A Victorian Millinery Catalogue from 1896. Mad hatter disease, or mad hatter syndrome, is a commonly used name for hatmakers whose felting work involved prolonged exposure to mercury vapours. By the turn of the 20th century, mercury poisoning among British hatters had become quite rare.
“Mad as a hatter” In 18th and 19th century England, mercury was used in the production of felt, which was used in the manufacturing of hats common of the time. People who worked in these hat factories were exposed daily to trace amounts of the metal, which accumulated within their bodies over time, causing some workers to develop dementia caused by mercury poisoning. Thus the phrase “Mad as a Hatter” became popular as a way to refer to someone who was perceived as insane.
MAD AS A HATTER 19th century Mercury used to be used in the making of hats. This was known to have affected the nervous systems of hatters, causing them to tremble and appear insane. Mercury poisoning is still known today as Mad Hatters disease.
The saying “mad as a hatter” dates back to the 19th century, when mercuric nitrate was used in the millinery industry to turn fur into felt. Hatters working in poorly ventilated factories breathed in toxic fumes, and prolonged exposure led to mercury poisoning with symptoms— such as trembling, memory loss, depression, irritability, and anxiety—that are still described as “mad hatter’s disease.”
The Hatter and the March Hare are initially referred to as "both mad" by the Cheshire Cat, and both first appear in the seventh chapter of Alice's Adventures in Wonderland, which is titled "A Mad Tea-Party".