Researchers studying medieval texts at the University of Southern Denmark have uncovered a hidden danger in the bindings of at least three different historical documents that date back to the 1500 and 1600s. A green pigment found in the bindings of these books was discovered to be enriched in arsenic, so much so that they would poison anyone coming into physical contact with them. Naturally, one wonders how the arsenic found its way into these medieval books.

First, for the non-chemists among us: what is arsenic?

Arsenic is one of the elements—atomic number 33 on the periodic table. In nature, it is usually found in minerals that contain sulfur and some other metals. However, it can also be found in a pure crystal form. Throughout the world, arsenic has been employed as an ingredient in many pesticides. It is actually beneficial as a dietary supplement in trace amounts. It is only at elevated levels where it becomes a health problem.

Short-term exposure at high levels can produce symptoms including but not limited to vomiting, watery and bloody stools, and severe abdominal pain. Prolonged exposure can result in general numbness, skin lesions, chronic diarrhea, heart disease, and a variety of different types of cancer. Scientific research has shown that arsenic-induced cancer results from it impeding the DNA repair pathways in the cell.

Globally, arsenic contamination of groundwater that feeds water supplies has been responsible for long-term poisoning of communities with one of the worst incidents reported in Bangladesh and described by the World Health Organization as the “largest mass poisoning of a population in history.”

But how did arsenic end up in medieval texts? The answer is actually somewhat surprising.

In the pre-Victorian era, arsenic was quite commonplace in various paints, dyes, and wallpaper. It was even found in children’s toys and cosmetics at the time. It was employed as a pesticide for meats and vegetables, where they would be sprayed and dipped in solutions containing arsenic. Indeed, the particular arsenic containing green pigment discovered on the pages of these books was well-known as “Paris green” or “emerald green.”

Despite being somewhat familiar with arsenic as a poison, its use was not discontinued until Victorian times. Indeed, one doctor aware of the dangers regarding arsenic was quoted in 1857 as saying “a great deal of slow poisoning is going on in Great Britain.” Given these historical uses of arsenic-containing substances, it has been speculated that the origin of the arsenic contamination of the texts was the intentional application of an arsenic-containing green pigment for aesthetic reasons or potentially as an anti-pesticide.

The arsenic contamination of the medieval texts was discovered when researchers were conducting an in-depth analysis of the book bindings. Their interest revolved around an old practice dating back to 1536 where parchment from banned books was recycled as binding material for new books. Therefore, fragments of historical texts have been partially resurrected underneath cover pigments.

The research team used X-ray beams to detect ink remnants containing the elements of calcium, copper, and iron. This approach has been previously used to examine other artifacts including paintings and pottery. But this time, the X-rays unexpectedly detected massive amounts of arsenic, which was responsible for the green coloring on the books being examined.

With the established health hazard of these medieval books, scientists have recommended that further study of these documents require several precautions. In the future, archivists and readers of such texts will be asked to use plastic gloves to prevent physical contact with potential sources of arsenic. Efforts will be made to scan the content of the texts and deposit them into online databases, so they can be digitally read free of any possible exposure.

To prevent the development of an inhalation hazard, the books themselves will be stored in dark, dry, well-ventilated rooms to prevent humidity and light from catalyzing the formation of an airborne version of arsenic known as arsine. Arsine gas represents a significant health hazard as it can induce kidney failure by popping red blood cells. These precautions are likely to be applied to other historical books dating from the pre-Victorian time period where arsenic was more commonly employed. Taken together, it appears that it can be truly said that these “books can kill.”