The human eye can distinguish more than 2 million distinct colors. But scientists studying smell now say they have their vision colleagues beat: The human nose, they say, can distinguish more than a trillion different smells.
Yes, trillion with a T.
That new figure displaces a much more modest estimate. Until now, smell researchers have been saying the human nose can distinguish about 10,000 smells.
But Andreas Keller at The Rockefeller University says that number comes from a discredited idea from the early 20th century, which asserted that there are four primary smells, the way there are primary colors. That's just plain wrong.
"And I found that so interesting and ridiculous that I thought it would be time to do an experiment and test how many odors there really are," Keller says.
So he and his colleagues at The Rockefeller University and the Howard Hughes Medical Instituteput together an experiment. They started with 128 chemicals with distinct smells, and started mixing them together —as many as 30 chemicals per bottle.
"Subjects come in and they're given three little bottles with odors in them," Keller explains. Two of the bottles are identical. The third is slightly different. "And the task is simply to tell which one is different."
Through this process, the scientists found that the human nose could do a remarkable job of distinguishing very small differences between smells. Once they figured out the percentage of these mixtures a person could distinguish, they called in a mathematician, who figured out how many possible unique odors you can make from elaborate mixtures of these 128 chemicals.
"And that's how the number of 1 trillion came about," Keller says.
It's true that a human being would never encounter a trillion different smells in a lifetime. "And most of the smells we tested in our test were probably never smelled by a human being before," Keller says.
But the human nose has presumably evolved to be able to detect tiny differences among smells — say to tell the difference between fresh food and something that's just beginning to spoil. That could be a matter of life and death.
So why do this experiment?
Written By: Richard Harris
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