Humans can Smell more than 1 Trillion Odors, Researchers Say

Humans can detect more than 1 trillion odor mixtures, a new study suggests.

The study, conducted by researchers at Rockefeller University and colleagues, challenges the common assumption that humans can sniff just 10,000 different kinds of odors. According to the research team, even 1 trillion might be an underestimation.

Avery Gilbert, an olfactory expert who wasn't part of the study, said that the 10,000-smell estimate came from a manuscript published by two American chemists in 1927, The Washington Post reported. Their odor classification system was based on fragrant, acid, burnt and caprylic (goat-smelling).  They used the four odors to build various types of scent-mixtures.

As a comparison, humans can see between 2.3 and 7.5 million colors and hear about 340,000 tones.

 "The message here is that we have more sensitivity in our sense of smell than for which we give ourselves credit. We just don't pay attention to it and don't use it in everyday life," said Andreas Keller, of Rockefeller's Laboratory of Neurogenetics and Behavior and lead author of the study.

Odor detecting patches made of five or six million yellowish cells pick up different kinds of scents. These cells are present high up in the nose.

We rarely smell an isolated odor. Our olfactory senses are regularly bombarded by a mixture of different scents. So, how did researchers test this multi-faceted sense?

To create the odor mixtures, researchers first isolated 128 odor molecules responsible for common scents such as orange and spearmint and then mixed them in different proportions.

For the study, researchers conducted an olfactory sense test on 26 people. Each person was given three vials with two containing the same scent. Participants were asked to choose the odd one out.

Data analysis showed that humans can pick out several kinds of smells.

 "It turns out that the resolution of the olfactory system is not extraordinary - you need to change a fair fraction of the components before the change can be reliably detected by more than 50 percent of the subjects," said Marcelo O. Magnasco, head of the Laboratory of Mathematical Physics at Rockefeller. "However, because the number of combinations is quite literally astronomical, even after accounting for this limitation the total number of distinguishable odor combinations is quite large."

According to researchers, modern living has led to loss of smelling ability in humans. We have some 350 functional genes for receptors dealing with smells whereas other mammals such as mice have 1,000 genes for olfactory receptors.

The study is published in the journal Science.