A new study published in the journal Science finds that the human sense of smell can detect more than 1 trillion odors, far exceeding the number previous studies have indicated.
Humans can discriminate several million different colors and nearly half a million sounds or tones, so by showing we can discriminate over 1 trillion odors, the study places the human sense of smell in a different league altogether.
The authors write that the study "demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far outperforms the other senses in the number of physically different stimuli it can discriminate."
Study leader Dr. Andreas Keller, of the Laboratory of Neurogenetics and Behavior at Rockerfeller University in New York, NY, says even 1 trillion may be an underestimate:
"The message here is that we have more sensitivity in our sense of smell than for which we give ourselves credit. We just don't pay attention to it and don't use it in everyday life."
Head of the Lab, Prof. Leslie Vosshall, says it was generally believed that the range of the human sense of smell was around 10,000, and adds:
"Everyone in the field had the general sense that this number was ludicrously small, but Andreas was the first to put the number to a real scientific test."
The quality of an odor is a complex thing. The smells we come across in everyday life actually comprise many different molecules, of which we sense only a few. For example a rose scent has 275 molecules, but we only sense a small proportion of them.
Sense of smell is more complicated to study
The human sense of smell can detect more than 1 trillion odors.
This means studying sense of smell is not as straightforward as examining sight or hearing. It is multidimensional in comparison.
For the study, Dr. Keller and colleagues invited volunteers to sniff vials of odors that held different mixes of 128 odor molecules responsible for scents ranging from spearmint to orange to anise.
The vials contained combinations of 10, 20 and 30 of the 128 odor molecules with different proportions of them in common.
The volunteers were presented with three vials at a time. Two of them contained identical mixtures of odor molecules, and the third one was different. They were asked to pick the odd one out.
The researchers found that while some volunteers were much better at picking the odd one out than others, on average they could distinguish between mixtures that shared as much as 51% of the same components. Above this proportion, most volunteers struggled to pick the odd one out. This was the case regardless of whether the mixtures contained 10, 20 or 30 odors.
Even 1 trillion may be an underestimate
From these results, the researchers extrapolated an estimate of the total number of distinguishable mixtures, arriving at the 1 trillion figure.
However, the team believes even 1 trillion may be an underestimate, because the real world contains many more odors that can be mixed in many more different ways.
Dr. Keller suggests our ancestors made more use of their sense of smell. We now hold our noses high above the ground, and by having daily showers and storing our food in a refrigerator we have effectively cut ourselves off from many of the odor encounters much valued by our forebears.
"This could explain our attitude that smell is unimportant, compared to hearing and vision," he adds.
In December 2013 Medical News Today reported a study that suggested we each live in a unique odor world. There, the researchers found as much as 30% of the large array of human smell receptors differs between any two individuals.
Written by Catharine Paddock PhD