The old saying goes that human nose can only distinguish 10,000 different odors, leaving humans at the bottom of the sense of smell in the animal kingdom. But a new study published in Science on March 20, Humans Can Discriminate More than 1 Trillion Olfactory Stimuli, suggested the human sense of smell far outperforms the other senses in the number of different stimuli it can detect, giving the nose more credit than previously thought.
A typical human nose has 400 olfactory receptors, causing researchers to speculate that it is capable of smelling many more than the 10,000 scents people used to believe. The number of receptors matters—the three color receptors in human eyes enabling the visual detection of up to 10 million colors.
Even testing the 10,000 different scents in the old belief would be impossible, so the design of the test must be smart. Researchers created mixtures of 128 different scent molecules. Although each molecule resembles familiar odors such as grass or citrus, when combined the mix smells unfamiliar. Each volunteer was given three vials of scents, two of which are the same. The volunteers were asked to identify the unique scent. The frequency of successful identifications by volunteers were counted and used to extrapolate the total number of scents an average human can detect out of all mixtures of 128 molecules. The result is one trillion but researchers said the number can easily be higher because there are more than 128 molecules in the nature.
Comparing to sound and sight, the sense of smell seems used less often and thus considered to have lesser importance by many people. This new discovery of the amazing scent distinguishing ability of the nose may encourage people to use this sense more attentively, as it deserves more credit than once believed.
Scientists, however, have been aware of the remarkableness of the human nose for decades and have incorporated its mechanism into making artificial or electronic noses.
Each of the 400 receptors in human nose can respond to a wide range of volatile compounds. The recognition of a smell seems to emerge from the combination of many such responses from the tens of millions of cells in the human olfactory system, just like the recognition of a face relies on identification of various distinct characteristics. This distributed approach contributes to the efficiency and versatility of the olfactory system and the electronic noses it inspired, which are way more superior to the traditional expensive and single-functioned sensor, which is built to detect a single chemical substance.
The traditional electronic nose, widely used in food industry to detect rotten ingredients, consists of an array of a dozen or so different polymer films. The electrical conductivity of these film changes when subjected to different chemicals, therefore each film respond in a characteristic way when the array of files is exposed to a particular scent.
New electronic nose can be an array of transistors made out of various organic semiconductor materials, instead of polymer films. This high sensitivity of transistors can greatly increase the number of odors the device can detect. The manufacture process of the organic semiconductors can also significantly reduce the cost of such device to only a few dollars. In the future with further cost reduction, electronic noses may be built into food containers and pharmaceutical packaging, for the best food safety monitoring.
The finding that human sense of smell can detect trillions of odors not only gives the human nose more credit than once thought, but also opens more possibilities to the new technologies that draw inspiration from it.
By Tina Zhang