What if you had no sense of smell?

Sniff, sniff! What can you smell right now?

Maybe toast, soap or flowers. What if your nose could not smell anything at all?

Take a slow sniff. Maybe you can smell toast, soap, a pet, or rain on the sidewalk. Smells are around us all day, even when we don't notice them.

Now imagine waking up with no sense of smell at all. Would your breakfast taste the same? Would you notice if milk had gone bad? In this lab we follow three clues: what smell does for flavor, how smell warns us, and how a dog's amazing nose compares with ours.

Smell is a chemical sense: tiny molecules released by things around us are detected by specialized cells in the nose, which signal the brain. We use it constantly, often without noticing, to enjoy food, recognize places and people, and detect hazards.

This thought experiment removes the sense of smell and asks what changes. It follows three threads: the contribution of smell to flavor, its role as a warning system, and the far more sensitive nose of the domestic dog. Facts come from the National Institute on Deafness and Other Communication Disorders (NIDCD), the U.S. Energy Information Administration and PBS NOVA; consequences are reasoned ideas.

High up inside your nose are special smelling cells.

When you sniff, tiny bits of a smell reach them. They send a message to your brain: cookies!

High inside your nose is a small patch of special cells called olfactory sensory neurons. Each one has a receptor that catches certain smell molecules, the tiny bits that drift off things like coffee or pine trees. When they're caught, the cells send messages straight to your brain, which works out what the smell is.

Smells reach these cells in two ways. One is through your nostrils when you sniff. The other is from the back of your mouth: when you chew, food releases smells that travel up a channel from your throat into your nose.

NIDCD describes olfactory sensory neurons in a small patch of tissue high in the nasal cavity, connected directly to the brain. Each neuron has one type of odor receptor; odor molecules stimulate them, and the brain identifies the smell from the pattern. There are more smells than receptor types, and any one molecule may stimulate a combination of receptors.

Odors arrive by two routes: through the nostrils, and through a channel from the roof of the throat to the nose, which carries aromas released by chewing. A separate common chemical sense, built from nerve endings in the eyes, nose, mouth and throat, detects irritants such as onion and the coolness of menthol.

Hold your nose and eat a piece of apple. Can you tell what it is?

Without smell, food can taste plain. That is because smell helps make flavor!

Without smell, the biggest surprise would be at mealtimes. NIDCD explains that without the smell cells, foods tend to taste bland, and it would be hard to tell chocolate from orange. Your tongue would still taste, but much of the flavor would be missing.

You may have noticed this already: when a cold stuffs up your nose, smells can't reach the smell cells, so food tastes less interesting. Some people who think they have lost their sense of taste discover it is really their sense of smell.

Much of what we call taste is flavor, and flavor depends heavily on retronasal smell. NIDCD states that without olfactory sensory neurons, familiar flavors like chocolate or oranges would be hard to distinguish and food tends to taste bland or flavorless. A blocked channel, as in a stuffy nose, produces a temporary version of the same effect.

The complete inability to detect odors is called anosmia; in rare cases people are born without a sense of smell. NIDCD notes that some people who believe they have lost taste have in fact lost smell. Tongue-based taste would remain in our thought experiment, but the richness of flavor would not.

Smell can warn you. You might smell smoke, or milk that has gone bad.

Then you tell a grown-up!

Smell is also a warning system. NIDCD says your nose can alert you to the smoke of a fire, food that has spoiled, or a leak of natural gas.

Here's a clever fact: natural gas, used for heating and cooking in many homes, has no smell of its own. The U.S. Energy Information Administration explains that gas companies add a harmless chemical called mercaptan so that leaks smell like rotten eggs. People decided to give gas a smell precisely because noses are such good detectors!

NIDCD describes smell as often serving as a first warning signal: of smoke from a fire, spoiled food, or a natural gas leak or dangerous fumes. Without smell, these early warnings would be lost, and a person would need to rely on other cues, such as alarms, sight and date labels; that adaptation is our reasoning.

The gas example shows how much society relies on noses. According to the U.S. Energy Information Administration, natural gas is colorless, odorless and tasteless, so gas companies add mercaptan, a harmless chemical that smells like rotten eggs, to make pipeline leaks detectable by smell.

Dogs have amazing noses! They can smell things we can't smell at all.

A dog's nose has many, many more smelling cells than yours.

If your nose is a good detector, a dog's is a super one. PBS NOVA reports that dogs have up to 300 million smell receptors in their noses, compared with about 6 million in ours. The part of a dog's brain that works out smells is, for its size, about 40 times bigger than ours.

Dogs' noses are built differently, too. When a dog breathes in, a fold of tissue sends some of the air to a special smelling area at the back of the nose, separate from the air going to the lungs.

PBS NOVA (2012) reports that dogs have up to 300 million olfactory receptors versus about 6 million in humans, with a brain region for analyzing smells that is proportionally 40 times larger. Scientists estimate dogs' sense of smell at 10,000 to 100,000 times as acute as ours.

Anatomy helps: bioengineer Brent Craven's airflow models found that about 12 percent of inhaled air detours into a recessed olfactory region lined with scroll-like turbinates, while exhaled air leaves through side slits, letting dogs sniff almost continuously. Compared with a dog, every human is a weak smeller, yet our smaller sense still shapes flavor and safety.