1 Trillion Scents

Humans have powerful sight and hearing. We are able to pick out several million distinct colors and almost half a million separate tones. But how powerful is our sense of smell?

A study from 1927 found that humans could detect fewer than 10,000 different odors, and for nearly a hundred years that number went undisputed. But now scientists have discovered that the human sense of smell is much keener than they ever thought.

A New Way to Measure Smell

Sight and hearing are easy to measure. Color, for example, is determined by the wavelength of light, while sound is made up of specific noise frequencies.

But smell isn’t just about being able to detect energy—smell is influenced by millions of different chemicals, each of which activates receptors in the nose differently. For example, the unique smell of a rose is formed by a combination of 275 different chemicals.

Andreas Keller, a research associate in The Rockefeller University's Smell Study, and his team knew that measuring every known odor-producing chemical would be impossible, so they selected 128 compounds in an attempt to establish the lower limit of the human olfactory system’s capabilities.

“There are probably several millions or billions of different odorous molecules ... many more than the 128 we used. It's not known, and an active field of research, how they combine,” said Keller.

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Weaving a Tapestry of Scents

First, Keller's team had to make sure that all the scents they used were equally intense. “People get two odors at different dilutions and are asked which one is stronger,” Keller explained. “The dilutions are adjusted until half of the time people say odor A is stronger and half of the time they say odor B is stronger.”

With the 128 compounds chosen, Keller’s team asked 26 subjects to sniff 260 combinations of 10, 20, or 30 compounds. For each trial, the subjects were given three vials, two of which contained the same mixture, and one of which contained a different mixture. They had to pick out which vial contained the different-smelling mixture. For most of the trials, the two mixtures contained many of the same compounds, with just a few differences.

An effect called "resolution" refers to a subject's ability to tell certain types of scents apart from others. The higher the subject’s resolution, the higher the percentage of overlap between the two mixtures could be while the subject was still able to tell the difference between them.

More than half of the subjects could readily identify the difference between mixtures with less than 75 percent overlap. Some could identify mixtures with 75 to 90 percent overlap, and none could identify mixtures with more than 90 percent overlap.

Next came the math. Overall, subjects were able to accurately identify the differences between mixtures about half the time. The number of possible 30-compound mixtures that could be made from 128 compounds was 100 octillion. When averaged across the possible combinations of 10-, 20-, and 30-compound mixtures, more than one trillion different scents were left that the subjects could tell apart from one another.

And that’s just the lower limit, derived from only 128 base odors. Keller thinks the upper limit may be much, much higher. “I'm comfortable guessing that [the lower limit] is more than a million times more than one trillion,” he said. “I'm currently working on a study that aims at estimating an upper limit.”

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No Two Noses Are the Same

Why is the human sense of smell so important? The strongest human sense is vision, which uses more than one-fifth of the brain’s processing power. By comparison, our olfactory bulbs—tiny nubs where smell is processed—make up less than 1 percent of the brain's volume. 

Keller has a theory. “Imagine food slowly spoiling,” he said. “Being able to tell that very small difference between still-OK food and already spoiled food is the difference between a nutritious meal and food poisoning. I can't think of a comparable situation in which discriminating one hue of orange from a very similar hue of orange would result in an equal difference in fitness.”

There’s also vast individual variation in how sensitive a person’s sense of smell is. One subject in the study could detect the difference in fewer than 10 million smells, while another could tell the difference between an impressive 10 octillion smells, a full tenth of all the possible combinations in Keller’s study.

“Each human being (except for identical twins) smells the olfactory environment with a different set of receptors,” Keller said. “So the olfactory environment does smell different to each individual.”

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