Smell is an important sense. Certain smells, like your dad’s cologne, can help you recall a memory. Other smells, like smoke from a fire, can alert you to danger. When you can’t smell things you enjoy, like your morning coffee or spring flowers, life may seem dull.

As you get older, your sense of smell may fade. Your sense of smell is closely related to your sense of taste. When you can’t smell, food may taste bland. You may even lose interest in eating.

Taste, also called gustation, and smell, also called olfaction, are the most interconnected senses in that both involve molecules of the stimulus entering the body and bonding to receptors. Smell lets an animal sense the presence of food or other animals—whether potential mates, predators, or prey—or other chemicals in the environment that can impact their survival. Similarly, the sense of taste allows animals to discriminate between types of foods. While the value of a sense of smell is obvious, what is the value of a sense of taste? Different tasting foods have different attributes, both helpful and harmful. For example, sweet-tasting substances tend to be highly caloric, which could be necessary for survival in lean times. Bitterness is associated with toxicity, and sourness is associated with spoiled food. Salty foods are valuable in maintaining homeostasis by helping the body retain water and by providing ions necessary for cells to function.

Tastes and Odors

Both taste and odor stimuli are molecules taken in from the environment. The primary tastes detected by humans are sweet, sour, bitter, salty and umami. The first four tastes need little explanation. The identification of umami as a fundamental taste occurred fairly recently—it was identified in 1908 by Japanese scientist Kikunae Ikeda while he worked with seaweed broth, but it was not widely accepted as a taste that could be physiologically distinguished until many years later. The taste of umami, also known as savoriness, is attributable to the taste of the amino acid L-glutamate. In fact, monosodium glutamate, or MSG, is often used in cooking to enhance the savory taste of certain foods. What is the adaptive value of being able to distinguish umami? Savory substances tend to be high in protein.

All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules into the air from its surface, it has no smell. And if a human or other animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor subtypes that work in various combinations to allow us to sense about 10,000 different odors. Compare that to mice, for example, which have about 1,300 olfactory receptor types, and therefore probably sense more odors. Both odors and tastes involve molecules that stimulate specific chemoreceptors. Although humans commonly distinguish taste as one sense and smell as another, they work together to create the perception of flavor. A person’s perception of flavor is reduced if he or she has congested nasal passages.

Reception and Transduction

Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity (as illustrated below). The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area about 5 cm² in humans. Recall that sensory cells are neurons. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors, distributed among hundreds of different receptor types that respond to different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about 100 million, most dogs have about 1 billion, and bloodhounds—dogs selectively bred for their sense of smell—have about 4 billion. The overall size of the olfactory epithelium also differs between species, with that of bloodhounds, for example, being many times larger than that of humans.

Olfactory neurons are bipolar neurons (neurons with two processes from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins, and it is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia, and the receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus.

Smell and Taste in the Brain

Olfactory neurons project from the olfactory epithelium to the olfactory bulb as thin, unmyelinated axons. The olfactory bulb is composed of neural clusters called glomeruli, and each glomerulus receives signals from one type of olfactory receptor, so each glomerulus is specific to one odorant. From glomeruli, olfactory signals travel directly to the olfactory cortex and then to the frontal cortex and the thalamus. Recall that this is a different path from most other sensory information, which is sent directly to the thalamus before ending up in the cortex. Olfactory signals also travel directly to the amygdala, thereafter reaching the hypothalamus, thalamus, and frontal cortex. The last structure that olfactory signals directly travel to is a cortical center in the temporal lobe structure important in spatial, autobiographical, declarative, and episodic memories. Olfaction is finally processed by areas of the brain that deal with memory, emotions, reproduction, and thought.

Taste neurons project from taste cells in the tongue, esophagus, and palate to the medulla, in the brainstem. From the medulla, taste signals travel to the thalamus and then to the primary gustatory cortex. Information from different regions of the tongue is segregated in the medulla, thalamus, and cortex.

Your sense of smell—like your sense of taste—is part of your chemosensory system, or the chemical senses.

Your ability to smell comes from specialized sensory cells, called olfactory sensory neurons, which are found in a small patch of tissue high inside the nose. These cells connect directly to the brain. Each olfactory neuron has one odor receptor. Microscopic molecules released by substances around us—whether it’s coffee brewing or pine trees in a forest—stimulate these receptors. Once the neurons detect the molecules, they send messages to your brain, which identifies the smell. There are more smells in the environment than there are receptors, and any given molecule may stimulate a combination of receptors, creating a unique representation in the brain. These representations are registered by the brain as a particular smell.

