In July 2018, three poisonous books were found in a library at the University of Southern Denmark. Dated to the 16th and 17th centuries, these books had green covers painted with high concentrations of arsenic. Back then, arsenic was often used in dyes for wallpapers, books, and clothes. Researchers suggest that the chemical was meant to protect the texts from mice and bookworms. Today, arsenic is known for its poisonous properties when inhaled or touched. But never fear, these dangerous books have been moved to specially ventilated cabinets.