Humans can smell short-lived odorous molecules better

April 20, 2020

Fast reacting volatile chemicals with short atmospheric lifetimes can be smelled more sensitively than persistent ones

The sense of smell is one of our oldest senses. Olfaction has always been one of mankind´s important ways of interaction with the environment—an essential tool that helped us identify food and dangers. However, human noses don´t detect all odorous molecules equally well. Odorous molecules are mostly volatile organic compounds such as alcohols, aldehydes, esters and ketones. For example, ethyl butyr-ate forms the typical pineapple smell and is found in many fruits such as strawberries and apples.
We can sense some of these organic compunds at extremely low concentrations, while others hardly at all.
Two scientists from Max Planck Institute for Chemistry in Mainz have now a possible explanation why. According to their findings, the sensitivity of our nose to chemicals in the air has evolved over time making us become most sensitive to fast reacting odorous molecules with short atmospheric lifetimes. These volatile chemical compounds react quickly with other elements in the atmosphere. As a result, they fall apart and disappear again for our nose. According to the scientists, these compounds are usually the ones that help us detect the exact position of a certain food or an enemy.

This surprising discovery came after Akima Ringsdorf and Jonathan Williams compared a smell sensitivity chemical database with an atmospheric chemistry database for the first time. The researchers used a database of olfactory thresholds created over decades by Japanese scientists. They then correlated this data with the available atmospheric chemistry databases that document the lifespan of organic molecules in the air. The scientists discovered that fast reacting chemicals with short atmospheric lifetimes can be smelled more sensitively than persistent ones. For many chemical families such as alcohols, aldehydes, and esters the correlation is striking.
Alcohols, aldehydes, esters and ketones are all associated with food (e.g. fruit) while nitrogen and sul-fur compounds are associated with urinary waste products and fire, respectively. “Thus, it can be speculated that the relationship between odour thresholds and the lifetime of these elements may have developed through the ability to find food or to avoid danger”, says Williams, a group leader at the Max Planck Institute for Chemistry.

"A good sense of smell is a big advantage when searching for food. If you want to find a nutritious ripe fruit in a dense dark forest you need to be able to follow a smell gradient as it twists and turns in turbulent air streams", continues Williams. Such smell gradients are stronger and therefore easier to follow for fast reacting chemicals.
The correlation uncovered in this study that has recently been published in the Journal Philosophical Transactions of the Royal Society, also means that odor thresholds can be predicted by atmospheric lifetime measurements and vice versa. “The results we have seem to support the general hypothesis that there is a link between atmospheric chemistry and the sense of smell, which nobody have thought of before”, continues Williams.

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