Read Online What the Nose Knows: The Science of Scent in Everyday Life by Avery Gilbert - Free Book Online Page B
Book: What the Nose Knows: The Science of Scent in Everyday Life by Avery Gilbert
Read Free Book Online
Authors: Avery Gilbert
Ads: Link
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?
T HE 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 that 1 to 2 percent of the U.S. population suffers from anosmia or hyposmia. In both cases, the most common cause by far is infectious disease. Severe colds, flu, and sinus infections inflame the tissues lining the nose and kill off sensory nerve cells. In severe cases, or after a lifetime of accumulated damage, areas that were once rich with nerve cells are replaced with nonsensory mucus membrane, and the tissue takes on a moth-eaten appearance.
Head injury is the second leading cause of smell loss. A blow to the head can sever some of the olfactory nerve fibers that travel to the brain through tiny holes in the base of the skull, at a location between the ears and behind the eyes. There’s an old (and possibly true) story about a waiter carrying a tray of food at head height. As he exits the kitchen, the swinging door slams the tray into his forehead. Being a professional, he maintains his balance and proceeds into the dining room. As he serves the dishes, he realizes he can’t smell a single one. The speed with which the waiter discovers his loss might be unusual—most people don’t notice for days or weeks—but the mildness of the damaging blow is not. It takes very little force to cause smell loss. I cringe when I see kids heading the ball in a soccer game. I wouldn’t bet on them becoming chefs or perfumers.
With the exception of a stuffy nose, smell loss is a long-term condition. Smell may return after a flu or sinus infection, as the damaged sensory cells are gradually replaced by new ones, but recovery can take months and your abilities may never return to their original levels. The probability of recovery declines with age. In cases of head trauma, the prospects are bleak; the severed nerve fibers rarely reconnect. Consider the results of a typical study: a year after their initial visit to the doctor, 32 percent of postinfection patients showed improvement, compared with only 10 percent of the post-injury group. The
T HE 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 that 1 to 2 percent of the U.S. population suffers from anosmia or hyposmia. In both cases, the most common cause by far is infectious disease. Severe colds, flu, and sinus infections inflame the tissues lining the nose and kill off sensory nerve cells. In severe cases, or after a lifetime of accumulated damage, areas that were once rich with nerve cells are replaced with nonsensory mucus membrane, and the tissue takes on a moth-eaten appearance.
Head injury is the second leading cause of smell loss. A blow to the head can sever some of the olfactory nerve fibers that travel to the brain through tiny holes in the base of the skull, at a location between the ears and behind the eyes. There’s an old (and possibly true) story about a waiter carrying a tray of food at head height. As he exits the kitchen, the swinging door slams the tray into his forehead. Being a professional, he maintains his balance and proceeds into the dining room. As he serves the dishes, he realizes he can’t smell a single one. The speed with which the waiter discovers his loss might be unusual—most people don’t notice for days or weeks—but the mildness of the damaging blow is not. It takes very little force to cause smell loss. I cringe when I see kids heading the ball in a soccer game. I wouldn’t bet on them becoming chefs or perfumers.
With the exception of a stuffy nose, smell loss is a long-term condition. Smell may return after a flu or sinus infection, as the damaged sensory cells are gradually replaced by new ones, but recovery can take months and your abilities may never return to their original levels. The probability of recovery declines with age. In cases of head trauma, the prospects are bleak; the severed nerve fibers rarely reconnect. Consider the results of a typical study: a year after their initial visit to the doctor, 32 percent of postinfection patients showed improvement, compared with only 10 percent of the post-injury group. The
Similar Books
