Mad hatter disease describes the symptoms of mercury poisoning, specifically its effect on the nervous system. These include paraesthesias, vision and hearing impairment, slurred speech, anxiety, hallucinations, irritability, depression, lack of coordination, and tremors. The condition was observed among workers in the hat-making industry in the 19th century. Chronic mercury exposure was common in hatters who used a mercury solution during the process of curing animal pelts. Poor ventilation in the workshops of the time resulted in the hatters breathing in the fumes of this highly toxic metal, leading to an accumulation of mercury in the workers' bodies. Metal toxicity was poorly understood and the broad range of symptoms were also associated with insanity.
The phrase mad as a hatter may have been derived from the condition, and is commonly associated with Lewis Carroll's character the Mad Hatter in Alice's Adventures in Wonderland. While the character's eccentricities differ from those suffering from mercury poisoning (the Hatter was likely inspired by Theophilus Carter, a furniture dealer), Lewis Carroll grew up near the town of Stockport, where hatting was the dominant trade.