Aside from the occasional paper cut or perhaps killer litter, books aren't typically a source of danger.
But for a family living in Taizhou in China's Jiangsu province, their love of reading was slowly poisoning them - literally.
At the end of last year, the family- which consists of a couple and their child- developed symptoms of formaldehyde poisoning, according to a report from Jiangsu Television.
The adults suffered from constant coughing, while their child had rhinitis, an inflammation of the mucous membrane.
The couple spent about 8,000 yuan (S$1,630) on medical treatment but nothing worked.
Furthermore, doctors weren't sure what was causing their illness, the television report said.
However, after doing some research, the woman realised that the air in their home could be the source of the problem.
She requested municipal authorities to test the air quality in their apartment and the results found excessive levels of formaldehyde in every room. In some areas, the concentration of formaldehyde was dangerously high.
According to China's national standards, anything over 0.08 milligrams of formaldehyde per square metre is considered unhealthy.
The concentration of the poisonous gas in the couple's bedroom was 0.10 milligrams per square metre, and the highest levels of formaldehyde came from their bookshelves.
Reaching up to 0.26 milligrams per square metre, the collection of books was identified as the main source of formaldehyde pollution, South China Morning Post reported.
Formaldehyde is a colourless, yet strong-smelling gas, and is found in many products ranging from wallpaper and paints to cosmetics, clothing, carpet, pressed-wood products, and even beer.
It is also used to make some nail polishes and embalming fluid.
High levels of the chemical can damage the respiratory and immune systems.
In this case, the family was affected by the formaldehyde present in the printing inks used in books, magazines and newspapers.
The couple, who are avid readers, said to Jiangsu Television that they bought books online three to four times a month.
Piles of books were present in every room of the family's flat, and the husband told the broadcaster that they had accumulated tens of thousands of them.
Experts suggested the couple get rid of some of the books while doctors told them to limit their collection to one room and improve the ventilation in their home.