Concerns that the prescription was having ill effects on the emperors health and sanity were waved off by the imperial alchemists, who cited medical texts listing a number of the emperors conditions (including itching, formication, swelling, and muscle weakness), today recognized as signs and symptoms of mercury poisoning, as evidence that the elixir was effectively treating the emperors latent ailments. Xuānzong became irritable and paranoid, and he seems to have ultimately died in 859 from the poisoning. The phrase mad as a hatter is likely a reference to mercury poisoning among milliners (so-called "mad hatter disease"), as mercury-based compounds were once used in the manufacture of felt hats in the 18th and 19th century. (The Mad Hatter character of Alice in Wonderland was, it is presumed, inspired by an eccentric furniture dealer named Theophilus Carter. Carter was  not a victim of mad hatter disease although Lewis Carroll would have been familiar with the phenomenon of dementia that occurred among hatters.) In 1810, two British ships, HMS Triumph and HMS Phipps, salvaged a large load of elemental mercury from a wrecked Spanish vessel near Cadiz, Spain. The bladders containing the mercury soon ruptured. The element spread about the ships in liquid and vapor forms. The sailors presented with neurologic compromises: tremor, paralysis, and excessive salivation as well as tooth loss, skin problems, and pulmonary complaints. In 1823 William Burnet, MD published a report on the effects of mercurial vapor. Triumphs surgeon, Henry Plowman, had concluded that the ailments had arisen from inhaling the mercurialized atmosphere.
In certain communities that survive on fishing, rates of mercury poisoning among children have been as high as 1.7 per 100. Signs and symptoms
Common symptoms of mercury poisoning include peripheral neuropathy, presenting as paresthesia or itching, burning, pain, or even a sensation that resembles small insects crawling on or under the skin (formication); skin discoloration (pink cheeks, fingertips and toes); swelling; and desquamation (shedding or peeling of skin).Mercury irreversibly inhibits selenium-dependent enzymes (see below) and may also inactivate S-adenosyl-methionine, which is necessary for catecholamine catabolism by catechol-O-methyl transferase. Due to the bodys inability to degrade catecholamines (e.g. adrenaline), a person with mercury poisoning may experience profuse sweating, tachycardia (persistently faster-than-normal heart beat), increased salivation, and hypertension (high blood pressure).Affected children may show red cheeks, nose and lips, loss of hair, teeth, and nails, transient rashes, hypotonia (muscle weakness), and increased sensitivity to light. Other symptoms may include kidney dysfunction (e.g. Fanconi syndrome) or neuropsychiatric symptoms such as emotional lability, memory impairment, or insomnia.Thus, the clinical presentation may resemble pheochromocytoma or Kawasaki disease. Desquamation (skin peeling) can occur with severe mercury poisoning acquired by handling elemental mercury. Causes
Consumption of fish containing mercury is by far the most significant source of ingestion-related mercury exposure in humans, although plants and livestock also contain mercury due to bioconcentration of organic mercury from seawater, freshwater, marine and lacustrine sediments, soils, and atmosphere, and due to biomagnification by ingesting other mercury-containing organisms.