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

Alice in Wonderland character
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hammering the head during hat adjusting
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empty hat symbols empty head
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mercury poisoning
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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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