According to a study published on Oct. 13 in the International Journal of Osteoarchaeology, the oldest evidence of mercury poisoning was discovered in 5,000-year-old human bones recovered from pits and caves in Spain and Portugal.

Human bones were obtained from 23 different archaeological sites and studied by experts. Researchers believe that the overuse of cinnabar causes mercury poisoning.

The bones came from 370 skeletons of people who lived at various times throughout 5,000 years.

After testing various bones, particularly the femur and the humeral bone between the shoulder and the elbow, abnormally high quantities of mercury were discovered.

“Although cinnabar (HgS) is a likely cause of mercury poisoning and toxicity for people exposed to this mineral from mining or use as a paint or pigment, not all sites investigated here had cinnabar associated with the burials or other excavated areas.” according to the study obtained by Wiley Online Library.

“The use and abuse of cinnabar appears to have been pervasive throughout the above-mentioned period, and particularly between 2900–2300 B.C.,” the October study found.

According to the research, the overuse of cinnabar was explained by the substance’s symbolic and likely religious value, which was sought after, traded, and widely employed in various ceremonies and social customs.

Researchers found mercury levels as high as 400 parts per million in some of the remains, far higher than the WHO’s acceptable threshold of 1 or 2 ppm in human hair, as LiveScience reported.

The researchers said that the exceptionally high levels of mercury could be due to exposure to cinnabar, a poisonous mercury sulfide mineral. When reduced to a fine powder, Cinnabar is a brilliant red pigment that has long been used to make paint pigments.

According to the statement, humanity began mining Almadén’s cinnabar resources around 7,000 years ago, during the Neolithic period.

By this time, cinnabar “was sought after, traded and extensively used in a variety of rituals and social practices,” the authors stated in their October study.

The study revealed that some people probably breathed or swallowed vast amounts of mercury-laden cinnabar for ceremonial purposes, either mistakenly or intentionally.

Mercury is one of the top ten substances of “major health concern,” according to the World Health Organization (WHO).

The WHO added that people can be exposed to low quantities of mercury today if they eat certain types of fish or shellfish.

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