Mad as a hatter has a grim source: erethism, a neurological disorder caused by chronic mercury exposure in the felt hat trade. Hatters brushed animal fur with orange mercuric nitrate, a process called carroting, then shaped and shrank it over boiling vats in steamy, poorly ventilated rooms that released mercury vapor. Inhaled, the vapor slips past the blood-brain barrier. Victims grew pathologically shy, anxious, irritable and paranoid, while losing teeth, hair and nails and developing a violent intention tremor known as the hatter’s shakes or the Danbury shakes, after the Connecticut hat center. The method traced to 17th century French Huguenots, who carried it to England.

In New Jersey hat towns such as Newark, Orange and Bloomfield, Dr. J. Addison Freeman described the disease in 1860 and pleaded simply for ventilation. An 1878 inspection of 25 Newark firms found at least a quarter of hatters sick, yet the inspector blamed drink. France acted in 1898, but in 1934 the US Public Health Service estimated 80 percent of American felt makers had mercurial tremors. The US banned mercury in hatting only in 1941, when the military needed it for detonators, and the industry switched to hydrogen peroxide.

• How the disease’s own symptoms left workers too fearful to challenge employers
• Employers on record saying ventilation cost money they did not want to spend
• Hatter’s consumption: tuberculosis drawing health officials’ attention away from mercury
• Modern mercury exposure in chloralkali plants and small-scale gold mining, and urine biomonitoring
• Johnny Depp’s orange-tinted Mad Hatter and Theophilus Carter, Lewis Carroll’s likely model