Why do bad smells smell good?

Smell is fundamental to our experience and linked with a variety of emotional responses. Some smells repel us, while some invite us in. They’re a vital part of our world and our survival strategy. Without smell we become more depressed and, conversely, when we’re depressed we can’t smell as well

Smell seems to be the most confusing of our senses. Different people react differently to the same odors, and they even identify them as coming from different things. Our scentscape does not seem to have a lot of objectivity, but our olfactory system has been very well-studied by scientists over the past century. They have come up with some fascinating theories on how we perceive smells and why an easily identifiable smell can be hard to come by.

Smells have weight

Our perception of scent is directly caused by specific chemicals in the air. Long before we were human, our bodies have been able to react to different chemicals in the environment. Many animals emit tiny traces of unique chemicals to communicate with each other. These chemical communication methods might be used to find prospective mates, learn who has been eating the best food, or maybe to ward off competitors and let scavengers know that an area is defended.

Your perception of smell is created by the interaction of scent molecules with your brain. The human olfactory system works primarily by inhaling outside air in which scent molecules have been dissolved. These scent molecules then react with smell receptors on nerve endings in the olfactory epithelium near the top of our nose. The nerves send signals to the olfactory bulb, which is part of the brain. 

The olfactory bulb is connected to the rest of the brain a little differently than other senses. Unlike sight and touch, which are usually processed by the thalamus, smell bypasses the thalamus and goes directly to the amygdala and hippocampus, the brain’s emotion and memory centers. As a result, it is very easy to create emotions and memories tied to smells, and often unavoidable.

Smell receptors are created by specific genes and bind to specific classes of scent-creating molecules. Some people may have “specific anosmia,” a genetic condition that occurs when the gene for a receptor for one class of smell molecules isn’t expressed so they can’t perceive certain smells.

In addition to the activity of the olfactory receptors, smells can also affect the trigeminal nerve. The trigeminal nerve runs between the brain and the rest of the head.  It can detect temperature and other physical properties. The hot sensation of peppers and the cooling sensation of mint are both created by smell molecules interacting with the trigeminal nerve.

Your olfactory system is very sensitive to certain smells, able to detect concentrations of some molecules as low as one part in five billion. The odor threshold for different smells varies considerably. It is extremely dependent on the tiniest changes in the unique chemistry of a smell molecule and the combination of incoming smells. Some researchers estimate we can differentiate between over a trillion different smells.

Chemistry of a smell molecule

All scent-creating molecules have a few things in common. All of them are volatile, which means they tend to evaporate into the air at room temperature. Without this characteristic, they would be unable to get into your nose. Also, all scent molecules are capable of interacting with the organic carbon-based molecules that constitute smell receptors, which means that most of them are organic as well. Just like many other systems in the body, olfactory nerves are very sensitive to specific chemical configurations.

Take these two molecules, geraniol on the left and 2-heptanone on the right:

These two look very similar, carbon chains with an oxygen atom. But our nose picks up geraniol as the smell of flowers (the name comes from the flower geranium) while 2-heptanone is the smell of blue cheese. Two totally different smells, one from a plant that is regarded near-universally as pleasant, the other from a mold with a controversial degree of agreeability, yet both are similar configurations of carbon, hydrogen and one oxygen atom. The sensitivity of our olfactory system and how odor contextually interacts with memory and emotion makes them quite different. Even if you like blue cheese, if you smelled it coming from a geranium it would seem wrong and probably make you believe it was contaminated.

For another example on how sensitive our olfactory system is, take these two molecules, S-furaneol on the left and R-furaneol on the right :

They are basically the same in almost all contexts. The subtle difference is the methyl group and hydrogen on the lower left of the images here have swapped positions. R-furaneol is the smell of strawberries, but S-furaneol has a very weak almost unidentifiable smell. The receptor that can detect the smell of strawberry is so specific that a tiny conformational change is the difference between a perception of sweet sugary jam and nothing.

Some organic compounds are created by living things specifically for their odorous effect and some just happen to interact with our systems to create smells. This molecule with an amine group on either side is called putrescine:

As you might have guessed it’s one of the smells of rotting flesh. It’s primarily a byproduct of bacteria breaking down the amino acids that make up proteins, but to us, it’s a signal to avoid something that is infested with bacteria. To a carrion-eating turkey vulture, it’s the smell of a good meal.

Our experiences influence our noses

The different reactions of humans and scavengers to rotting flesh are not as much of a result of preferred diet as you might think. We have about 400 different types of receptors in our olfactory epithelium and each one is influenced not just by our experiences but also our human genes, though most of our reactions to smells are learned.

