Hat-making in the 18th and 19th centuries was a hazardous business, because it involved the use of many chemicals, one of which was the toxic substance mercury. Working in poorly ventilated rooms, hat-makers breathed in so much mercury fumes that a good number of them were driven out of their wits by mercury-induced brain damage. Mercury poisoning among hat-makers is widely believed to be the origin of the proverbial saying “mad as a hatter”. Even the character of the Hatter in Lewis Carroll's iconic Alice's Adventures in Wonderland exhibited psychotic behavior similar to a person suffering from mercury poisoning.
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John Tenniel's illustration of the tea party scene in “Alice's Adventures in Wonderland”. Accompanying Alice at the table is March Hare, the Dormouse and the Hatter.
Source: amusingplanet.com