In medieval Europe , mercury was used in medicine and manufacturing. Later, hatmakers commonly cured felt using a form of mercury called mercurous nitrate. As the hatmakers inhaled mercury vapors over time, many experienced neurological symptoms of mercury poisoning

What made the Mad Hatter go mad?

The origin of the phrase, it’s believed, is that hatters really did go mad. The chemicals used in hat-making included mercurous nitrate, used in curing felt. Prolonged exposure to the mercury vapors caused mercury poisoning.

Is the Mad Hatter insane?

Colloquially used to describe an eccentric person, “mad as a hatter” is based on a problem that arose in the 1800s when hat companies used lead in the hat-making process. The lead got into their systems and they went insane, hence the term “mad as a hatter”.