The process of learning to experience smell starts in the womb. Whatever our mothers were eating when pregnant influences what we find appealing after birth. However, children must still learn whether a smell is good or bad. While their parents might find rancidity offensive and bananas appealing, most children will find any new smell offensive. By age eight most kids have the same smell preferences as their parents.

Context can be an important factor in the appeal of a smell, as well. The smell of cabbage coming from your shoes might be extremely offensive, but coming from the stovetop might make you hungry. The US military has allegedly tried for years to create a universally despised odor to use as a non-lethal crowd dispersal weapon, and could only find a few scents that everyone hates. They were sulfur-based compounds like those found in feces, which may also activate the trigeminal nerve to make them truly inescapable.

The trigeminal nerve

The way we think about and label smells in our minds appear to influence our ability to perceive them, a concept called linguistic determinism. Just putting the wrong label on a smell can influence how it is perceived. This is not the same as with other senses, such as sight, where we can accurately identify colors and shapes with very little ambiguity over and over without context having much impact.

This might all be a function of how language influences our thinking, as it’s likely that some languages just don’t give us the vocabulary to properly think about smells. Trygg Engen, a prominent smell researcher, found that English speakers associate lemon odor with air freshener, bathroom freshener, berries, magic markers, candy, citron/citrus, Lemon-fresh Pledge, hard candy, pine, orange, some kind of fruit, or a wide variety of other responses. Similarly, he found that cinnamon might be associated with spiciness, sweetness, bayberry, candy, Red Hots, smokiness, wine, potpourri, and many other words.

What is interesting about this aspect of our olfactory experience is that it very likely depends on our language’s ability to accurately label unique sensations from our olfactory receptors. To this point, yet another study sought to find a language that could better identify smells. These researchers found such a language among the Jahai people who live in areas of Malaysia and Thailand. Jahai and English both have specific terms for color, but only Jahai has similarly specific terms for smell. They found that the Jahai could identify smells just as well as they or English-speakers identify color.

Same smell different person

The primary reason there seems to be so much ambiguity in our perception of smell is likely as a result of our omnivorous diet and adaptable biology. Humans are generalists, we can eat a wide variety of foods and live in a wide variety of environments. Other animals may be specialists, who eat only one food and live in one type of environment. Specialist animals react to predators and other threats without prior knowledge. Since we are generalists and threats may take different forms in different environments, our reactions are passed down more socially and experientially instead of genetically. For example, we develop an aversion to the smell of food after being poisoned just once. Get sick on pizza once and the smell of mozzarella, tomato sauce, oregano, and toasted bread will be tragically and unfortunately offensive for a long time. The same would not be true for a Koala that got sick from eucalyptus leaves, the only food in its diet.

A dog displaying generalist behavior while hunting for truffles

We do have some genetic predeterminations with smells that are not learned. Sweetness is universally a pleasant taste, bitterness is always an acquired taste. Very young babies like sweetness and are drawn to the sweetness of breast milk, but are repulsed by bitterness, which may be poison.

Aromatherapy is a controversial method of using smell to influence health outcomes. The scientific evidence that it has any effect is lacking, and the therapeutic impact of smell molecules on our biology has not been shown. This is likely because aromatherapy works for some people by reminding them of positive memories. The smell of apples might remind someone of a safe childhood, or roses the smell of perfume on an affectionate loved one. Lavender might be relaxing because we associate it with a neat and clean room, or the linger scent of mint with the feeling that our teeth have just been brushed. But since everyone has different associations with different smells, just perceiving a scent is unlikely to make any specific person experience the desired effect. Though if you find a smell does have a positive effect there are practically no drawbacks to adding it into your space, but you might want to stay away from synthetic scents or scented candles.

Some people even like smells that are generally considered bad or even poisonous, like gasoline or paint fumes. The research is still being conducted, but there are two possible reasons for this. One is what is described above, that perhaps gasoline evokes positive memories of road tripping or paint the memory of a good day’s work. The other is that substances like gasoline and paint contain volatile organic compounds like benzene and toluene. When these chemicals enter the body they release dopamine and can cause a euphoric effect (in addition to raising the risk of cancer, neurological problems, and respiratory disease). So these smells may be appealing or even addictive in a similar way to opiates, which also release dopamine.

Our olfactory system is very old and very powerful and evolution has linked it tightly to our survival. The adaptable neurology of the human brain allows us to experience smells in a way that is best for the particular living circumstances in which we find ourselves.

At Molekule we are always doing research on how our PECO technology impacts all sorts of things floating through the air, and that includes smell molecules like d-limonene, the scent of citrus. Read more on our blog about what’s in your air and how it impacts your body.


Pictures of blue cheese, the trigeminal nerve, and the dog sniffing for truffles are courtesy of Wikimedia Commons, part of the Wikimedia Foundation. Molecules made with MolView.

Written by