There is an oft-referenced factoid, reproduced everywhere from websites to textbooks, that the human nose can distinguish between 10,000 smells. But a recent investigation has revealed this under-investigated figure to be off. Like way, way, way off.
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The actual figure? "On the basis of the results of psychophysical testing, we calculated that humans can discriminate at least 1 trillion olfactory stimuli." That's according to a study led by Rockefeller University's Andreas Keller, the results of which appear in the latest issue of Science.
As the researchers explain, the claim that humans can discriminate 10,000 odors, while rooted in a (very old) study, had never been empirically validated. So Keller and his colleagues set out to test it themselves. Ed Yong explains their process over at NatGeo (emphasis added):
To estimate the bounds of our sense of smell, Keller had to get creative.
He gave volunteers three jars, two of which contained the same smell. Their job was to find the odd one out. The team made the smells from the same pool of 128 ingredients, which were mixed together in groups of 10, 20 or 30. They then paired these mixtures up so that some pairs had no ingredients in common, some were almost identical, and most were somewhere in between. Each volunteer sat through 260 of these discrimination tests.
After crunching the numbers, the team found that when the pairs of mixtures overlapped by less than 51 percent, most of the volunteers could tell the difference between them. And if they overlapped by less than 57 percent, most of them were distinguishable. This means that the average person can tell the difference between 1.7 trillion (that's 1,700,000,000,000) different combinations of 30 ingredients.
"It's one of those moments you live for as a scientist: reframing a problem and finding the solution out in left field," says Avery Gilbert, a smell scientist who first tracked down the origins of the spurious 10,000 number.
What's more, Keller and his team conclude that the 1.7-trillion-smell figure is a conservative estimate, and is actually a lower-bound to the limits of human olfactory potential. As the researchers explain:
The actual number of distinguishable olfactory stimuli is likely to be even higher than [1.7-trillion] for three reasons. First, it is currently not known how many odorous moleculres there are or how many of them can be discriminated from others. However, there are considerably more possible odorous molecules than the 128 different components that we used. Second, components can be combined in mixtures of more than 30 components. Third, even mixtures with the same components can be distinguished if the components are mixed at different rations.