One in a trillion. The vials of smells that volunteers sniffed in the new study were only a tiny sampling of all the possible smells that humans can distinguish.
A rose, a fresh cup of coffee, a wood fire. These are only three of the roughly 1 trillion scents that the human nose and brain are capable of distinguishing from each other, according to a new study. Researchers had previously estimated that humans could sense only about 10,000 odors but the number had never been explicitly tested before.
“People have been talked into this idea that humans are bad at detecting smells,” says neurobiologist Leslie Vosshall of Rockefeller University in New York City, who led the new work. “So these findings should give the whole human race a confidence boost.”
Humans detect smells by inhaling air that contains odor molecules, which then bind to receptors inside the nose, relaying messages to the brain. Most scents are composed of many odorants; a whiff of chocolate, for example, is made up of hundreds of different odor molecules. Understanding how people process the complex information contained in scents—or memories of smells—offers a window into how the human brain functions.
Vosshall says she and others in the field had long guessed that the number of detectable scents often cited in the literature, based on rough calculations made in the 1920s of the known groups and ranges of smells—claiming that humans could distinguish 10,000 odors—was way off. So her lab decided to test it once and for all. They took 128 odor molecules that represented a wide range of smells and started combining them into unique mixtures containing 10, 20, or 30 different components. Then, they recruited volunteers from the community, aged 20 to 48, to start sniffing the mixtures. “The people we invited to do this study were not professionals; they were not wine tasters or perfumers,” Vosshall says.
Each volunteer was given three smell-containing vials at a time—two that were identical and one that was a slightly different mixture—and then was asked which was the odd one out. On average, if the components varied by more than 50%, the scientists found, people could distinguish the smells as different. When Vosshall’s team crunched these numbers, extrapolating how many different combinations of the 128 odorants an average person could differentiate, they arrived at an average of 1 trillion smells.
Individual performance, however, varied, they report online today in Science. The researchers calculated that the least successful smeller in the study would be able to smell only 80 million unique scents. And the best performer had a far more sensitive sense of smell, likely able to distinguish more than a thousand trillion odors.
The ability to distinguish a trillion scents from one another when they’re paired up, though, doesn’t mean that humans can identify a trillion different scents, says neurologist Jay Gottfried of the Northwestern University Feinberg School of Medicine in Chicago, Illinois. “Even if humans can distinguish that many odors based on these projected mixtures, I don’t know if there are really 1 trillion unique odors in the world that we would need to be discriminating.”
Gottfried adds, however, that the study brings up interesting questions regarding how complex smells are sensed by the nose and brain. “In general, it highlights a growing interest in how combinations of odors—rather than single odor molecules at a time—are sensed and processed.”
Vosshall and her colleagues are pursuing some of these questions, including whether certain combinations of odors are indistinguishable despite being very different at a molecular level. But for now, she just hopes the new findings encourage people to take another sniff at the world around them.
“Knowing we have these capabilities, I hope people, as they go about their business, start saying, ‘Hey, I can smell all these things.’ Maybe the companies that make scented products will start making greater use of the human capacity and develop cleaners and perfumes with new, more interesting scents,” she says. “Maybe we’re going to start using those corners of our smell capacity that have just not been exercised lately.”
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