Alice in Wonderland’s Mad Hatter did a pretty good job of demonstrating the saying “mad as a hatter,” but where did this phrase come from? While its origins do center around hats, they are a little less delightful than an eccentric who throws lots of tea parties, instead involving urine, syphilis, and the element mercury.

“Mad as a hatter” is a colloquial English phrase thrown about lightheartedly to describe a person whose behavior is peculiar, but erethism – the neurological disorder also known as “mad hatter syndrome” – is no joke.

What is mad hatter syndrome?

The element mercury has many uses for humans, including gilding (S-Town listeners may remember this), purifying gold, and – you guessed it – hat making. However, as a potent neurotoxin, it also presents many hazards to those who work with it.

Inhalational mercury poisoning was first reported in medicine before Lewis Carrol penned "Hatta" in Through the Looking-Glass, but its connection to hat making wouldn't be established until sometime later. However, as doctors' understanding of mercury poisoning grew, they became more familiar with the tell-tale symptoms of the neurological disorder erethismus mercurialis, which would later become known as mad hatter disease or mad hatter syndrome. They include:

• Headaches
• Low self-confidence
• Personality changes
• Tremors
• Delirium

While Carroll was well-read in matters of medicine, as detailed in a 1984 BMJ correspondence from TML Price titled “Did the Mad Hatter have mercury poisoning?”, it seems unlikely that he knew about the risk of mercury poisoning for hatters. Instead, the author suggests, it’s possible he may have had first-hand experience of a real-life worker with mad hatter syndrome.

“Erethism and hatters' shakes are surely in evidence when [Hatta] appears as a witness at the trial of the Knave of Hearts,” wrote Price. “He shakes so much that both his shoes fall off, and he certainly shows excessive timidity, diffidence, loss of confidence, and anxiety, to such an extent that he bites a piece out of his cup instead of the bread and butter.”

“With his extraordinary ability to observe and portray the oddities of human behavior and his interest in clinical matters I would expect him to describe with great accuracy the type of madness found in these unfortunate people, which he seems to me to have done.”

Why did hat makers get mad hatter syndrome?

Mercury was a key ingredient in the art of felt hat making as it could toughen animal fur fibers, allowing them to matt together more robustly for a firmer hat. Felt hat makers would use mercury nitrate – Hg(NO?)? – for the process, known as secretage or carroting, and it was used in this way for around a century.

It seems the felt hat making industry had a history of opting for unusual substances in their craft, as when the trend reached France, makers ditched camel urine – something that was commonplace among hatters elsewhere – in favor of their own piss. Hat Realm reports that among them, one wee-wielding hat maker appeared to churn out finer felt than his competitors, and it was later revealed he was taking a mercury compound to treat syphilis.

The discovery led to hatmakers cutting out the middleman (quite literally), forgoing wee in favor of mercury nitrate. This orange substance – which gave the process its name carroting – wasn’t too harmful until it was time to shape the hat’s felt using steam.

Hat makers steaming their mercury-nitrate-infused felt hats would then breathe in vapors of the compound, exposing them to mercury that accumulated across their careers. Early signs of the mad hatter syndrome they would later develop included losing teeth, trembling and mood swings, but could extend to hallucinations, delusions, and paranoia.

Danbury, a prolific hat making city in Connecticut in the 19th century, experienced so many cases that mercury-induced tremors became known as “the Danbury shakes,” reports History. As the damaging long-term effects of using mercury in this way became more apparent, carroting with mercury nitrate was eventually banned.

A rather somber tale, then, for Carroll’s endearing Hatta, but at least Alice was able to provide some memorable words of reassurance.

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