Evidence status: PLAUSIBLE

Lewis Carroll gave the world its most famous eccentric hat-maker, but he did not invent the expression “mad as a hatter”.

Real hatters really did suffer neurological illness caused by mercury.

Whether that occupational disease actually created the phrase is a slightly harder question.

The Short Answer

Hat-makers once used mercury compounds in the manufacture of felt. Chronic mercury exposure could cause tremors, speech problems, irritability and other neurological and psychiatric symptoms. The occupational hazard was real.

“Mad as a hatter”, however, was already in print decades before Lewis Carroll published Alice's Adventures in Wonderland in 1865.

Mercury poisoning offers a very plausible explanation for the expression, but language historians have not established a conclusive documentary link proving that poisoned hatters directly created it.

What We Know

Hat-making could be dangerous work.

Felt manufacture historically involved mercury compounds, including mercuric nitrate. In poorly ventilated workshops, repeated exposure could damage workers' nervous systems.

NIOSH describes symptoms among hatters including tremors, slurred speech, irritability, shyness and depression.

So the underlying phenomenon behind the story is not folklore.

Hatters really did become ill from mercury exposure.

The expression itself is also older than Carroll.

An occurrence has been identified in an Irish newspaper from 1827, while Blackwood's Edinburgh Magazine used “mad as a hatter” in 1829. It appears again in Thomas Chandler Haliburton's writing during the 1830s.

Carroll's Hatter therefore arrived after the expression was already established.

How It Became a Habit

Alice's Adventures in Wonderland enormously strengthened the cultural association between hatters and madness.

Interestingly, Carroll never actually calls his character the Mad Hatter as a formal name.

He is simply the Hatter, although the Cheshire Cat tells Alice that both the Hatter and the March Hare are mad.

Popular culture did the rest.

Eventually the character and the pre-existing expression became so intertwined that many people assume one created the other.

The Story Everyone Tells

The standard explanation runs:

Hatters used mercury.

Mercury poisoned them.

They behaved strangely.

People therefore began saying someone was “mad as a hatter”.

It is logical, memorable and supported by a genuine occupational disease.

What the Evidence Actually Says

This is where Strange by Habit has to resist going one step further than the documents permit.

The mercury poisoning is DOCUMENTED.

The phrase's existence before Lewis Carroll is DOCUMENTED.

But the claim that mercury poisoning specifically caused the expression is PLAUSIBLE rather than proved.

Phrase researchers have proposed other possibilities, including older dialect meanings connected with words resembling hatter, and the very earliest uses do not explicitly explain the phrase by reference to poisoned craftsmen.

So saying “mercury made hatters ill” is factual.

Saying “therefore we know exactly why the idiom was invented” is not.

Different Elsewhere

The strange history of mercury exposure was not limited to Britain.

Hat-making centres elsewhere experienced the same occupational hazard. In Danbury, Connecticut — once an important American hat-making centre — mercury-related tremors became notorious enough to acquire the nickname “Danbury shakes”.

This helps explain why the connection between hatters and neurological illness would have seemed entirely believable to nineteenth- and early-twentieth-century audiences.

Why It Stuck

Lewis Carroll probably deserves much of the credit for making the image immortal.

Once generations of children had met an eccentric tea-drinking Hatter in Wonderland, “mad as a hatter” no longer needed an explanation.

The character had become the explanation — even though the expression came first.

Related Curiosities

Sources & Further Reading

  • U.S. National Institute for Occupational Safety and Health — mercury exposure and neurological illness among hatters.
  • Phrase Finder — 1829 usage and the uncertainty surrounding the exact etymology.
  • Word Histories — an earlier 1827 Irish newspaper example.
  • Medical review of historical occupational mercury exposure.

Last checked: 19 September 2026