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Generally, the chemical breakdown of compounds within paper leads to the production of the smell of old books. The musty smell of an old book is the result of hundreds of volatile organic compounds released into the air from the paper. Paper contains cellulose and smaller amounts of lignin, with much less lignin in more modern books than in books from more than one hundred years ago.

In trees, lignin helps bind cellulose fibres together. Oxidation reactions break lignin down into acids, which then help break down cellulose. Modern, high quality papers are chemically processed to remove lignin, but cellulose in the paper can still break down at a much slower rate because of acids in the surroundings. These acid hydrolysis reactions produce a wide range of volatile organic compounds, many of which are likely to contribute to the smell of old books.

Acetic acid and furfural are known volatile degradation products of paper. The 15 most abundant volatile compounds present in all chromatograms included acetic acid, benzaldehyde, 2-ethylhexanol and furfural. Books published after the mid-1800s emit more furfural, and its emission generally increases with publication year relative to older books composed of cotton or linen paper. Furfural is a product of the dehydration of sugars. 2-Ethylhexanol is a branched, eight-carbon chiral alcohol and a colorless liquid that is poorly soluble in water.

The particular blend of compounds depends on the original composition of the object, including paper substrate, applied media and binding. Papers containing rosin and wood fiber are the most rapidly degrading types found in old books. The scientists identified 15 VOCs that seem good candidates as markers to track the degradation of paper. No single compound, or family of compounds, can be pointed to as the cause of the scent.

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