Smells reach the olfactory sensory neurons through two pathways. The first pathway is through your nostrils. The second pathway is through a channel that connects the roof of the throat to the nose. Chewing food releases aromas that access the olfactory sensory neurons through the second channel. If the channel is blocked, such as when your nose is stuffed up by a cold or flu, odors can’t reach the sensory cells that are stimulated by smells. As a result, you lose much of your ability to enjoy a food’s flavor. In this way, your senses of smell and taste work closely together.

Without the olfactory sensory neurons, familiar flavors such as chocolate or oranges would be hard to distinguish. Without smell, foods tend to taste bland and have little or no flavor. Some people who go to the doctor because they think they’ve lost their sense of taste are surprised to learn that they’ve lost their sense of smell instead.

Your sense of smell is also influenced by something called the common chemical sense. This sense involves thousands of nerve endings, especially on the moist surfaces of the eyes, nose, mouth, and throat. These nerve endings help you sense irritating substances—such as the tear-inducing power of an onion—or the refreshing coolness of menthol.

It’s important to be aware of odors around you. You need to be able to detect smoke, gas leaks, spoiled food, and vapors from potentially dangerous household chemicals.

• Smoke — check your smoke detectors once a year to make sure they work.
• Gas leaks — make sure you have a gas detector in your home.
• Spoiled food — throw out food that’s been in the refrigerator too long.
• Household chemicals — make sure there is fresh air where you live and work.

These safety measures are especially important if your sense of smell has diminished.

Your sense of smell enriches your experience of the world around you. Different scents can change your mood, transport you back to a distant memory, and may even help you bond with loved ones. Your ability to smell also plays a key role in your health. If your ability to smell declines, it can affect your diet and nutrition, physical well-being, and everyday safety.

Whether coffee brewing, pine trees in a forest, or smoke from a fire, the things we smell are actually tiny molecules released by substances all around us. When we breathe in these molecules, they stimulate specialized sensory cells high inside the nose. Each of these sensory cells has only one type of odor receptor—a structure on the cell that selectively latches onto a specific type of “smelly” molecule. There are more smells in the environment than there are odor receptors. But a given molecule can stimulate a combination of these receptors, creating a unique representation in the brain of a particular smell.

“It’s estimated that the number of odors that people can detect is somewhere between 10,000 and 100 billion, or even more,” says Dr. Gary Beauchamp, a taste and smell researcher at Monell Chemical Senses Center in Philadelphia. We all have different combinations of odor-detecting cells in our noses, he explains, so people vary greatly in their sensitivity to smells. “In fact, when you or I smell the same physical thing, our perceptions may be very different,” Beauchamp says.

Because smell information is sent to different parts of the brain, odors can influence many aspects of our lives, such as memory, mood, and emotion. For thousands of years, fragrant plants have been used in healing practices across many cultures, including ancient China, India, and Egypt. Aromatherapy, for example, aims to use essential oils from flowers, herbs, or trees to improve physical and emotional well-being.

To date, there’s little scientific evidence supporting aromatherapy’s effectiveness for most health issues. Yet memories of smell can be vivid and long lasting, which may have a positive effect.

“Lavender is a good example, which is touted as a relaxation odor,” Beauchamp says. “But the question is: Is that a relaxation odor because we’ve had past experience with this particular odor where we’ve been relaxed, and so we’ve learned the association?” Scientists continue to examine how different types of aromatherapies might affect our health and well-being.

Smell is also important for your perception of taste. Chewing your food releases aromas that travel from your mouth and throat to the nose. Without smell, we can detect only 5 basic tastes: sweet, salty, bitter, sour, and umami (savory). But our brains incorporate information from both taste and smell receptors to create the perception of many different flavors.

Some people may think they’ve lost their sense of taste if food begins to taste bland or slightly “off.” But in fact, they may have lost their ability to smell.

Many things can cause smell loss. A stuffy nose, or a harmless growth in the nose (called a polyp) can block air and thus odors from reaching the sensory cells. Certain medications, like some antibiotics or blood pressure pills, can alter smell. These effects are usually temporary. Your smell should come back once you’ve recovered or stopped the treatments.

But some things can cause a long-lasting loss of smell. A head injury or virus, for example, can sometimes damage the nerves related to smell. And your ability to smell may naturally fade as you get older.

“A good sized majority of people don’t know they have a problem with their sense of smell,” says Howard Hoffman, a public health expert at NIH. A national health and nutrition survey recently revealed that 12% of adults have a smell dysfunction. The problem increases with age, with 39% of those ages 80 and older showing a deficit.

