Why is the Mad Hatter mad?

Johnny Depp as a Mad Hatter 

The character of Lewis Carroll - the embodiment of illness, which is referred to as the "Mad Hatter's disease” or mercurialism. Both terms refer to the poisoning by mercury vapor after prolonged exposure. A large number of cases of this occupational disease was first recorded in the 19th century and it was noted in people working in the workshops making hats. Mercuric nitrate was used to soften the hair in the process of making felt. Poorly ventilated rooms lead people to excessively inhaling fumes that adversely affects the nervous system. Symptoms of poisoning (hallucinations, impaired vision and hearing, emotional instability, tremor of limbs, etc.) were perceived as madness and insanity. In England at that time the saying became common "as mad as a hatter" - "mad as a hatter."

A prototype of the Mad Hatter, was likely an eccentric carpenter and furniture dealer Theophilus Carter, who was famous for his invention - the bed-alarm clock, hurling people into the tub with cold water, as well as for the manner of wearing the cylinder-hat shifted to the back.

Via MintTea