brain biology smell olfaction anosmia

COVID-19 Stinks!

Many people who have recovered from COVID-19 report a loss of their sense of smell. How important is your sense of smell, really?

COVID-19 stinks for so many reasons. On August 26, 2020, the BBC reported a worldwide death toll at 820,264 out of 23,895,181 total confirmed cases [1]. Even those who recover from COVID-19 often experience symptoms such as difficulty breathing, diarrhea, or chills [2]. But one symptom has been almost laughed off, as if it should be considered a blessing in disguise: loss of smell. The loss of your ability to smell odors in your environment is called anosmia. COVID-19 stinks, and it has resulted in many people who know firsthand how much it stinks to lose their ability to smell [2].

Figure 1. Total confirmed COVID-19 cases in the world as of September 1, 2020. This map shows a large number of cases in the USA, Brazil, Chile, and South Africa, but warns that limited testing reduces the total case count. Source: Our World in Data, Wikimedia Commons.

Why do we even want to be able to smell? I’d be happy if I never had to smell a dumpster or morning breath ever again. But we often disregard the many ways losing your sense of smell would make you unhappy:

  1. Death: One major use for the human sense of smell is to avoid poisoning ourselves [3]. Eating spoiled or contaminated food can result in death or sickness; meats, fruits, dairy, and vegetables all give off a distinct odor when they’re no longer safe to eat. By smelling the odor of rotten food, we know to avoid rather than ingest it.

  2. Social Isolation and Depression: Have you ever cut a conversation short because someone’s breath was unbearable? What about avoiding someone with extreme body odor? A lack of ability to smell applies to your own odors, and many people experiencing anosmia report decreased quality of life in relation to social interactions [3]. Even those who overcome personal odor obstacles still miss out on shared experiences of smelling the ocean, a campfire, or dinner on the grill.

  3. Lack of Taste: Have you ever noticed that food is less flavorful when you have a cold? Remember being told to plug your nose when you take medicine you don’t like the flavor of? When your nose is clogged up and/or your sense of smell is impaired, food just doesn’t have the flavor that it otherwise would. This is because your sense of smell is tied to taste [3]. Many people experiencing anosmia report reduced enjoyment of food and dining experiences. This can lead to an altered diet, overeating (often reported in women), or undereating (often reported in men) [3].

  4. Anxiety: People experiencing anosmia often also experience anxiety [3]. Anosmia makes you aware that you miss signals such as smoke from a fire or a gas leak in your house that other people will respond to. Lacking this information can easily put someone in an anxious and/or hyper vigilant state.

Knowing that anosmia can have such severe ramifications, why do we discount our sense of smell so much? Humans are very visual animals. We rely heavily on our eyes to interpret the world around us. Try to smell it from someone else’s point of view. Rodents primarily experience the world through their sense of smell and the touch of their whiskers. Their eyes are underdeveloped compared to ours. But they don’t need their eyes nearly as much as they need their nose. Rodents are generally nocturnal and communicate tons of social and predatory information using odors. We have all evolved to best suit our own lifestyles, so humans primarily pay attention to visual cues while rodents selectively attend to olfactory cues [4]. We must keep in mind that whatever we discover from studying rodent olfaction has different salience, evolution, and environmental factors than what we would experience as humans. Yet, as rodents are driven by olfactory cues, insights from rats may be particularly valuable in the study of anosmia.

Figure 2. Top view of a rat brain facing right. The olfactory bulbs are in the oblong structures at the far right of the figure, close to where the animal’s nose would be. Source: Paul Starosta, Getty images.

Figure 3. Side view of a human brain facing right, with the olfactory bulbs highlighted in yellow. Source: Science photo library, Alamy stock photos.

The proportions of a rodent brain directly align with this priority of olfactory information. They have giant olfactory bulbs. Those are the rugby ball-shaped parts at the front of the brain closest to the nose, optimally positioned for the processing of olfactory information from the nose (Figure 2). In contrast, the human olfactory bulbs take up a much smaller percentage of the brain (Figure 3).

Ultimately, humans don’t prioritize our sense of smell over our other senses. We’re not set up to do so. But maybe we shouldn’t take it for granted either.

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[1] The Visual and Data Journalism Team, BBC News (August 26, 2020) Covid-19 pandemic: Where are the global coronavirus hotspots? Accessed August 26, 2020.

[2] Centers for Disease Control and Prevention (May 13, 2020) Symptoms of Coronavirus. Accessed August 26, 2020.

[3] Boesveldt S, Postma EM, Boak D, Welge-Luessen A, Schöpf V, Mainland JD, Martens J, Ngai J, Duffy VB (2017) Anosmia-A Clinical Review. Chem Senses. 42(7): 513–23.

[4] Carlson KS, Gadziola MA, Dauster ES, Wesson DW (2018) Selective Attention Controls Olfactory Decisions and the Neural Encoding of Odors. Current Biology. 28(14): 2195-205.

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