For nearly a century, scientists assumed the human nose was capable of discerning about 10,000 different odours. Turns out, that number was missing a whole bunch of zeroes—new research shows that the human nose can detect over 1,000,000,000,000 distinct scents. Tell your dog to stop being so smug about his smelling abilities.
In a paper published yesterday in Science, Howard Hughes Medical Institute's Leslie Vosshall and colleagues took on the 10,000 scents estimate that sprang up in the 1920s and wasn't really based on any science. "Objectively, everybody should have known that that 10,000 number had to be wrong," Vosshall said.
Consider that the human eye perceives 10 million different colours using just three types of light receptors. By comparison, the human nose has 400 different receptor types. Clearly, our stink-sensing capabilities were seriously underestimated.
Since running human tests on a trillion different scents would probably take a nearly literal eternity, Vosshall and her team used some crafty extrapolation. Starting with 128 unique odourant molecules, the team crafted completely novel, never-before-smelled scents composed of 10, 20, or 30 odour ingredients. They then grouped the novel scents into batches of three (two identical, one different) and asked test subjects to identify which smell smelled different from the other two smells.
"We didn't want them to be explicitly recognisable, so most of our mixtures were pretty nasty and weird," Vosshall told MedicalXpress. "We wanted people to pay attention to 'here's this really complex thing – can I pick another complex thing as being different?'"
Each volunteer smelled 264 different groupings of odours. Then, the team extrapolated the data to determine how many different scents the average human could discern if presented with the full gamut of combinations possible using the 128 ingredients. Vosshall compares the method to taking a census—rather than knocking on every door, you survey a representative group and use your findings to make a larger estimate.
Since there are way more than 128 scents in the real world, that 1 trillion estimate is actually on the low end. And while you probably don't encounter a trillion different odours in a given day (I hope), the capability to discern new scents means your nose is ready for whatever changes your environment presents.
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