The human nose can detect more than 1 trillion different smells, according to new research – suggesting we are much better at telling odours apart than we previously thought.
The study published in the journal Science claims the human olfactory system can detect many more smells than the longstanding received wisdom of 10,000.
“[Our study] replaces that previous number of 10,000 with a much more realistic and [much] higher number and shows the human sense of smell does have a good capacity to discriminate,” said lead researcher Dr Andreas Keller of Rockefeller University.
Unlike the auditory system, which can be measured in frequency, the olfactory system is tricky to assess. The fact that most odours are composed of many different chemicals causes more difficulties.
Keller’s team created odours with varying degrees of similarity and tested if subjects could spot the difference. They used a collection of 128 different chemicals to concoct three groups of odours containing 10, 20 or 30 different components.
For each group, they created odours with the same number of components but a different composition, thereby varying the degree of similarity between pairs of odours.
Three vials were then given to each participant – two bearing the same odour, and one containing a different odour with the same number of components, but a different degree of similarity. They were then asked to inhale and decide which was the odd one out.
Analysing the results from 26 participants, each of whom compared in total 264 pairs of odours with varying degrees of similarity, the team found that the greater the degree of overlap in their composition, the harder it was for participants to tell two odours apart. No one could discriminate odours with more than 90% of overlap, although at least half of the participants could tell odours apart when the degree of overlap was less than 75%.
Individual performance varied greatly, with the calculated number of 30-component mixtures that participants could tell apart varying by as much as 21 orders of magnitude between two participants.
“If one would make a strong statement about that, one would have to do different types of tests,” said Keller.
“I think a huge part of that variability is due to the genetic variability in the odourant receptors that bind to the odours.”
From the results, Keller and his team were able to work out the resolution of the olfactory system which they coupled with the total number of possible odour mixtures to state that humans can discriminate more than a trillion different odours of 30 components.
However despite the enormous magnitude of their figure, Keller believes the result is actually an underestimate of human olfactory abilities, pointing out there are far more odorous molecules than the 128 studied, while odours may also contain more than 30 components, with varying concentrations.
“There hasn’t been a previous estimate before of how many different smells the olfactory system might be able to pick up and how fine the discrimination might be in the same way as there has been for other senses so it is a bit of a step forward there,” said Dr Peter Brennan from the University of Bristol who was not involved in the study.
“I think it is just emphasising the fact that smells can really vary in minute ways and they are very, very complex. Our sense of smell is able, if it is important for us, to pick up on these small differences.
“But in the everyday world those small differences normally are not interesting and we actually ignore them because we are generalising it to a particular odour that we can put a label on, such as strawberry or banana.”
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