Biology textbooks are riddled with passages relating how bad humans are at perceiving odors. As the oft-quoted statistic goes, humans can only perceive '10,000 odors', a number that sits particularly well with some dog-lovers, who like to remind us that canines have 300 million odor receptors, while humans only sport 6 million. But a study in 2014 revealed that humans might not be as olfactorily challenged as we once thought because, as it turns out, we can perceive more than 1 trillion odors—and that's a conservative estimate. 1
The original belief that humans' sense of smell is worse than that of other animals—dogs, mice, moles and even sharks was based on a 19th century hypothesis about free will that has more in common with phrenology than with our modern understanding of how brains work. John McGann, a neuroscientist who studies olfaction at Rutgers University, recently revealed how we ended up with this myth. The truth is humans are actually pretty good at smelling our world. 2
“We're discovering to our delight, that the human smell system is much better than we were led to believe”
The comparison that the human sense of smell is widely considered to be weak and undeveloped is an unproved hypothesis. McGann traces the origins of this false belief back to comparative19th-century neuroanatomical studies by Paul Broca, a French physician. A modern look at the human olfactory bulb shows that it is rather large compared with those of rats and mice, which are presumed to possess a superior sense of smell. In fact, the number of olfactory bulb neurons across 24 mammalian species is comparatively similar, with humans in the middle of the pack, and our sense of smell is similar to that of other mammals. 3
McGann joins a growing list of scientists who argue that human olfaction is nothing to sniff at. We can follow smell trails. We can discriminate between similar odors and detect a wide range of substances, sometimes more sensitively than rodents and dogs can. We live in a rich world of scents and sensibility where odors deeply influence our emotions and behavior. 4
“We're discovering to our delight, that the human smell system is much better than we were led to believe,” McGann said. “It may be different than other mammals' but actually in ways that suggest that it could be more powerful than mice, rats and dogs.”
What matters may not be the size or the space in the brain devoted to smelling, but other things like the ways our smell or brain systems are wired or used. 2
Other Smelly Items
When you correctly identify the smell of a strawberry, you have sorted through about 300 different components at a concentration of 10 parts per million. Scientists know that each of the approximately 10,000,000 receptors in your nose responds to more than one different molecule and that the same molecule can trigger more than one receptor. Concentration and proportion are both important. A substance that smells agreeable when dilute may be unappealing in higher concentrations, and a particular mix of substances can have a very different odor if the proportions of those chemicals are changed. 5
One study reveals for the first time that activating the brain's visual cortex with a small amount of electrical stimulation actually improves our sense of smell. This improves performance on a task that requires participants to identify the odd odor out of a group of three. This result is interesting because it shows for the first time, that on a basic level the brain structures involved in different senses are really quite interconnected in everyone—more so than previously understood. 6
In the moment before you stop and smell the roses, it's likely your brain is already preparing your sensory system for that familiar floral smell. Research from Northwestern University provides evidence that the brain uses predictive coding to generate 'predictive templates'--setting up a mental expectation of a scent before it hits your nostrils. Predictive coding is important because it provides humans with a behavioral advantage in that they can react more quickly and more accurately to stimuli in the surrounding environment. 7
Scientists have discovered we can distinguish a person's age by scent alone. They conclude our noses are able to distinguish older people from middle-aged and younger ones not based on the scent of mothballs or denture cream but based on the smell of their body odor. No one is entirely sure what makes the smell of older adults different from that of younger people. Studies have found that certain chemicals are present in different levels in older body odor, suggesting that these compounds may serve as biomarkers for old age, but the relevance of these chemicals to age determination has yet to be tested.8
Lastly, back to animals. A cat named Oscar, a resident of the Steere House Nursing Center in Rhode Island, possessed the extraordinary gift of knowing instinctively when the end of life is near. Oscar would notice when patients with end-stage dementia were about to die. Somehow the cat picked up on the 'sweet smell' of death, various types of ketones known for their fragrant aroma. For an interesting coverage about this cat and folks with dementia, read Making the Rounds With Oscar by David Dosa.
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References
Arielle Duhaime-Ross, “Humans can distinguish more than 1 trillion odors,” theverge.com, March 20, 2014
Joanna Klein, “Humans have a poor sense of smell” It's just a myth.” The New York Times, May 11, 2017
John P. McGann, “Poor human olfaction is a 19th-century myth,” Science, 356, 6338, May 12, 2017
Ed Yong, “The myth that humans have poor smell is nonscents,” The Atlantic, May 11, 2017
Doris R. Kimbrough, “How we smell and why we stink,” ChemMatters, December 2001
“Open your eyes and smell the roses: activating the visual cortex improves our sense of smell,” Science Daily, February 28, 2012
“Sniffing out the brain's predictive power,” northwestern.edu, October 6, 2011
Christie Wilcox, “The nose knows: telling age based on scent,” scientificamerican.com, May 30, 2012
Jack Dini is author of Challenging Environmental Mythology. He has also written for American Council on Science and Health, Environment & Climate News, and Hawaii Reporter.
Biology textbooks are riddled with passages relating how bad humans are at perceiving odors. As the oft-quoted statistic goes, humans can only perceive '10,000 odors', a number that sits particularly well with some dog-lovers, who like to remind us that canines have 300 million odor receptors, while humans only sport 6 million. But a study in 2014 revealed that humans might not be as olfactorily challenged as we once thought because, as it turns out, we can perceive more than 1 trillion odors—and that's a conservative estimate. 1
The original belief that humans' sense of smell is worse than that of other animals—dogs, mice, moles and even sharks was based on a 19th century hypothesis about free will that has more in common with phrenology than with our modern understanding of how brains work. John McGann, a neuroscientist who studies olfaction at Rutgers University, recently revealed how we ended up with this myth. The truth is humans are actually pretty good at smelling our world. 2
