A Victorian Millinery Catalogue from 1896. Mad hatter disease, or mad hatter syndrome, is a commonly used name for hatmakers whose felting work involved prolonged exposure to mercury vapours. By the turn of the 20th century, mercury poisoning among British hatters had become quite rare.
“Mad as a hatter” - In and century England, mercury was used in making felt for hats. Workers in hat factories were exposed daily to trace amounts of the metal, which eventually caused mercury poisoning in some, resulting in dementia.