If you plan on doing lots of summer reading this year, be sure to keep the safety basics in mind: Always keep your page-turning fingers hydrated; never enter an unfamiliar fictional world without a compass; and — most important — watch out for poisonous books.

Odd as it may sound, works on paper can actually be toxic — even deadly — if they’re colored with the wrong pigments. A team of researchers at the University of Southern Denmark (SDU) recently rediscovered this peculiar bane of bibliophiles when they pulled three Renaissance-era manuscripts from the school library’s rare-book collection, put them under an X-ray microscope and found themselves face-to-face with glowing green arsenic.

“We took these three rare books to the X-ray lab because the library had previously discovered that medieval manuscript fragments, such as copies of Roman law and canonical law, were used to make their covers, Jakob Povl Holck, a research librarian at SDU, and Kaare Lund Rasmussen, an associate professor in physics, chemistry and pharmacy, wrote in The Conversation.  “It is well documented that European bookbinders in the 16th and 17th centuries used to recycle older parchments”.

The problem was, all three book covers were caked in an “extensive layer” of green paint that made reading the underlying text impossible with the naked eye. So, Holck and Rasmussen used a technique called micro X-ray fluorescence to shine a pinhole-thin beam of light onto the manuscripts, hoping to highlight specific elements (like calcium or iron) baked into the underlying ink. Instead, they found arsenic.

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(“Purest Green…”)

Arsenic is a natural metalloid element found all over Earth’s crust — however, when combined with other elements like hydrogen and oxygen, it becomes deadly poisonous. “This chemical element is among the most toxic substances in the world and exposure may lead to various symptoms of poisoning, the development of cancer and even death,” Holck and Rasmussen wrote. “The toxicity of arsenic does not diminish with time”.

Read more: Live Science