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Making Sense of Smell

by Berkeley Wellness  

Many people fear—and may even have nightmares about—becoming blind or deaf or not being able to speak, but few of us have such primal dread about losing the ability to smell or taste. The least understood and appreciated senses, smell and taste are nonetheless essential to our interactions with the world and the pleasures we derive from it. They can trigger some of our most intense emotions and memories. The sense of smell, in particular, can be a lifesaver—for instance, when it recognizes that something is burning, gas is leaking, or food has spoiled.

It’s hard to discuss the sense of taste by itself, since smell is such an integral part of tasting. In fact, most people who complain that they can’t taste food actually have an impaired sense of smell (you may notice this when you have a bad cold or if you hold your nose while eating).

While complete loss of smell or taste is relatively rare, less severe impairments are common, affecting about half of all people over 65. Fortunately, there often are ways to treat the underlying conditions or at least compensate for the losses. Here we discuss smell; a separate article will tackle taste.

Olfactory facts

The sense of smell, called olfaction, is far less developed in humans than in other mammals, but a person with a healthy sense of smell can still detect at least 10,000 different odors. In 2004 two American scientists won the Nobel Prize for unraveling the mystery of how we can recall and distinguish so many odors. It’s a complex process, starting when odor molecules in the air are inhaled into the nose or when they waft up into the nasal cavity from food or drinks in the mouth. The molecules bind to receptors in olfactory nerve cells high up in the nose, which send signals to specialized parts of the brain called olfactory bulbs.

Women tend to have a keener sense of smell than men, and it’s particularly acute at the time of ovulation. Recent research has found that women have far more nerve cells in their olfactory bulbs; social factors may also increase their olfactory awareness. Besides having a less keen sense of smell, men tend to suffer more olfactory problems than women. This is partly due to the fact that more men have smoked, which can seriously damage the sense of smell. And the damage may persist long after a person quits smoking.

If you are constantly or repeatedly exposed to an odor, your ability to perceive it usually declines—this is called adaptation. This decrease in sensitivity can last a few weeks after the odor is removed. Interestingly, people are more likely to adapt to an odor if they believe it is a natural essence, rather than a potentially hazardous chemical, even when they are being exposed to exactly the same odor.

Odors can affect the release of certain neurotransmitters in the brain, which can, for instance, produce relaxation, alter mood, or even reduce pain. However, odor preferences vary in different parts of the world and from person to person, depending on cultural and genetic factors and on associations people may have with certain smells. Lavender, for instance, may calm one person, annoy another, and have no effect on a third.

By the way, though you may never notice it, the two nostrils shift their workload back and forth during the day. What’s more, one nostril is dominant, usually on the same side as the dominant hand.

When the nose knows not

Like many other bodily functions, odor perception starts to decline gradually after about age 40 in most people. Many things can impair the sense of smell temporarily or permanently, but often there’s no known cause other than advancing age. In most cases the ability to smell is merely diminished (this is called hyposmia). Less common is total loss of smell (anosmia), as well as misperception of smells (dysosmia) or perception of phantom smells (phantosmia).

Among the leading causes of loss of smell:

  • Nasal obstruction—resulting from allergies, infections, sinusitis, polyps, or trauma, for instance.
  • Problems with olfactory cells— caused notably by smoking, toxic chemicals, medication (such as some antibiotics and blood pressure drugs), chemotherapy, radiation therapy, or zinc nasal gels or sprays.
  • Problems affecting the nervous system, such as dementia, Parkinson’s disease, head injury, neurosurgery, diabetes, hypothyroidism, alcohol abuse, or certain nutritional deficiencies.

Olfactory problems can also simply be a marker for poor overall health. This was seen in a 2014 study in the online journal PLOS ONE, in which 3,000 people ages 57 to 85 were given a smell test using five common odors (rose, orange, leather, fish, and peppermint). The 3 percent who could identify only one—or none—of the odors had by far the highest death rate over the next five years; even the 19 percent who identified only two or three odors were at elevated risk. The researchers controlled for factors such as age, sex, smoking, nutrition, and initial health status. This doesn’t mean you’re doomed to die prematurely if your sense of smell is impaired, but it is something that deserves medical attention.

Nothing to sniff at

If you’re experiencing a decrease in your sense of smell and it isn’t due to something obvious like a cold or allergies, ask your health care provider about it. Not only can it make food less appealing and cut you off from many of life’s other pleasures, it may be a symptom of an undiagnosed medical condition (such as diabetes or nasal polyps) that can be treated, or a side effect of medication that can possibly be replaced. Depending on the results of the initial medical evaluation and testing, you may be referred to an ear, nose, and throat (ENT) doctor or other specialist.

If your sense of smell remains seriously impaired, it can endanger your safety. So it’s essential to take extra precautions at home by having good smoke alarms and, if you cook or heat with gas, a gas detector.