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[Text: QQs: Why do old books smell so good?]

Michael: Walk into an old library or secondhand bookstore, you'll be surrounded by that comforting old book smell. You know the one: some people describe it as a little musty, with hints of vanilla, or coffee, or even newly cut grass. Or maybe you prefer the smell of new books, which 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. See, paper is made up of wood pulp, so it has a lot of organic compounds, which are just chemicals that contain carbon. Specifically paper has a lot of the polymer cellulose, which is a long chain of the molecule glucose, and that's bound together with the help of lignin, another complex organic polymer found in plant cells, and over time these chemical compounds react to things like light, heat, and moisture in their surroundings and start breaking down. In the process they release volatile organic compounds, or VOCs, which easily vaporize and enter the surrounding air.

There are different kinds of these VOCs, and which ones are released depends on how the manufacturer made the paper and bound the book. If you detect a hint of almond, you're probably smelling benzaldehyde, a ring of carbons connected to another carbon that's double bonded to an oxygen. It's naturally found in almonds which explains the scent.

A vanilla like fragrance is thanks to vanillin, the main compound that gives vanilla its smell and flavor. If you smell something sweet, it's likely because of ethylbenzene, a ring of carbons connected to a short carbon chain that's often used to manufacture plastic. It's also in things like inks and paints. If you're detecting a light floral aroma, you're probably smelling 2-ethyl hexanol, a kind of alcohol that's often used in solvents, but also in flavors and scents. New books released different kinds of VOCs because modern manufacturing processes use different kinds of chemicals like hydrogen peroxide to bleach the paper and alkyl ketene dimers to make people a little 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 like whether they've been exposed to smoke or had water damage. And learning more about old book smells can help historians determine which ones are degrading and need to be better preserved and protected. So it turns out we can learn a lot from the smells in books, not just the words in them.

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