Today, Mainland is at home in the lab at the Monell Chemical Senses Center in Philadelphia, where he focuses on the study of primary smells. Specifically, he is striving to develop quantitative systems that would assign a number to every odor, similar to the way that colors can be coded into Pantone, CMYK and RGB models. Different industries could then use different odor systems just as designers use color systems. Mainland foresees applications in fields from disease diagnosis to consumer goods and education.
He recently collaborated with Google to create the world’s first scent-describing artificial intelligence tool, the Principal Odor Map. POM charts how a molecule’s structure correlates to its scent; the resulting map lets the AI program predict how similar molecules will smell. Using the scent of a mosquito repellant, POM has already found more than a dozen similar molecules with equal repellency.
Mainland’s interest in olfaction dates back to the College, when he first learned that while minutely different shades of color could be labeled, nobody had done this kind of foundational research into smell. “It was a very exciting area to go into, because so much was unknown,” he says. “You could see the path ahead.”
The sense of smell in fact has long perplexed scientists. Human eyes have only three types of color receptors (red, green and blue), but the average person has about 350 kinds of odor receptors. These perch on sensory neurons deep in the nostrils and fire odor information to olfactory bulbs in the brain. Scientists have yet to unravel how they work together or even what scents which receptors process.
Also, people smell things differently. The reasons why elude Mainland and other odor scientists though he says genetic and cultural differences drive some of the variations. “There’s a very famous example — androsterone,” he says. “A third of the population can’t smell it. Another third smells it as a sweet or sandalwood odor. The rest smell it as urine.”
Based on the current research, Mainland believes that 50–100 odors form the primary building blocks of olfaction, similar to the way that three primary colors create millions of hues. Once these foundational scents are sniffed out, there could be huge benefits including cost savings. For example, food companies sometimes must reformulate products that are under development at great expense, after testing reveals they contain unsafe ingredients that cause a certain smell. “If you could make sure the primary odors are safe, you could make many different smells from that core set, and that would shift the field significantly,” he says.
He imagines a future in which schoolchildren are taught to identify smells and “paint” with them as they do with colors. “All of us in western industrialized countries are bad at naming odors,” he says. “If we had a set of primary odors to talk about and could mix them, a lot more people would do things with odor that aren’t being done now.”
Mainland predicts that in five years a device will exist that detects odors and tells users what to do about them. Such a device would have a far wider olfactory range than airport security sniffing devices that only recognize explosives. “Whether this will be in a commercial product or limited to an industrial application is not clear to me,” he admits. He foresees the food industry using sniffing gear to scan for spoiled food. And according to Mainland, a “handful” of diseases will one day be diagnosed by smells undetectable to the human nose.
The anosmia (long-term smell loss) that affects 15 percent of Covid-19 sufferers has boosted interest in Mainland’s field. The virus kills cells that aid olfactory neurons and stifles the growth of new receptors. Current treatments include smell training and nasal steroid sprays, but he says, “Neither of those are fantastic. They almost certainly do no harm, and there’s some indication of positive effects. We’re still in need of much better treatments.”
And while Mainland’s favorite smell is rhubarb pie, there’s one smell — vanilla — that is beloved around the world. The reason why is nose-opening.
“There’s a vanilla-smelling compound in breast milk,” he says. “We learn early on to associate it with nutrition.”
George Spencer is the former executive editor of Dartmouth Alumni Magazine. He lives in Hillsborough, N.C.