TIL the term "Mad as a Hatter" likely came from hat makers using mercury and the resulting mercury poisoning.
Mercury poisoning of hat-makers – In 18th and 19th century England, mercury was used in the production of felt, which was commonly used in the hat-making trade at the time. Long-term use of mercury products often resulted in mercury poisoning-induced erethism among hat-makers.[1][2] In the late 19th-century United States, a notable example occurred in Danbury, Connecticut, where hat making was a major industry. Instances of erethism were so widespread among hat-makers, the condition became known locally as the “Danbury Shakes.” It was characterized by slurred speech, tremors, stumbling, and in extreme cases hallucinations.
