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The oldest evidence of mercury poisoning in humans was discovered in 5,000-year-old bones.

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The high mercury levels were most likely produced by exposure to cinnabar, which is used to make a bright red paint pigment.

According to a new study, the earliest evidence of mercury poisoning has been discovered in 5,000-year-old human bones buried in Spain and Portugal.

According to the World Health Organization, naturally occurring heavy metals can have harmful effects on the body, including the neurological, digestive, and immunological systems (WHO). As a result, the WHO ranks mercury as one of the top ten substances of "serious health concern."

According to the WHO, humans are most usually exposed to some degree of mercury when they eat certain fish or shellfish. However, the levels are often modest. But how widespread was mercury poisoning in the past?

To find this out, the researchers examined human bones from 23 distinct archaeological sites in Spain and Portugal, including pits and caves. The bones belonged to 370 people who lived at diverse periods throughout 5,000 years.

Analysis of some of the bones, particularly the humerus, the arm bone between the shoulder and the elbow, found extremely high amounts of mercury, levels that could not have been created by food or post-mortem decomposition.

According to a release, the researchers identified mercury levels of up to 400 parts per million in some of the remains, which is far higher than the 1 or 2 ppm that the WHO considers normal levels in human hair.

According to the researchers, the extraordinarily high amounts of mercury were most likely produced by exposure to cinnabar, a poisonous mercury sulpsulphidehide mineral that, when ground into a fine powder, has a vivid red hue and has historically been used to manufacture paint pigments. In fact, Almadén, Spain, is home to one of the world's largest cinnabar mines.

According to the statement, people began utilizing Almadén's cinnabar trove in the Neolithic period, some 7,000 years ago. The highest quantities of mercury were found in remains dating from 2900 B.C. to 2300 B.C., or from the late Neolithic to the middle Copper Age (which was a transition between the Neolithic and Bronze Age)

Cinnabar had become a highly symbolic, likely sacred substance at this time, "which was sought after, traded, and extensively used in a range of rituals and social practices," the scientists said in the study, which was published Oct. 13 in the International Journal of Osteoarcheology.

The cinnabar powder was used to paint rooms, embellish sculptures, and even spread over the deceased in tombs from this time period discovered in southern Portugal and Andalusia. According to the statement, it's possible that people breathed or swallowed huge amounts of mercury-filled cinnabar inadvertently or on purpose for ritualistic purposes.

According to the study, by the end of the Copper Age and the beginning of the Bronze Age, or around 2200 B.C., the use of cinnabar had declined dramatically.