Why do old books have that smell?

Open an old book and you can often smell it before you've read a word. Woody. Dusty. Sometimes sweet or musty.

It's easy to blame dust, or just "oldness". But part of what you're smelling is chemistry.

The book is giving off molecules.

A book isn't chemically still. Its paper is mostly cellulose, and some papers contain more lignin than others. As paper ages, mainly through reactions with water and oxygen that acids speed up, it releases volatile organic compounds, or VOCs: molecules that escape easily into the air. Ink, glue and binding add their own.

It isn't one smell. It's a mixture.

The blend depends on what the book is made of and where it has been kept. Researchers have linked furfural to the breakdown of cellulose, and compounds such as benzaldehyde and vanillin to the breakdown of lignin, a wood component more plentiful in some wood-pulp papers. Acetic acid, the acid in vinegar, is another common emission.

We experience the mixture as one smell.

Your nose doesn't pick out individual compounds; it gets the blend. In a 2017 study, seven untrained volunteers inside the historic library at St Paul's Cathedral in London chose words from a supplied list; woody, smoky, earthy and vanilla came up most. But they could see wooden shelves and old books, and the researchers noted that sights and suggested words can shape what people report.

Simplified. Every book gives off its own mixture, depending on what it is made of, how it has been stored and how far it has degraded. The compounds shown are examples measured in the cited studies, not a full list, and no single one is the old-book smell. The words for the smell are descriptions, not chemical results, and are not matched to individual compounds.

To a conservator, the smell is information.

In 2009, a team measured the compounds given off by 72 historic papers and compared them with each paper's chemistry. Patterns in the emissions were linked to how much lignin the paper contained, how far its cellulose had broken down, and how acidic it was. That study still needed small samples, heated in the lab.

In 2012, researchers sampled the air in eight major libraries and archives in the UK and Ireland. Furfural was the only compound consistently higher where paper collections were stored. The authors suggested that acetic acid and furfural together might act as a non-invasive marker of paper decay. They also placed thin strips between the pages of books to collect emissions directly, without removing a sample from the book.

An old book's smell carries clues to what its paper is made of and how it is ageing. Reading them takes lab instruments, not a nose.

When an old book smells old, you're picking up molecules released as its materials slowly change: some of the same molecules researchers analyse for clues to its condition.