”Mad as a hatter" In 18th and 19m 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 m refer to someone who was erceived as insane.