By Will Dunham
WASHINGTON (Reuters) - What does your nose know? A lot more than you might expect.
Scientists studying the breadth of people's sense of smell said on Thursday the human nose can discern far more than the 10,000 different odors 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 odor 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 Behavior 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 odor, rose blossom, beer, rotten egg, paint, cut grass, spoiled milk, fresh popcorn, dog breath, burning wood, ammonia, grilled meat, orange peel, pine, excrement, cinnamon, exhaust fumes, cookies and skunk spray.
Research has shown that people can distinguish several million different colors and about 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 only about 10,000 odors, 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 odor molecules with a wide spectrum of scents including ones smelling like citrus, mint, garlic, leather, tobacco and others, and then concocted mixtures for the volunteers. When combined into random mixtures of many odor molecules, the scents became largely unfamiliar.
"In general, I would say 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 odor 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 odor molecules.
From this, they estimated that an average person can discern more than a trillion different odors. The researchers said even this number may be too low because many more odor 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 odor 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 odor 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.
"I think that we evolved to discriminate very similar odors: Food - and the same food with the slightest hint of being spoiled - can be the difference between a nutritious meal and food poisoning," Keller added.
People sense smells when odor molecules enter the nose and are detected by the 400 olfactory receptors. The information is then sent to the brain, including areas responsible for emotions and memory. Scents can have a deeply emotional effect, reminding people of a loved one or a long-ago place.
"There are so many people, and it makes me so sad, who live purposely in a completely odor-free universe. They don't wear perfume. They wash their clothes in scent-free detergent. Offices have no smells anymore," Vosshall said. "So I hope people will go outside and start smelling things."
(Reporting by Will Dunham; Editing by Tom Brown)