The human nose can distinguish at least 1 trillion different odours, millions more than previously estimated, U.S. researchers said Thursday.
For decades, scientists accepted that humans could detect only 10,000 scents, putting the sense of smell well below the capabilities of sight and hearing.
"Our analysis shows that the human capacity for discriminating smells is much larger than anyone anticipated, said study co-author Leslie Vosshall, head of Rockefeller University's Laboratory of Neurogenetics and Behavior.
The previous estimate for the nose's capabilities -- which are carried out with the help of 400 olfactory receptors -- dated to the 1920s and was not backed by data.
Researchers have estimated that the human eye and its mere three receptors can distinguish several million colours and that the ear can discriminate 340,000 sounds.
"For smell, nobody ever took the time to test," Vosshall said.
To conduct their research, scientists subjected 26 participants to mixtures made with 128 different odorant molecules that individually might evoke grass, citrus or various chemicals, but were combined in groupings of up to 30.
"We didn't want them to be explicitly recognizable, so most of our mixtures were pretty nasty and weird," Vosshall said.
"We wanted people to pay attention to 'here's this really complex thing - can I pick another complex thing as being different?'"
Volunteers would sample three vials of scents at a time -- two that were the same and one that was different -- to see if they could detect which was the outlier, completing 264 such comparisons.
Although volunteers' abilities varied greatly, they could on average discern the difference between vials with up to 51 percent of the same components, with fewer volunteers detecting a difference once the mixtures shared more components.
Researchers then extrapolated how many odours the average person could detect if all possible combinations of the 128 odorants were sampled, coming to their estimate of at least one trillion.
Lead researcher Andreas Keller, also of Rockefeller University, said the number is almost certainly too low given that there are numerous other odorants that can mix in countless ways in the real world.
He said our ancestors relied more on the sense of smell, but that refrigeration and the development of personal hygiene have limited odours in the modern world.
"This could explain our attitude that smell is unimportant, compared to hearing and vision," Keller said.
He added that upright posture, which raised humans' noses far from the ground where odors often emanate, may also have contributed.
The sense of smell is closely linked to human behaviour and the researchers stressed that studying it could shed light on how the human brain processes complex information.
The study was published in the journal Science.