What the Nose Knows: The Science of Scent in Everyday Life Read Online Free PDF

people who were odor-blind to the stinky-feet smell of isovaleric acid were relatively insensitive to similar smells. He proposed that “sweaty” was a primary odor in the same way that red is a primary color. Amoore sought out molecules with similar shapes and smells that he thought might be the basis of other primary odors. (He eventually proposed seven of them: camphoraceous, musky, floral, pepperminty, ethereal, pungent, and putrid.) While he did succeed in finding other instances of selective odor blindness, Amoore’s notion of primary odors did not hold up under rigorous sensory testing. In the end, the structural features of a molecule are not a reliable guide to the psychological realities of odor categories.
    The latest attempts at odor classification use a technique called semantic profiling, an approach pioneered by the fragrance chemist Andrew Dravnieks in the 1960s, and still used today. Researchers hand people a long list of smell descriptors and have them check off as many as apply to a given odor sample. The hope is that with enough descriptors, smells, and statistical analysis, a pattern will emerge. And indeed patterns do appear—it is possible to point to groups of odors that share similar descriptions. The trouble is, this leaves us back where we started from—odors are described similarly because they smell similar. What we really want to know is, Why do they smell similar? For now, scientists are stumped—the molecular structure of odors isn’t the answer, nor can we conjure categories from lists of adjectives. As a result, researchers today are reluctant to propose anything like the grand classifications of the past.
     
    I F HISTORY IS littered with the wrecks of Universal Classifications of Smell, we can still learn something from surveying the ruins. What they have in common is a surprisingly limited number of elementary categories: either 4, 6, 7, or 9, depending on who you like. The mind-boggling variety of smells in the world is reducible to a manageable handful of nameable odor classes, just as the brain carves the range of visible light into a handful of focal colors. Suppose one adopted the standard perfumery categories as an approximation of the pleasant sectors of smell space; this amounts to one or two dozen classes (woody, floral, fruity, citrus, etc.). What more would one need to encompass the stinks and stenches of the world? The fecal category would cover a lot of territory—from benign horse manure to the intolerable air in a rock concert privy. A category for urinous could include the sour smells in a nursing home and the heavier fourth-quarter reek of urinals at an NFL game. We’d have to add a class for retch-inducing smells—vomit and really stinky feet—and another for fishiness in all its gradations. Skunk, sulfur, and burning rubber could constitute yet another class. Finally, the putrid stench of rotting meat probably deserves its own banner. These six classes would capture most of the bad smells abroad in the world. Which is more amazing—the huge number of possible odors, or the tiny number of odor types?
     
    C AN SUCH A stripped-down system of classification handle the olfactory complexities of the real world? It turns out that the human brain already does a pretty good job of reducing that complexity. The Australian psychologist David Laing was the first to tackle the relevant question: How many smells can we pick out of a complex mixture by nose alone? He began with a set of distinctive odors such as spearmint, almond, and clove, each easily identifiable on its own. He created mixtures—beginning with combinations of two odors at a time—and asked people to identify as many components as they could. The more odors he added to the mixture, the more difficult it became to identify even a single ingredient within it. The degree of difficulty was surprising. For example, in a mixture of three or more odors, fewer than 15 percent of people could identify even one