Human Nose Can Detect a Trillion Smells

(Sciencemag) - 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.