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Patrick Guilfoile: How well do you smell?

Smell has long been the Rodney Dangerfield of the senses, afforded little respect. In part, this is due to the perception that our sense of smell is less refined. For example, previous work has suggested that humans can detect perhaps 5 million colors and nearly a half-million tones, yet the prevailing wisdom is that we can detect only about 10,000 odors. However, the foundation for our limited ability to detect odors was shaky, based on “back of the envelope” calculations dating to the 1920s.

Scientists from The Rockefeller University in New York decided to test whether the poor status of our sniffers was deserved. The scientists started with 128 distinct odors, including strawberry and thyme scents, anise, vinegar, cinnamon, citrus, eucalyptus, and others. Researchers mixed individual scents in groups of 10, 20, or 30 components. This gave an enormous number of possible odors, considering each mixture as a distinct odor. For example, with mixtures of 10 of the 128 possible odors, there would be almost 300 trillion different scents. They then tested the ability of volunteers to discriminate different odors, based on how much overlap one mixture had with another. For example, two mixtures might share 9 of 10 components, so they would have 90% overlap. On the other hand, mixtures that shared only 3 of 10 components would have only 30% overlap.

Researchers presented subjects with vials containing three scents, two that were identical and one that was different, but overlapped to some extent with the identical odors. The subject opened the vials, and then tried to identify which one was distinct. In total, over three separate ~hour-long visits, each person attempted to discriminate between 264 different odor mixtures. The researchers found that, on average, their subjects could differentiate odors that had 51% overlap. This meant that, based on the number of possible mixtures, the average person could discriminate at least 1 trillion different scents.

There was quite a bit of variation between individuals, confirming that some people have better sniffers than others. Based on the testing, one individual did only a little better than chance at distinguishing odors, suggesting a limited sense of smell. Another individual could only identify about 80,000,000 different odors, whereas the most proficient of the 26 subjects could potentially distinguish 10 octillion (1,000,000,000,000,000,000,000,000,000) unique scents.

At least for some of us, science has now shown that the nose really does know a lot about the world around us.

More information is available in the article by C. Bushdid and others “Humans can discriminate more than 1 trillion olfactory stimuli” Science, March 21, 2014 343: 1370-1372

PATRICK GUILFOILE has a doctorate in bacteriology and is the associate vice president at Bemidji State University.