Why Do You Love That Smell?

The distinctive smell of old books has captivated readers and historians for centuries. Some people describe it as a little musty with hints of vanilla, coffee, or even newly cut grass. On the other hand, the smell of new books can seem crisp and fresh. But what causes books to have such distinctive scents? Well, it comes down to a handful of chemical compounds found in the paper, ink, and bindings of the book.

Paper is the fundamental material in most books, and it's made up of wood pulp, which contains a variety of organic compounds. Organic compounds are chemicals that contain carbon, and paper is rich in the polymer cellulose. Cellulose is a long chain of glucose molecules bound together, often with the help of lignin, another complex organic polymer found in plant cells.

Over time, these chemical compounds in paper react to various environmental factors like light, heat, and moisture, which can cause them to break down. As they break down, they release volatile organic compounds (VOCs) into the surrounding air. The specific VOCs released depend on the paper's composition and how the book was manufactured.

If you detect a hint of almond in the book's smell, you're probably smelling benzaldehyde. Benzaldehyde is a compound found in almonds, which explains the almond-like scent. A vanilla-like fragrance is thanks to vanillin, the primary compound that gives vanilla its smell and flavor. If you detect something sweet, it's likely due to ethyl benzene, a compound often used in solvents, inks, and paints. If you're smelling a light floral aroma, it's probably due to ethyl hexanol, a type of alcohol used in solvents, flavors, and scents.

New books release different kinds of VOCs because modern manufacturing processes use various chemicals like hydrogen peroxide to bleach the paper and alkyl ketine dimers to make the paper somewhat water-resistant. Scientists and historians can use these volatile organic compounds to learn more about the age and condition of older books or to reveal parts of their history, such as exposure to smoke or water damage. Understanding the scents of old books can help historians determine which ones are degrading and need better preservation and protection.

The smell of rain, often described as petrichor, is another distinctive scent that many people find calming and refreshing. Despite rain being just water, it has a unique smell associated with freshness and cleanliness. Petrichor gets its name from the Greek words "petra" (rock) and "ichor" (the blood of the gods), signifying the earthy aroma that rain imparts.

The primary source of the rain smell is not rocks themselves but rather plants, especially during dry periods. Some plants release oils rich in fatty acids when water is scarce. These oils contain compounds like palmitic and stearic acids, which contribute to the fresh, vegetal scent. These oils accumulate on soil and rocks, and when rain falls, they are released into the air as fragrant volatile compounds.

Another component of the rain smell is geosmin, a chemical released by soil bacteria. Geosmin is responsible for the earthy aroma often associated with rain. It's a type of alcohol that can give some vegetables and wines their loamy, earthy flavor. Soil bacteria release geosmin when they die or go dormant, especially during hot, dry conditions. When rain finally comes along, it disperses geosmin into the air in the form of a fine mist.

In addition to the plant-derived and bacterial components of the rain smell, ozone plays a role. Ozone (O3) is produced when lightning ionizes oxygen molecules in the air, causing them to recombine into groups of three oxygen atoms. Ozone has a distinctive, bracing scent that contributes to the invigorating quality of petrichor.

The combination of these natural compounds creates the pleasant scent associated with rain. Interestingly, the strength of the rain smell is often more pronounced if it hasn't rained in a while, as the accumulated oils and geosmin have had time to build up on the soil and rocks. So, the next time it rains in your area, take a moment to appreciate the chemistry that contributes to the earthy, plant-like, and ozone-scented experience.

While rain may be known for its soothing and earthy aroma, the smell of gasoline is quite different and can be polarizing. Gasoline, a common fuel used in automobiles, is primarily composed of hydrocarbon molecules and various additives to ensure the engine's smooth operation. One of these molecules, benzene, is responsible for the distinctive smell of gasoline.

Benzene is a chemical compound with a unique odor that gives gasoline its characteristic scent. When we encounter the smell of gasoline, it's often due to benzene's presence. However, our perception of this smell can vary based on our individual experiences and associations.

Our sense of smell is closely connected to our emotions and memories. We tend to like familiar smells and those we have positive associations with. Benzene triggers the release of the feel-good chemical dopamine in our brains, which can create a sense of euphoria when we encounter its smell. People may have formed positive associations with the scent of gasoline from activities like road trips, visits to convenience stores, or other enjoyable experiences. These positive associations can make some individuals find the smell of gasoline appealing.

However, it's important to note that inhaling too much benzene can be harmful. Prolonged exposure to high levels of benzene can damage our DNA, leading to various health issues, including a weakened immune system, reproductive problems, or even cancer. Gasoline contains other compounds with less pleasant smells, such as long-chain amines, which can have a fishy or rotten odor, and long-chain carboxylic acids, which some people describe as smelling like goats. These less pleasant odors in gasoline might explain why not everyone finds the smell of gasoline appealing.

Ultimately, the perception of the gasoline smell depends on individual preferences, associations, and sensitivities. While some people may enjoy the aroma of gasoline due to positive associations, it's essential to recognize the potential health risks associated with prolonged exposure to the volatile organic compounds found in gasoline. Practicing safety precautions, such as minimizing the time spent near gasoline fumes, can help protect against potential health hazards.

New cars, in particular, have a distinct smell, often referred to as the "new car smell." This smell is a combination of various volatile organic compounds (VOCs) released from the materials used in the car's interior, including the upholstery, plastics, adhesives, and other components. The new car smell can be appealing to many people, and it's often associated with the excitement of having a brand-new vehicle.

The specific VOCs that contribute to the new car smell can vary depending on the materials and manufacturing processes used by the carmaker. Some common VOCs found in new cars include formaldehyde, benzene, and styrene. These compounds are part of the complex mixture of chemicals released into the car's interior.

However, it's important to note that some of these VOCs are classified as carcinogens. Formaldehyde and benzene, for example, are known carcinogens, which means there is strong evidence that they can cause cancer under certain conditions. The levels of these VOCs in a new car's interior can vary, and exposure to them is typically at low levels, so the health risk is generally considered low for the average consumer. Still, it's crucial to be aware of potential health