Pictured above are two people looking through a book of wallpaper samples. As activities go, this is probably among the most harmless there is; sure, there's a slight risk of a paper cut if you decide to feel your way through the samples, as the people above are, but beyond that, there's no enhanced risk of injury. Wallpaper may be expensive and it can often be rather ugly, but in no sense of the word is it dangerous.

Well, not anymore, at least. Go back far enough, and wallpaper could kill people -- and, unfortunately, did so pretty often.

The culprit? The color green.

The problem began in the mid-1700s. A chemist named Carl Wilhelm Scheele discovered a copper derivative that was particularly vivid. As Victorian-era historian Lucinda Hawksley told the Atlantic, "if you think about the brilliance of copper and the way that a patina begins to color metal, it’s a beautiful color," close to that of the Statue of Liberty (which, being plated in copper, was originally a dull bronze). Fast forward about a century and Scheele's discovery -- known as Scheele's Green -- was used as a pigment for all sorts of artistic works, particularly among the European elite. And, in particular, wallpaper with a green base or green accents typically used Scheele's Green to achieve the hue desired by designers and customers alike.

Unfortunately, Scheele's Green was soft on the eyes but not nearly as kind to the ears, nose, throat, and the rest of the human body. The pigment was made from copper arsenite, which is to say, it's made of arsenic. And arsenic is a toxin -- it can cause nausea, vomiting, abdominal pain, and more. Continued exposure to arsenic can ultimately result in death. And at the time of arsenic-laden wallpaper,  there was only a limited understanding of the dangers of the element. Many people understood that you couldn't safely eat it, but few understood that arsenic was also dangerous if inhaled. And that's exactly what happened with the wallpaper. The Saint Louis Art Museum explains: 

Despite its vivid and eye-catching nature, doctors eventually discovered that arsenical wallpaper could kill. The ink often flaked off the paper only to be inhaled by those nearby, while moisture, abrasion, or heat caused the release of toxic vapors. Increasing reports of mysterious illnesses and deaths of small children or even entire families gained the attention of the general public in the mid-19th century. It was not until the late 1860s that doctors connected those maladies with the presence of luminous green paper on the walls.