mad hatter disease

"During the 18th to 20th centuries, hat makers used mercury to stiffen felt for hats. They used a type of mercury called mercuric nitrate and worked in poorly ventilated rooms. Over time, the hatters inhaled mercury vapors. Many developed symptoms of chronic mercury poisoning, including psychosis, excitability, and tremors. These symptoms became so common in hatters that the phrase “mad as a hatter” was born."