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Study Finds Humans Can Smell at Least 1 Trillion Odors

  • Jessica Berman

FILE - A visitor smells a Sir Paul Smith climbing rose at the Chelsea Flower Show in London, May 21, 2013.

FILE - A visitor smells a Sir Paul Smith climbing rose at the Chelsea Flower Show in London, May 21, 2013.

One trillion.

That’s the number of scents researchers have determined humans can detect through their sense of smell. Scientists say the figure is probably a conservative estimate.

Popular and scientific literature since the 1920's has put the number of odors detectable by humans at around 10,000.

But, in a new study published in the journal Science, researchers at Howard Hughes Medical Institute and Rockefeller Institute in New York City calculated the number of odors detectable by humans as much, much higher.

Andreas Keller was co-author of the study which challenged volunteers to sniff scents in three tiny bottles. The researchers mixed 128 odor molecules in combinations of 10, 20 and 30 molecules.

The resulting subtle odors were both pleasant, including grass and citrus, and distasteful, such as sweaty socks. Two of the bottles were an identical blend. The third bottle was altered slightly.

Extrapolating from how often the volunteers could identify the different scent, the scientists calculated humans could discriminate at least one trillion odors. Keller believes the human nose is sensitive enough to detect more than that.

“Obviously in reality there are many more than 128 molecules that you can mix up and you can mix them in mixtures of more than 30,” he said.

How does that compare to dogs, which are held up as the "gold standard" of scent identification? Keller says our sense of smell is probably at least as good as a canine’s. Our noses are just higher than a dog's.

“In my opinion, the main difference between you and your dog when you go for a walk is that the dog’s nose during walking is always very close to the ground, which is where the interesting smells are," Keller said.

"So, the reason the dog picks up all those interesting smells and you don’t isn’t that the dog’s nose is better than you. It’s that the dog’s nose is closer to where the action is,” Keller said.

Researchers hope to use their findings to shed light on how the brain processes sensory information.