Origin of The Expression: "To Be Mad as a Hatter"

Hat makers have been associated with madness since the early 19th century. At the time, felt hats were the fashion all over Europe, and the processes by which hatters processed cheap animal fur into felt substance made much use of mercury, a substance that causes brain damage after prolonged exposure.
Working in poorly ventilated workshops and breathing in mercury fumes, took its toll on the hat-making business, which lost many men to brain damage and madness. To this day, the symptoms of mercury poisoning, which include trembling, loss of teeth, loss of coordination, slurred speech, loss of memory and depression, are still known as "mad hatter syndrome".