Howard Hughes Medical Institute (HHMI) scientists  have tested the olfactory capacity of human volunteers and found that humans are capable of discriminating at least one trillion different odors — not just 10,000 different odors, a  number first proposed decades ago and not backed by data.

HHMI investigator Leslie Vosshall, who studies olfaction at the Rockefeller University, and Andreas Keller, a senior scientist in her lab at Rockefeller University, devised a strategy to present their research subjects with complex mixtures of different odors, and then ask whether their subjects could tell them apart.

In a paper in Science, the author note that researchers have carried out psychophysical experiments with color or tone discrimination tasks to estimate the average resolution of the visual and auditory systems. From these experiments, those researchers estimated that humans can distinguish between 2.3 million and 7.5 million colors and ~340,000 auditory tones.

A test of odor discrimination

The authors decided to do a corresponding test of humans’ ability to discriminate odors. Vosshall and Keller used 128 different odorant molecules to concoct their mixtures. The collection included diverse molecules that individually might evoke grass, or citrus, or various chemicals. But when combined into random mixtures of 10, 20, or 30, Vosshall says, they became largely unfamiliar.

“We didn’t want them to be explicitly recognizable, so most of our mixtures were pretty nasty and weird,” she says. “We wanted people to pay attention to ‘here’s this really complex thing  — can I pick another complex thing as being different?’”

The scientists presented their volunteers with three vials of scents at a time: two matched, and one different. Volunteers were asked to identify the one scent that was different from the others. Each volunteer made 264 such comparisons.

Vosshall and her colleagues tallied how often their 26 subjects were able to correctly identify the correct outlier. From there, they extrapolated how many different scents the average person would be able to discriminate if they were presented with all the possible mixtures that could be made from their 128 odorants.

In this way, they estimated that the average person can discriminate between at least one trillion different odors.

Vosshall says she doubts individuals are exposed to a trillion smells on a daily basis. “But I like to think that it’s incredibly useful to have that capacity, because the world is always changing,” she says. Plants are evolving new smells. Perfume companies are making new scents.

“You might move to some part of the world where you’ve never encountered the fruits and vegetables and flowers that grow there. But your nose is ready. With a sensory system that is that complex, we are fully ready for anything,” she says.

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Abstract of Science paper

Humans can discriminate several million different colors and almost half a million different tones, but the number of discriminable olfactory stimuli remains unknown. The lay and scientific literature typically claims that humans can discriminate 10,000 odors, but this number has never been empirically validated. We determined the resolution of the human sense of smell by testing the capacity of humans to discriminate odor mixtures with varying numbers of shared components. On the basis of the results of psychophysical testing, we calculated that humans can discriminate at least 1 trillion olfactory stimuli. This is far more than previous estimates of distinguishable olfactory stimuli. It demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far outperforms the other senses in the number of physically different stimuli it can discriminate.