Well it seems that yours truly was invited to give "expert" analysis on the Spike T.V. show 1000 Ways to Die.  This show was aired last January and told the story of a Mad Hatter.  You see, mercury was used in processesing beaver fur in order to felt it.  This practice caused merucury fumes to overwelm the hatters and with this constant contact, the hatters would eventually get mercury poisoning which would affect their brains and would eventually cause MADNESS.
Hence, the term Mad as a Hatter was coined.  Hope you catch it!