Biology textbooks are riddled with passages relating how bad humans are at perceiving odors. As the oft-quoted statistic goes, humans can only perceive "10,000 odors" — a number that sits particularly well with some dog-lovers, who like to remind us that canines have 300 million odor receptors, while we only sport 6 million. But a new study published today in Science reveals that humans might not be as olfactorily challenged as we once thought because, as it turns out, we can perceive more than 1 trillion odors — and that's the conservative estimate.
To find out how many odors we can distinguish, researchers asked 26 participants to put their noses to the test. During each experiment, study participants were asked to smell the contents of three vials that the scientists had mixed themselves using 128 different odor molecules. Two of the vials contained the same mixture, while one did not. The participants' task was to identify the odd mixture. Then, using the statistics obtained during the tests, the researchers were able to determine that people can distinguish two odors when their components differ by more than half.
In other words, if less than 50 percent of the molecules that make up two odors are identical, your nose will be able to tell.
Billions of odorous molecules in existence
Armed with this information, the researchers decided to do a little math: if they could calculate how many different types of odor mixtures on the planet differ in their components by more than half, then they would also be able to determine how many odors we can perceive. Of course, that task would have been nearly impossible because there are billions of different odorous molecules in existence. So, the scientists decided to limit the math to mixtures containing up to 30 different molecules. The number "1 trillion" therefore embodies the number of odors you can create by mixing 10, 20 and 30 molecules belonging to a source batch of 128.
As study co-author and neuroscientist Andreas Keller put it in an email to The Verge, the number "doesn't reflect the entire odor universe." But it does give us a much better idea of what humans can actually sniff out.