page_title: Erethism
section_title: History among hatters
hierarchical_section_title: Erethism / History among hatters
caption_attribution_description: English: From Charles Knight's Pictorial Gallery of the Arts, England, 1858. Fur industry, hat-making, Canadian voyageurs. The manufacturing of beaver-hair top hats from Canada-produced pelts.
context_page_description: Erethism, also known as erethism mercurialis, mad hatter disease, or mad hatter syndrome, is a neurological disorder which affects the whole central nervous system, as well as a symptom complex, derived from mercury poisoning. Erethism is characterized by behavioral changes such as irritability, low self-confidence, depression, apathy, shyness and timidity, and in some extreme cases with prolonged exposure to mercury vapors, by delirium, personality changes and memory loss. People with erethism often have difficulty with social interactions. Associated physical problems may include a decrease in physical strength, "headaches, general pain, and tremors after exposure to metallic mercury" as well as an irregular heartbeat.
  Mercury is an element that is found worldwide in soil, rocks, and water. People who get erethism are often exposed to mercury through their jobs. Higher risk jobs include construction, industrial work, and working in factories. Some elemental and chemical forms of mercury are more toxic than other forms. The human fetus and medically compromised people are the most susceptible to the toxic effects of mercury.
context_section_description: Especially in the 19th century, inorganic mercury in the form of mercuric nitrate was commonly used in the production of felt for hats.  During a process called carroting, in which furs from small animals such as rabbits, hares or beavers were separated from their skins and matted together, an orange-colored solution containing mercuric nitrate was used as a smoothing agent. The resulting felt was then repeatedly shaped into large cones, shrunk in boiling water and dried. In treated felts, a slow reaction released volatile free mercury. Hatters (or milliners) who came into contact with vapours from the impregnated felt often worked in confined areas.
  Use of mercury in hatmaking is thought to have been adopted by the Huguenots in 17th-century France, at a time when the dangers of mercury exposure were already known. This process was initially kept a trade secret in France, where hatmaking rapidly became a hazardous occupation.  At the end of the 17th century the Huguenots carried the secret to England, following the revocation of the Edict of Nantes. During the Victorian era the  hatters' malaise became proverbial, as reflected in popular expressions like "mad as a hatter" (see below) and "the hatters' shakes".
  The first description of symptoms of mercury poisoning among hatters appears to have been made in St Petersburg, Russia, in 1829. In the United States, a thorough occupational description of mercury poisoning among New Jersey hatters was published locally by Addison Freeman in 1860. Adolph Kussmaul's definitive clinical description of mercury poisoning published in 1861 contained only passing references to hatmakers, including a case originally reported in 1845 of a 15-year-old Parisian girl, the severity of whose tremors following two years of carroting prompted opium treatment. In Britain, the toxicologist Alfred Swaine Taylor reported the disease in a hatmaker in 1864.
  In 1869, the French Academy of Medicine demonstrated the health hazards posed to hatmakers. Alternatives to mercury use in hatmaking became available by 1874. In the United States, a hydrochloride-based process was patented in 1888 to obviate the use of mercury, but was ignored.
  In 1898, legislation was passed in France to protect hatmakers from the risks of mercury exposure. By the turn of the 20th century, mercury poisoning among British hatters had become a rarity.
  In the United States, the mercury-based process continued to be adopted until as late as 1941, when it was abandoned mainly due to the wartime need for the heavy metal in the manufacture of detonators. Thus, for much of the 20th century mercury poisoning remained common in the U.S. hatmaking industries, including those located in Danbury, Connecticut (giving rise to the expression the "Danbury shakes").
  Another 20th-century cohort of affected hatmakers has been studied in Tuscany, Italy.
caption: Some of the steps in the manufacture of felt hats are illustrated in this image from 1858.
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