“Quality of life issues from smell loss affect people differently depending upon their situation,” Hoffman says. “The effects can be enormous.” Food can become less enjoyable. You may lose interest in eating or change your eating habits, consuming a less healthy diet.

People who’ve lost their sense of smell sometimes try to boost flavor by adding more salt or sugar to their foods. But these additions might cause problems for those at risk for certain medical conditions, such as high blood pressure, kidney disease, or diabetes. Talk with your doctor if you think a smell deficit might be affecting your quality of life.

Smell loss can also put you in harm’s way if you don’t notice a “warning” smell. The recent national health and nutrition survey found that 1 in 10 people couldn’t identify the smell of smoke, and about 15% couldn’t identify the smell of natural gas. “As people get older, those rates go up,” Hoffman says. For those ages 70 and older, 20% couldn’t identify the smell of smoke, and 31% couldn’t recognize gas odor.

“With age, there is a decline in the ability to smell to some extent in the nose, but much more in the brain itself,” says Dr. Davangere Devanand at Columbia University, an expert on neurodegenerative diseases and smell loss. “The main reason appears to be that the functioning of the brain regions involved in smell and memory become impaired as we grow older.”

But problems with your ability to smell may be more than normal aging. They can sometimes be an early sign of serious health conditions, such as Parkinson’s disease, Alzheimer’s disease, or multiple sclerosis. Devanand’s group is currently studying the relationships between smell dysfunction and Alzheimer’s disease.

If your food doesn’t smell or taste the way you think it should, talk to your doctor. Health care providers can give you a “scratch and sniff” smell identification test to help assess the kind of smell disorder you might have. This test alone can’t diagnose more serious health problems, but it can be informative when used alongside other tests.

Smell may be the most mysterious of our 5 senses, Beauchamp says. “We know quite a bit about smell loss and can diagnose this fairly well. But, for the most part, we have no treatments that are reliable and widely accepted” for long-lasting cases of smell loss. Some studies suggest that smell training may help you improve your ability to discriminate and identify odors. It may stimulate growth of new receptors or improve your brain’s ability to interpret low levels of odors, Beauchamp explains. But researchers are still learning how and whether this works.

Like all of your senses, your sense of smell plays an important part in your life. If you think you’re experiencing a loss of taste or smell, see your health care provider. There may be ways to help fix the problem. If not, your doctor can help you learn to cope with the changes in smell and taste.

Smell Loss and Safety

If you’ve lost your ability to smell, it’s important to find other ways to detect:

• Smoke. Check your smoke detectors once a year to make sure they work.
• Gas leaks. Make sure you have a gas detector in your home.
• Spoiled food. Throw out food that’s been in the refrigerator too long and practice other basic food safety. Learn more at www.foodsafety.gov.
• Household chemicals. Make sure there’s fresh air where you live and work.

Many problems cause a loss of smell that lasts for a short time. This temporary loss of smell may be due to:

• A cold or flu that causes a stuffy nose. The ability to smell will come back when you’re better.

• Coronavirus infection, which sometimes causes a new loss of smell. You should regain your sense of smell after all other symptoms have gone away, however if it doesn’t return, seek medical care as soon as possible.

• Allergies. Try to stay away from things you’re allergic to, like pollen and pets. Talk to your doctor about how to manage your allergies.

• A harmless growth (called a polyp) in the nose or sinuses that gives you a runny nose. Having the growth removed may help.

• Some medications like antibiotics or blood pressure medicine. Ask your doctor if there is another medicine you can take.

• Radiation, chemotherapy, and other cancer treatments. Your sense of smell may return when treatment stops.

Some things can cause a long-lasting loss of smell. A head injury, for example, can damage the nerves related to smell.

Sometimes, losing your sense of smell may be a sign of a more serious disorder, such as Parkinson’s disease, mild cognitive impairment, or Alzheimer's disease. Be sure to tell your doctor about any change in your sense of smell.

If the foods you enjoy don’t smell or taste the way you think they should, talk to your doctor. He or she might suggest you see a specialist who treats people with smell and taste problems.This kind of doctor is called an otolaryngologist (oh-toh-lar-ing-gol-uh-jist), also known as an ENT (which stands for ear, nose, and throat). An otolaryngologist works on problems related to the ear, nose, and throat, as well as the larynx (voice box), mouth, and parts of the neck and face. The doctor may ask:

• Can you smell anything at all?
• Can you taste any food?
• When did you first notice the problem?
• Is the problem getting worse?
• Have you been told that you have allergies or chronic sinus problems?
• What medicines do you take?

There are likely ways to help fix the problem. If not, the doctor can help you cope with the changes in smell and taste.