Mercury is a heavy metal that’s found in the earth’s crust. It’s toxic to people, so even low levels of exposure can cause health problems.

Prolonged exposure can lead to chronic mercury poisoning. This may be called mad hatter disease or mad hatter syndrome.

Mad hatter disease causes severe neurological effects known as erethism. It can lead to symptoms like headaches, behavioral changes, and weakness.

Mad hatter disease is uncommon. Chronic mercury poisoning is more likely to affect people who are exposed to mercury at work. Young children and people who eat a lot of fish also have a higher risk.

To learn more about mad hatter disease, read on. We’ll explore the history behind its name, the symptoms, and treatment options.

During the 18th to 20th centuries, hat makers used mercury to stiffen felt for hats. They used a type of mercury called mercuric nitrate and worked in poorly ventilated rooms.

Over time, the hatters inhaled mercury vapors. Many developed symptoms of chronic mercury poisoning, including psychosis, excitability, and tremors. These symptoms became so common in hatters that the phrase “mad as a hatter” was born.

In the United States, mercury was used in hat making until 1941.

The symptoms of mercury poisoning depend on your level of mercury exposure and the types of mercury a person is exposed to. Mad hatter disease is marked by symptoms of prolonged exposure.

Early symptoms of mercury poisoning include:

Later symptoms include more severe symptoms, such as:

Today, mad hatter disease and chronic mercury poisoning is uncommon in the United States. In 2013, just 24 out of the 1,300 mercury cases in the country caused moderate to major problems.

This is due to efforts to reduce human exposure, like removing mercury from the hat making process.

Mad hatter disease is caused by prolonged mercury exposure. The exact method of exposure varies by the form of mercury:

  • Elemental mercury. Elemental mercury vapors may be inhaled in workplaces like dental offices, smelting sites, and mining operations. Thermometers and fluorescent lights also contain elemental mercury.
  • Inorganic mercury. Inorganic mercury is used in medicines, skin creams, and products as preservatives. Exposure can happen if the mercury is inhaled, consumed, or applied on the skin.
  • Organic mercury. People are commonly exposed to organic mercury by eating fish and shellfish containing methylmercury.

Hatters were specifically exposed to mercuric nitrate, a form of inorganic mercury. However, the term “mad hatter disease” may be used to describe neurological symptoms due to chronic mercury poisoning in general.

There are certain factors that increase your risk of getting chronic mercury poisoning. This includes:

Young age

Fetuses and children have a higher risk of mercury exposure.

A fetus in the womb can be exposed to mercury if the mother eats fish containing mercury. Because the fetus is still growing, it’s more likely to develop neurological effects from mercury.

Children are more likely to experience mercury exposure through inhaled vapors. This is due to their smaller lung capacity. It’s also common for children to play on floors, where mercury spills might occur.

Workplace exposure

Some work environments contain mercury. People who work in these settings are more likely to develop poisoning over time.

This includes environments like:

  • dental offices
  • smelting sites
  • mining facilities
  • fishing operations

High fish intake

Eating fish is the most common way people are exposed to methylmercury. Consuming a lot of fish increases your risk of developing poisoning.

The risk is higher for larger fish, which contain higher amounts of methylmercury. This includes:

  • bigeye tuna
  • swordfish
  • king mackerel
  • shark
  • tilefish
  • marlin

Eating the above fish is not recommended for:

Treatment includes stopping mercury exposure, along with:

  • oxygen
  • bronchodilators
  • fluids

The aim of treatment is to reduce the concentration of mercury in the body.

Your doctor might also give you medicine to increase mercury excretion through the urine or gastrointestinal tract. The best option depends on the type of mercury involved.

It’s possible to reverse chronic mercury poisoning. The condition will resolve once the mercury is eliminated from the body.

In general, with proper treatment, most people recover from mercury poisoning. Their specific outlook depends on their level of mercury exposure.

If a person doesn’t receive treatment, they may experience:

  • kidney problems
  • respiratory failure
  • permanent lung damage
  • hypoxia (tissues do not receive enough oxygen)
  • death

The sooner treatment begins, the better the outlook.

If you think you’ve been exposed to mercury at home, speak with a doctor. This includes exposure to broken items that have mercury, such as thermometers.

You should also talk to a doctor if you work in an environment that contains mercury.

Seek medical help if you notice the early signs of mercury poisoning, including:

Mad hatter disease is a form of chronic mercury poisoning. Depending on the level of exposure, it can cause symptoms like vomiting, skin rashes, tremors, twitching, and excitability.

The condition is called “mad hatter disease” because it commonly affected hat makers in the 18th to 20th centuries. They used mercury in the hat making process and developed mercury poisoning. Fortunately, hatters stopped using mercury in 1941.

If you think you’ve been exposed to mercury, speak with a doctor immediately. Mercury poisoning is reversible with proper treatment. This includes oxygen, bronchodilators, fluids, and medicine to help your body excrete mercury.