Belfast Telegraph

Wednesday 20 August 2014

Humans sniff out a trillion smells

The characteristic scent of a rose has 275 components

We may not compete with Lassie, but humans have a greatly underestimated sense of smell, new research has shown.

In fact, scientists now believe the human nose can sniff the difference between one trillion different odours - probably more.

Previously experts thought for decades that we could discriminate between 10,000 smells at the most.

The figure is cited in scientific journals and quoted in popular magazines. But the estimate, made in the 1920s, was more of a guess than a scientific finding.

"Our analysis shows that the human capacity for discriminating smells is much larger than anyone anticipated," said Dr Leslie Vosshall, from Howard Hughes Medical Institute in the US, who co-led the new study.

"I hope our paper will overturn this terrible reputation that humans have for not being good smellers."

Previous research has shown humans can distinguish between 2.3 milion and 7.5 million different colours and around 340,000 audible tones.

But testing smelling ability is much more difficult because odours are so complex. For instance, the characteristic scent of a rose has 275 components, with only a small percentage dominating the flower's perceived smell.

To overcome this problem Dr Vosshall's team asked volunteers to distinguish between smelly mixtures with some components in common.

The trick was to use the percentage of overlap between more than one mixture to measure the sensitivity of an individual's sense of smell.

Like skilled perfumers , the scientists created their mixtures using 128 different odorant molecules.

Individually, the molecules evoked well known scents such as grass, oranges, aniseed and spearmint. But combined together randomly, they became largely unfamiliar.

Volunteers were given three vials to sniff, two containing identical mixes, and were asked to pick the odd one out. Each of the 26 volunteers made 264 comparisons.

The tests, reported in the journal Science, allowed the scientists to extrapolate how many different scents an average person could detect if presented with all the possible mixtures available from the 128 odorants.

The result was "at least" a staggering one trillion.

"I think we were all surprised at how ridiculously high even the most conservative lower estimate is," said Dr Vosshall. "But in fact, there are many more than 128 odorants, and so the actual number will be much, much bigger."

She doubted that anyone would regularly be exposed to this many smells, but added: "I like to think that it's incredibly useful to have that capacity, because the world is always changing. Plants are evolving new smells. Perfume companies are making new scents.

"You might move to some part of the world where you've never encountered the fruits and vegetables and flowers that grow there. But your nose is ready. With a sensory system that is that complex, we are fully ready for anything."

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