In the classic children’s book Alice’s Adventures in Wonderland, Alice encounters a tea party hosted by a nonsensical and irritable hat-maker. Some hat-makers from the 1700s through the early 1900s really did behave as if insane, due to the long-term exposure to mercuric nitrate, used in the millinery industry to turn fur into felt.
The saying “mad as a hatter” dates back to the 19th century. 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 disease.”
beaver, Property#1/Flickr; hat, ©AMNH/C. Chesek