When you are packed on a commuter train, the body odour of fellow passengers can at times be hard to ignore. Now scientists have shed light on how our noses pick up on the stench.
While different scents can be highly evocative, only a fraction of our 400 or so odour receptors have been shown to be involved in the perception of specific smells.
“We’re still, I would say, surprisingly ignorant about what all the olfactory receptors do and how they interact with each other to encode olfactory percepts,” said Dr Joel Mainland, a co-author of the research from the University of Pennsylvania and the Monell Chemical Senses Center.
Among those that have been uncovered are receptors linked to the perception of cis-3-hexen-1-ol, a chemical that smells like freshly cut grass, and androstenone, a steroid hormone that either smells like urine, sandalwood, or is odourless depending on an individual’s genetic makeup.
Now scientists have discovered an odour receptor linked to the perception of body odour, and another linked to the perception of a musky scent.
Writing in the journal Plos Genetics, Mainland and colleagues, including researchers from Unilever, report how they asked 1,000 Han Chinese people to rate the intensity and pleasantness of 10 odours on a 100-point scale.
Among the odours was galaxolide, a synthetic musk, and 3M2H, one of about 120 chemicals that comprise body odour, but which carries the “characteristic scent” by itself.
The team then combined the results with an analysis of the whole genome of each participant.
The results confirmed previously discovered associations between the perception, including intensity, and genetic variants for specific odour receptors for three out of four of the scents, including androstenone.
The team, however, also found genetic variants linked to the perception of the musky scent of galaxolide and 3M2H, allowing them to identify the odour receptors involved in the detection of these scents in humans for the first time.
The findings were backed up by work in which six of the odours, some at different concentrations, were presented to 364 participants from New York City – the majority of whom were Caucasian – suggesting the role of the odour receptors may hold across populations and odour concentrations.
Mainland said an unusual finding was that many participants with two copies of a particular genetic variant for one odour receptor were unable to smell galaxolide, suggesting only a single receptor was involved in its detection. “It’s generally the rule that more than one receptor will play some role,” he said.
Mainland suggested the findings could, at least theoretically, have practical applications in the development of personal hygiene products – for example in blocking the perception of bad smells. But they are also of personal interest, as he is unable to smell 3M2H at normal concentrations.
“I would guess I have the non-functional version of this receptor [for 3M2H],” he said, adding he could still smell body odour – likely because he had receptors able to detect other chemicals released by a sweaty armpit.
The team also reports an analysis of 29 mutations that have been linked to the perception of particular odours, suggesting that, in general, genetic variants that occurred more recently in the evolutionary history of humans and other primates are found in odour receptors that appear to be less sensitive.
Mainland stressed, however, that it was unclear whether that meant the sense of smell had eroded, noting it was possible other receptors or processing mechanisms may have compensated.
Prof Matthew Cobb of the University of Manchester and author of Smell: A Very Short Introduction, said that by focusing on a handful of our smell receptor types, the study provided a scientific basis for what we know intuitively – that some people are more sensitive to certain smells, and this can affect whether the odour seems nice or nasty.
“Furthermore, it sheds light on a long debate in human and primate evolution – the extent to which sight has tended to replace smell over the last few million years,” he said. But, he added: “There are another 400 or so receptors to study, and the vast majority of our responses to odours remain a mystery.”