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Why did some California American Indians become ill after using cinnabar in war paint?

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A periodic table element box for mercury shows its symbol Hg, atomic number 80, atomic weight 200.592, electron configuration, and name. Icons indicate mercury is a transition metal, liquid at room temperature, has a rhombohedral crystal structure, and is weakly basic. — mercury Properties of mercury.

Some California American Indians experienced illness after using cinnabar in war paint because cinnabar is mercury sulfide (HgS), a compound that contains mercury.1

While cinnabar itself is a mineral, mercury poisoning occurs when mercury compounds interact with body tissues and functions. Chronic exposure to mercury—including through absorption through the skin—can lead to serious health issues, such as tremors, numbness, inflammation of the mouth, and various neurological or systemic symptoms. When used as a pigment on the skin, the mercury present in the cinnabar could be absorbed into the body, leading to toxic effects.2