Old books from the 19th-century featured green covers with worrying levels of arsenic, prompting some libraries to remove them from the shelves.
Researchers at the University of Delaware also identified red and yellow covers that contain levels of mercury and lead using X-rays to test the chemical compounds.
The book publishers instead created book covers that contained green dye because it was a cheaper alternative to leather, but they were unaware the arsenic used to make the dye could be fatal.
France's National library removed four books with emerald green covers believed to contain arsenic, considering placing them back only after conducting tests.
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