What the Nose Knows - Avery Gilbert [23]
It is odd to think that a childhood’s worth of olfactory memories can be boiled down to a pocket chemistry set. Were the tomato fields of Davis and the cooking ketchup of the Hunt’s cannery—so powerfully evocative to me as a kid—just a particular shuffle of sixteen key odorants? Evidently so. Knowledge also brings insight. I now understand in molecular detail why my grade-school field trip to the Spreckels Sugar Company plant was such a stunning disappointment. As beets are processed into pure white refined sugar they first release geosmin (damp earth) and dimethyl disulfide (onion, cabbage, putrid). Later comes propionic acid (the pungent, rancid note in Swiss cheese and sweat), and finally hexenoic acid (musty, fatty). Those four notes were the heavy stew that oppressed my soul that day in third grade. Somehow, knowing that makes me feel better.
CHAPTER 3
Freaks, Geeks, and Prodigies
DON GIOVANNI: Zitto: mi pare sentir odor di femmina!
[Hush! I think I scent a woman!]
LEPORELLO: Cospetto! Che odorato perfetto!
[My, my! What a nose!]
DON GIOVANNI: All’aria mi par bella.
[And a pretty one at that.]
—MOZART, Don Giovanni
TAKE A FEW DOZEN PEOPLE AT RANDOM, AND YOU WILL find a range of olfactory talent that stretches from American Idol–tryout bad to unbelievably excellent. There are people who cruise untroubled past the fetid plumes of garbage cans and subway vents, and others for whom the faintest milk fart escaping from an elderly relative is a nasal crisis. Olfactory sensitivity (technically, the lowest concentration at which someone detects a smell) is just one dimension of smell talent; other factors include an awareness of smells, and the ability to identify them and discriminate among them. Extreme variability is a hallmark of odor perception, and sensory scientists have identified many factors that contribute to it. It is now possible to answer a fundamental question: Who has a good nose and who doesn’t?
THE FIRST THING to note is that people are not accurate judges of their own ability. When we asked people taking the National Geographic Smell Survey to rate their own sense of smell, we found a Lake Wobegon effect: most people were above average. The only way to assess someone’s ability impartially is with a smell test. These come in two types: identification tests and threshold detection tests. The former ask you to put names to odors, the latter ask you to detect progressively lower concentrations of a smell. Smell tests have been commercially available for years, but were formally recognized as a medical device by the FDA only in 2006; this may explain why they are an underutilized part of the physician’s diagnostic arsenal. The tests range from one-shot sniff tests, appropriate for quick screening during an office exam, to elaborate, hours-long testing with scores of odors that takes place in a research lab. Normal smelling is generally defined as a certain proportion of correctly identified odor samples, or a specific, very dilute concentration at which an odor should be smellable. An odd feature of smell tests is that the best one can score on them is “normal” there is no test that rates levels of excellence, no equivalent to a 150 IQ. In fact, there is not even an official medical term for smell genius.
Because smell tests are designed to identify people with dysfunctional noses, they are finely calibrated for degrees of underachievement. At the lowest end of the scale are people unable to smell anything at all; they suffer from anosmia, the technical term for complete smell loss. One notch up the scale are people with hyposmia, which is the olfactory equivalent of being hard of hearing; like deafness, it can be mild or severe. It has been estimated