Your nose is a paradox. In some ways the human sense of smell is astonishingly precise. For example, natural gas companies add a smelly molecule called n-butyl mercaptan to natural gas, which is odorless by itself, so that people can sniff gas leaks. All it takes is one n-butyl mercaptan molecule for every 10 billion molecules of methane to do the trick. To put this precision in perspective, imagine you are standing in front of two Olympic-size swimming pools. One of them contains a grand total of three drops of n-butyl mercaptan, and the other has none. Your nose could tell the difference.
But don’t get too smug, because in other ways your sense of smell is practically useless. To judge for yourself, find someone to help you run a simple experiment. Close your eyes while your partner raids your refrigerator and then holds different foods under your nose. Try to name each scent. If you’re like most people, you’ll bomb. In a number of studies, scientists have found that people tested on items in their own kitchens and garages give the wrong answer at least half the time. And as bad as we normally are at identifying smells, we can easily be fooled into doing worse. If orange food coloring is added to cherry-flavored soda, for example, people are more likely to say that it smells like oranges.
Noam Sobel of the Weizmann Institute of Science in Israel and his colleagues have been puzzling over this paradox for the past several years. What has been missing in the science of smell, they argue, is a meaningful way to measure it—an olfactory yardstick. Now they have built one.
That it has taken so long for someone to come up with a yardstick for smell is something of a scandal. Scientists who study vision, for example, know that light with a wavelength of 620 nanometers will appear a particular shade of orange. They know with perfect certainty that orange is closer in wavelength (and perceived color) to yellow than it is to green. And they have used such objective measures about light and vision to learn a great deal about the biology that allows us to see. Scientists who study smell have had no equivalently objective way to judge, for example, whether the smell of roses is closer to spearmint or vanilla.
Part of the reason for this lack of an odor yardstick may have been the common belief that the human sense of smell is crude. Dogs and other mammals have a better sense of smell than we do, but their prowess doesn’t mean our noses are useless. In fact, as Sobel and his colleagues demonstrated in a 2007 experiment, humans can do a pretty good impression of a bloodhound.
Sobel and company went into an open field and set down 30 yards of twine scented with chocolate. Then they brought together 32 people and gave them a mission: Follow the trail using nothing but your nose. The scientists put blindfolds on their subjects so they could not see the twine. Earmuffs blocked out sounds. Elbow pads, knee pads, and work gloves shielded them from tactile clues. Only their noses could provide them with information. The subjects got down on all fours about 10 feet from the start of the scent trail. Then they started to sniff.
Remarkably, most of the volunteers were able to find the twine. Even more remarkably, 21 of them were able to follow its scent from start to finish. Whenever they veered off course, they sniffed their way back. Not only did they track the scent impressively well, but they also improved with practice. The scientists arranged for some of the subjects to run the course for 45 minutes a day for three days; they cut their times and improved their accuracy.
When Sobel’s human bloodhounds put their noses to the ground, they drew in a gaseous cocktail of many different kinds of molecules—from the dirt, the grass, and anything else riding along in the air. Those molecules then latched onto olfactory receptors located on nerve endings in their nostrils. Only certain molecules, one theory holds, have the right shape to latch onto certain receptors. A given receptor can snag a number of different odor molecules, and a given odor molecule can latch onto several different receptors. Each nerve in a person’s nose builds all its receptors using just a single gene.
The olfactory neurons are the only ones in the central nervous system that are directly exposed to the air. When a receptor grabs a molecule, it causes an electric signal to travel the length of the neuron from the nasal lining to the smell-processing regions of the brain. There, the neuron converges with thousands of other neurons delivering their own signals. The brain does not just passively accept all these signals. If we learn how to tell two odors apart through one nostril, for example, we are able to tell them apart with the other nostril as well. The learning happens in the brain, not in the nose.
All this complex signal processing means that we can distinguish among thousands of different odor molecules. Sobel and his colleagues recently set out to pin down how that process works by determining the relationship between the structure of a molecule and the way it smells. The scientists began by building a database of 1,500 odor-producing molecules, cataloging 1,664 different traits—their size, the strength of the chemical bonds between their atoms, and so on.