Erethism is characterized by behavioral changes such as irritability, low self-confidence, depression, apathy, shyness[2][3] and timidity, and in some extreme cases with prolonged exposure to mercury vapors, by delirium, personality changes and memory loss.

[11] Historically, this was common among old England felt-hatmakers who had long-term exposure to vapors from the mercury they used to stabilize the wool in a process called felting, where hair was cut from a pelt of an animal such as a rabbit.

With continuing exposure, a fine tremor develops, initially involving the hands and later spreading to the eyelids, lips, and tongue, causing violent muscular spasms in the most severe cases.

Long-term, low-level exposure has been found to be associated with less pronounced symptoms of erethism, characterized by fatigue, irritability, loss of memory, vivid dreams, and depression (WHO, 1976).

Effects of chronic occupational exposure to mercury, such as that commonly experienced by affected hatters, include mental confusion, emotional disturbances, and muscular weakness.

[14] After chronic exposure to the mercury vapours, hatters tended to develop characteristic psychological traits, such as pathological shyness and marked irritability (see box).

[23] 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.

[27][28] 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 1878, an inspection of 25 firms around Newark conducted by Dr L. Dennis on behalf of the Essex County Medical Society revealed "mercurial disease" in 25% of 1,589 hatters.

Although Dennis did recommend the use of fans in the workplace he attributed most of the hatters' health problems to excessive alcohol use (thus using the stigma of drunkenness in a mainly immigrant workforce to justify the unsanitary working conditions provided by employers).

It does not seem to have occurred to them that all the efforts to keep up wages... [are] largely offset by the impairment of their health, due to neglect of proper hygienic regulations of their workshops... And when the fact of the workmen in the sizing room, who stand in water, was mentioned, and the simple and inexpensive means by which it could be largely avoided was spoken of, the reply was that it would cost money and hat manufacturers did not care to expend money for such purposes, if they could avoid it.

Some voluntary reductions in mercury exposure were implemented after Lawrence T. Fell, a former journeyman hatter from Orange who had become a successful manufacturer, was appointed Inspector of Factories in 1883.

This deadly communicable disease was rife in the extremely unhygienic wet and steamy enclosed spaces in which the hatters were expected to work (in its annual report for 1889, the New Jersey Bureau of Labor and Industries expressed incredulity at the conditions—see box).

Consequently, public health campaigns to prevent tuberculosis spreading from the hatters into the wider community tended to eclipse the issue of mercury poisoning.

For instance, in 1886 J. W. Stickler, working on behalf of the New Jersey Board of Health, promoted prevention of tuberculosis among hatters, but deemed mercurialism "uncommon", despite having reported tremors in 15–50% of the workers he had surveyed.

[27][35] While hatters seemed to regard the shakes as an inevitable price to pay for their work rather than a readily preventable disease, their employers professed ignorance of the problem.

Instead, it seems to have been the need for mercury in the war effort that eventually brought to an end the use of mercuric nitrate in U.S. hatmaking; in a meeting convened by the U.S. Public Health Service in 1941, the manufacturers voluntarily agreed to adopt a readily available alternative process using hydrogen peroxide.

Lewis Carroll's iconic Mad Hatter character in Alice's Adventures in Wonderland displays markedly eccentric behavior, which includes taking a bite out of a teacup.

[38] Carroll would have been familiar with the phenomenon of dementia among hatters, but the literary character is thought to be directly inspired by Theophilus Carter, an eccentric furniture dealer who did not show signs of mercury poisoning.