We Are Out-Smelling Ourselves

Jan. 16, 2015

By Medical Discovery News

Dogs are commonly known for their strong sense of smell. Their noses can detect when someone is pregnant, has cancer, even blood glucose levels. But the human nose is pretty amazing too. You may already know that smell is the strongest sense tied to memory – it can provoke intense responses and can seemingly bring back memories from long ago. Recently, a study has determined that our noses are capable of distinguishing between at least a trillion different scents, much more than previously believed.

The common theory was that we could only discern about 10,000 different odors, so this new estimate is a huge jump. Compare that with our ability to discriminate between 2.3 and 7.5 million different colors, which vary in wavelength and intensity, and about 340,000 different tones, which vary in frequency and loudness.

Back in 1927, a study proposed that there were four elementary odor sensations with sufficient resolution for humans to rate each elementary odor sensation on a nine-point scale. Doing the math, they estimated the number of discernable odors to be nine to the fourth power or 6,561, which was later rounded up to 10,000. Since then, that number has been accepted in both popular and scientific literature. But some scientists suspected that this number was too low. After all, we have just three receptors for light yet we can see millions of colors. Since human noses have about 400 different smell receptors that work in concert, the number of smells we can perceive is likely to be much higher the original estimation.

Natural odors are usually mixtures of large numbers of diverse components. The scent of a rose is produced by a mixture of 275 components, only some of which are chiefly responsible for the aroma. Researchers tested human ability to smell using 128 different molecules that are odorants, mixing them in many unique combinations. They then asked how much (or how little) two mixtures had to differ before the human nose could detect two separate smells.

Although they used many familiar smells like orange and spearmint, they intentionally mixed them to produce unfamiliar odors. Since this equals thousand to millions of odors, the study used methods similar to those used by political polls that use a sample to represent the general population. Each person was given three vials at a time, each consisting of 10, 20, or 30 different compounds. Two contained the same mix of odors while the third was different. Each subject was asked to smell about 500 different odorants in all.

Two vials of odorants had to differ by at least 49 percent to be distinguished. That means two vials of odorants could be 51 percent identical and humans could still tell them apart! Overall, scientists estimated that the human nose can distinguish at least 1 trillion scent combinations. Keep in mind, however, that this estimation is based on only 128 different odors. In the real world, many more odors exist in varying combinations so the number of smells discernible by the human nose could actually be much higher.  Of course, there was large variability in the abilities of different people to discern different smells.

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