BioEd Online

The human nose can distinguish at least 1 trillion different odours, a resolution orders of magnitude beyond the previous estimate of just 10,000 scents, researchers report today in Science.

Olfactory scientists have suspected a higher number for some time, with some suggesting that the number could be effectively infinite. But few studies have attempted to explore the limits of the human nose’s sensory capacity.

“It has just been sitting there for somebody to do,” says study co-author Andreas Keller, an olfactory researcher at the Rockefeller University in New York.

Keller and his colleagues prepared scent mixtures with 10, 20 or 30 components selected from a collection of 128 odorous molecules. Study participants were asked to identify the mixture that smelled differently in a sample set where two of three scents were the same. When the two scents contained components that overlapped by more than about 50%, participants struggled to discriminate between them. The authors then calculated the number of possible mixtures that overlap by less than 50% to get what they say is the lower limit of human olfactory discrimination.

“It’s an important question: how many odours can we discriminate?” says Donald Wilson, an olfactory researcher at the New York University School of Medicine, who calls the study “thrilling”.

A human nose has around 400 scent receptors. When the smell of coffee wafts through a room, for example, specific receptors in the nose detect molecular components of the odour, eliciting a series of neural responses that draw one’s attention to the coffee pot. But many of the details of how that sequence unfolds are still a mystery.

“The relationship between the number of odorants that we can discriminate and the number of receptors that we have is unclear,” says Noam Sobel, a neuroscientist at the Weizmann Institute of Science in Rehovot, Israel.

The absence of a boundary-setting dimension like wavelength for colour vision or frequency for sound has created a lag in the understanding of smell compared with other senses.

“It’s hard to organize odours,” Wilson notes. How, he asks, does one compare the musky scent of a US drugstore cologne like Axe body spray with a rival, such as Old Spice, or with something that smells of vanilla?

“My hope is that this helps to dispel the myth that humans have a bad sense of smell,” Keller says.