Human sense of smell far more refined than previously thought
Humans can discern an almost infinite number of smells - more than a trillion - scientists find in a study aimed at testing previous assumptions
What does your nose know? A lot more than you might expect.
Scientists studying the breadth of people's sense of smell find that the human nose can discern far more than the 10,000 different odours long cited as the outer limit of a person's 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.
As with sights and sounds, people are accosted with a multitude of smells, such as perfume, body odour, 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, biscuits and skunk spray.
Research has shown that people can distinguish several million different colours 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 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 others, and then concocted mixtures for the volunteers.
When combined into random mixtures of many odour 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 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.
The scents people encounter in everyday life are not usually a single odour molecule but rather a mix. For example, the scent of rose has 275 components.
The volunteers could usually distinguish between mixtures with up to half the same odour molecules, but they tend to have 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 odours: 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 odour 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 odour-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."