How Many Smells Can Humans Distinguish

According to Howard Hughes Medical Institute (HHMI) experts, the human sense of smell is equipped to recognize this huge array of smells after testing individuals' abilities to distinguish between complex fragrances blended in the lab.

It is estimated that humans are capable of discerning at least one trillion various odor types

In an environment where freshly popped popcorn and exhaust scents combine, and where the scent of flowers or wet paint mixes with sea breezes, it is estimated that the human nose can distinguish at least one trillion different odor types.

Humans are claimed to be able to distinguish between 10,000 different scents, and this has been known for decades. There are several references to this number in scientific literature, as well as in popular media. “It's the generally accepted number,” says HHMI investigator Leslie Vosshall, who studies olfaction at the Rockefeller University. “Our analysis shows that the human capacity for discriminating smells is much larger than anyone anticipated.” It was published in Science on March 21 of this year. “I hope our paper will overturn this terrible reputation that humans have for not being good smellers,” she says.

When it was first proposed in the 1920s that people could only smell 10,000 smells, it disturbed him because it wasn't backed up by any research. “Objectively, everybody should have known that that 10,000 number had to be wrong,” she says. Another problem was that humans were able to smell significantly fewer things than colors, which seemed counterintuitive. To see up to 10 million hues, three light receptors operate together in the human eye, says Vosshall. The average person's nose, on the other hand, contains 400 olfactory receptors.

But no one had ever examined the olfactory abilities of humans. “We know exactly the range of sound frequencies that people can hear, not because someone made it up, but because it was tested. We didn't just make up the fact that humans can't see infrared or ultraviolet light. Somebody took the time to test it,” Vosshall says. “For smell, nobody ever took the time to test.”

However, she and a prominent scientist at the Rockefeller University who was part of her team felt they could come up with a more accurate number. Their technique was to expose their research subjects with complicated combinations of several odors, and then ask if they could tell them separately after that.

As a result, 128 different odorant molecules were combined. When I looked over the collection, I noticed a range of molecules that evoked grass, citrus, or other chemicals on their own. They became familiar with each other when they were randomly mixed in groups of 10, 20, or 30. “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?'”

Two of the smells were identical, and one was distinct and were delivered to the volunteers at a time. Volunteers were required to have a distinct odor. Every single participant took part in these comparisons.

Only one out of 26 individuals in Voshall's study was able to correctly identify an outlier, according to the report. From this, researchers calculated how many different scent combinations each individual could detect from their 128 odorants if they were exposed to all of the possible blends of those odorants.“It's like the way the census works: to count the number of people who live in the United States, you don't knock on every single door, your sample and then extrapolate,” she explains. “That's how I like to think of this study. We knocked on a few doors.”

This led them to estimate that the average individual is able to distinguish between at least one trillion various odors in this way. “I think we were all surprised at how ridiculously high even the most conservative lower estimate is,” Vosshall says. “But in fact, there are many more than 128 odorants, and so the actual number will be much, much bigger.”

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.

Source: hhmi.org

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