Babies calm down more when exposed to the hospital gown of their own mother or another mother, compared to a gown worn by a non-mother or a clean gown. We are attracted to the smell of people who are immunologically different from us.

Are you convinced yet? Humans have more sensitive senses of smell than we usually realize, and it turns out that we take it for granted — until it’s gone.

Humans have more sensitive senses of smell than we usually realize, and it turns out that we take it for granted — until it’s gone.

Anosmia (remember learning that word?) has been afflicting millions of people during the pandemic. Loss of the sense of smell is, in fact, one of the earliest and most consistent symptoms of infection with the novel coronavirus and, critically, the symptom that most reliably differentiates it from other infections. As early as June 2020, researchers and clinicians were noting this unusual feature of the disease and soon were even suggesting that tracking reports of anosmia might be a better indicator of an upcoming surge in cases than coughs or fevers. (Later, a reported spike in online complaints about scented candles invited speculation that the complainers were suffering from COVID-19-induced anosmia.)

There are (at least!) three possible ways in which where you could bring this topic into your biology classroom. First, through the lens of epidemiology — how can this unusual symptom contribute to a better public health response? A comprehensive meta-analysis (that is, a study of studies that combines the results of many separate studies) published in December 2020 found that about 44% of COVID-19 patients self-reported loss of smell, but that 77% of patients who were given a smelling test (yes, it’s a thing) exhibited impaired senses of smell. Anosmia is often one of the earliest symptoms of coronavirus infection and sometimes the only symptom. Hauntingly, otolaryngologists (ear, nose, and throat doctors) were often the earliest healthcare workers to contract the virus because so many of their patients were coming to see them about their loss of smell.

What does this all mean for epidemiology? Perhaps we should be administering a brief smell test at the school door or airplane gate rather than checking temperatures. Perhaps we should be making decisions about restrictions on gatherings and business closings based on the evidence that reports of anosmia are a leading indicator, with the possibility that any restrictions would be more timely and therefore might not have to last as long. Certainly we should be making it easy for anyone who notices a change in their sense of smell to get tested, and public health announcements should emphasize that this symptom alone is a reason to self-isolate. These are potential policy areas that could be informed by our new understanding of the importance of anosmia in COVID-19.

Second, when you get to the part of the biology curriculum that deals with our senses, your students might find COVID-19-induced anosmia something that makes the subject particularly intriguing and personal. Our understanding of how humans smell is still in its infancy and the process of smelling appears to be an order of magnitude, at least, more complicated than that of our other senses. It’s a great way to learn about how a chemical signal is detected and then transmitted in a form that the brain perceives as a smell. When it comes to COVID-19 in particular, it turns out that it’s not the sensory neurons that are infected directly by the virus — it’s the cells that surround and support the neurons. With their support cells damaged or killed, the neurons can’t function. But when the infection is cleared up, those neurons can re-grow (how many neurons can do that?!), but the process happens slowly and sometimes with bizarre consequences. Smells come back very slowly or not at all, and patients find that some things smell bad that used to smell good (parosmia, remember?), or that they smell things that aren’t there (phantosmia!) — these symptoms can all be explained by delving into the physiology of these neurons and how they communicate with the brain.

Finally, there is the evolution angle. Although we humans think of ourselves as also-rans in the smelling Olympics compared to, say, dogs, it turns out that our sense of smell is incredibly acute. Why? When did the ability to smell evolve and why has it so clearly been selected for over billions of years? Billions of years, you say? Humans haven’t been around for billions of years! True. But the ability to sense chemicals — which is essentially what smelling is — was probably one of the first senses that evolved in bacteria. After all, being able to detect a nutrient and move toward it, or detect a toxin and move away from it would have high survival value. The ability to detect and distinguish chemicals is found throughout biology — if you’ve ever watched Shark Week you know that sharks can smell blood from up to a third of a mile away, and your latest picnic misery may be explained by the fact that mosquitos can detect exhaled carbon dioxide from 30 feet away. Even trees use chemical signals, mediated by soil fungi, to communicate with each other. Who says that science is boring? Turn your students loose on smell and I think they’ll really enjoy it once they get a whiff.

NCSE Executive Director Ann Reid
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Ann Reid is the Executive Director of NCSE.

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