Hatters in the 19th century typically made hats using felt that had been made using mercury nitrate. The inhalation of the residual vapours from the felt could potentially lead to mercury poisoning, one of the symptoms of which is brain damage and dementia.

It is often said that this is the origin of the phrase "mad as a hatter" and also the character of the Mad Hatter from Lewis Carroll's Alice's Adventures in Wonderland. It's not entirely clear that this is unambiguously the case, as there are also competing theories.

Anyway, I can assure you that it is the origin of the character of Death of Inhaling Hatmaking Chemicals. I thought it was time we saw more of this guy.