For what reason were hatters considered mad in the Victorian era?

Alice in Wonderland character
hammering the head during hat adjusting
empty hat symbols empty head
mercury poisoning
Mad hatter's disease is a common name for chronic mercury poisoning among hatmakers, who were often exposed to mercury vapors during felting works. Along with physical symptoms such as tremors or loss of hair and teeth, hatters tended to develop characteristic psychological traits, such as pathological shyness and marked irritability.
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reach: rather globaldiseaseshatshistorymadnessmedicinemental disordersVictorian era