Volunteers sniffed 250 different scent cocktails to help determine the limits of human odor detection.
photograph by Zach Veilleux, The Rockefeller University

What the Nose Knows

Humans can tell the difference between at least a trillion smells.

Every day, we’re confronted by a multitude of smells, good and bad: perfume, body odor, baking cookies, ripe garbage. But how many smells can the human nose actually distinguish? According to a recent study by HHMI Investigator Leslie Vosshall, it’s more than 1 trillion.

For decades, people believed that the human nose could discriminate between 10,000 different smells. That estimate, never empirically tested, didn’t sit right with Vosshall. “The number was from theoretical work in the 1920s that came to be uncritically accepted by scientists and nonscientists alike,” she says.

It also didn’t make sense that humans could detect fewer smells than colors. The human eye can perceive at least 2.3 million different colors using three types of light receptors. By comparison, the human nose has 400 olfactory receptors. Surely we should be able to smell more than 10,000 odors.

Andreas Keller, a senior scientist in Vosshall’s Rockefeller University lab, decided to determine a more accurate number. He selected 128 different odorant molecules that, when sniffed individually, could evoke smells such as mint or citrus. But when mixed in combinations of 10, 20, or 30, the odorants’ smells were unfamiliar.

Twenty-six volunteers were presented with three vials of these scent cocktails; two were identical, the third was different. It was the sniffer’s job to pinpoint the outlier. Each volunteer did this more than 250 times. On average, they could easily distinguish between mixtures with fewer than half their components in common; above that, discrimination became harder.

From the data, the team extrapolated how many different odors the average human can detect. Vosshall likens the process to a survey—rather than asking the entire country what presidential candidate they will vote for, you telephone a few thousand voters and use your findings to make an estimate of the entire population’s preferences based on this sampling. The number, published March 21, 2014, in Science, was 1.7 trillion—a conservative projection, as there are many more than 128 odorants in the world.

No one encounters a trillion smells in a day, so the ability to distinguish between so many odorant molecules isn’t really necessary. But being able to discriminate between similar smells, such as spoiled milk versus fresh milk, is certainly useful.