Thallium is a metal found in the Earth’s crust and can be obtained by smelting metal compounds. Today, most thallium is used in the production of electronics, especially semiconductors. It is also used for medical imaging and in the production of glass. Thallium contamination of the surrounding environment most commonly results from the smelting process, but it can also happen during transport or improper disposal. Once in the environment, it remains in the air, water, and soil without breaking down. It can enter the food chain because it is absorbed by plants and builds up in fish.

Eating food contaminated with thallium is the most likely way people in the United States would be exposed to it. Ingesting high levels of thallium over a short period of time can lead to symptoms such as vomiting, diarrhea, and hair loss. It can impair function of the brain, lungs, heart, liver, kidneys, and even lead to death. Little is known about the health effects of ingesting low levels of thallium over a long period of time. People who work in facilities that use thallium or live near waste sites containing thallium can also be exposed by breathing contaminated air or touching contaminated material. Workers exposed to thallium over many years have had nervous system impairments, including numbness in the extremities. Studies on laboratory animals have shown that exposure to high levels of thallium can cause reproductive and developmental defects, but it is not known if this also occurs in people.

Historically, thallium was a common ingredient in rat poisons and insecticides sold in the United States. Recognizing that it is highly toxic, the government banned its use in these consumer products in 1972. In fact, thallium is considered so dangerous that it is no longer produced in the United States. Many other countries also ban or restrict the production of thallium. While these are positive developments to keep people safe, at lease 210 Superfund sites are known to contain thallium, meaning it still poses a danger to people’s health today.