Range of smell spelled for a trillion different scents
The human nose is capable of some high-definition smelling, with a new study estimating we can distinguish around one trillion different scents.
Everyone has a mental catalogue covering an incredible array of smells; fresh popcorn, car exhaust, cut grass, sea breezes, wet dogs, paint and roast chicken to name a few. But tests in the lab suggest our palette is much more vast than many expected.
For some time, the oft-quoted figure of 10,000 different scents has been the accepted amount humans can smell.
Unfortunately, the 10,000 figures was reported in the 1920s and not backed by any data, but few could have imagined how far off it was.
“It's the generally accepted number,” says Leslie Vosshall, who studies olfaction at the Rockefeller University.
“Our analysis shows that the human capacity for discriminating smells is much larger than anyone anticipated... I hope our paper will overturn this terrible reputation that humans have for not being good smellers.”
Early on it was presumed that the number of different smells we can register must be large, considering that in vision just three light receptors together see about 10 million colours, and the average nose has about 400 olfactory receptors.
But no one had tested humans' olfactory capacity.
“We know exactly the range of sound frequencies that people can hear, not because someone made it up, but because it was tested. We didn't just make up the fact that humans can't see infrared or ultraviolet light. Somebody took the time to test it,” Vosshall says.
“For smell, nobody ever took the time to test.”
Rather than trying to go through every possible smell individually, researchers presented their subjects with complex mixtures of different odours, then asked whether they could tell them apart.
Volunteers were presented with three vials of scent at a time; two matched, and one different.
Volunteers were asked to identify the one scent that was different from the others, 264 times.
Researchers then tallied how often the 26 subjects were able to correctly identify the correct outlier, and then extrapolated how many different scents the average person would be able to discriminate if they were presented with all the possible mixtures.
They estimated that the average person can discriminate between at least one trillion different odours.
“I think we were all surprised at how ridiculously high even the most conservative lower estimate is,” Vosshall says.
“But in fact, there are many more than 128 odorants, and so the actual number will be much, much bigger.”
Vosshall says it is very unlikely a person would come across and recognise a trillion smells in their lifetimes, but the toolkit is still valuable.
“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,” she says.