Humans can distinguish at least one trillion different odours, millions more than previously anticipated, a new study has found.
For decades, it has been believed that humans are capable of discriminating between 10,000 different smells.
"It's the generally accepted number," said Howard Hughes Medical Institute investigator 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," she said.
Vosshall had long been bothered by the idea that humans were limited to smelling 10,000 odours - an estimate that was made in the 1920s, and not backed by any data.
For one thing, it didn't make sense that humans should sense far fewer smells than colours.
In the human eye, Vosshall explained, three light receptors work together to see up to 10 million colours. In contrast, the typical person's nose has 400 olfactory receptors.
But no one had tested humans' olfactory capacity.
Vosshall and Andreas Keller, a senior scientist in her lab at Rockefeller University, devised a strategy to present their research subjects with complex mixtures of different odours, and then ask whether their subjects could tell them apart.
They used 128 different odourant molecules to concoct their mixtures. The collection included diverse molecules that individually might evoke grass, or citrus, or various chemicals.
But when combined into random mixtures of 10, 20, or 30, Vosshall said, they became largely unfamiliar.
"We didn't want them to be explicitly recognisable, so most of our mixtures were pretty nasty and weird. We wanted people to pay attention to 'here's this really complex thing - can I pick another complex thing as being different?'" she said.
The volunteers were presented with three vials of scents at a time: two matched, and one different. Volunteers were asked to identify the one scent that was different from the others. Each volunteer made 264 such comparisons.
Vosshall and her colleagues tallied how often their 26 subjects were able to correctly identify the correct outlier.
From there, they extrapolated how many different scents the average person would be able to discriminate if they were presented with all the possible mixtures that could be made from their 128 odourants.
Researchers 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 said.
"But in fact, there are many more than 128 odourants, and so the actual number will be much, much bigger," she said.
The study is published in the journal Science.