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The nose knows

It’s a trillion times more accurate than we thought.

March 30, 2014 12:34:18 am

What does your nose know? A lot more than you might expect.

Scientists studying the breadth of people’s sense of smell said last week that the human nose can discern far more than the 10,000 different odours long cited as the outer limit of our olfactory abilities. They concluded that the human nose can differentiate an almost infinite number of smells — more than a trillion — based on their extrapolation of findings in laboratory experiments in which volunteers sniffed a large collection of odour mixtures.

“The single most important contribution of this research is that it revises this current idea that humans are terrible smellers,” said Leslie Vosshall, who heads the Laboratory of Neurogenetics and Behaviour at Rockefeller University in New York that conducted the study published in the journal, Science. “We’re very good smellers,” Vosshall added.

Just like with sights and sounds, people are accosted with a multitude of smells like perfume, body odour, rose blossom, beer, rotten egg, paint, cut grass, spoiled milk, fresh popcorn, dog breath, burning wood, ammonia, grilled meat, orange peel, exhaust fumes.

Research has shown that people can distinguish several million different colours and 340,000 audio tones, but the dimensions of the sense of smell had remained a mystery. There has been a notion around since the 1920s that people could discern about 10,000 odours, but that was based on faulty assumptions, these researchers said.

They designed experiments to try to zero in on the actual number. Their experiments involved 26 men and women of various racial and ethnic groups, ages 20 to 48. The volunteers were given three glass vials of scents at a time — two identical to one another and the third different — and were asked to identify the different one. Each did this with 264 different scents.

The researchers used 128 different odour molecules with a wide spectrum of scents, including ones smelling like citrus, mint, garlic, leather, tobacco, and then concocted mixtures for the volunteers. When combined into random mixtures of many odour molecules, the scents became largely unfamiliar. “They had unfamiliar smells that were neither very pleasant nor very unpleasant. I thought ‘fresh garbage’ is a good descriptor,” said Andreas Keller, a researcher in Vosshall’s lab who led the study.

The researchers tallied how often volunteers correctly identified the odour that differed from the other two, then calculated how many scents an average person would be able to discern if given all possible mixes of the 128 odour molecules. From this, they estimated that an average person can discern more than a trillion different odours. The researchers said even this number may be too low because many more odour molecules exist in the real world than the 128 used in the study and they can be combined in innumerable ways.

The scents people encounter in everyday life usually are not a single odour molecule but rather a mix. For example, the scent of rose has 275 components. The volunteers usually could distinguish between mixtures with up to half the same odour molecules, but had a much harder time when the mixtures shared more than half their components. Keller said he hoped the findings will make people appreciate their sense of smell more.

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