Mercury detected in the teeth of medieval lepers reveals how they were treated 900 years ago.

The use of mercury as a medical treatment in the Middle Ages left traces that can today be read in teeth. A team led by Elena Fiorin from the Sapienza University of Rome detected elevated levels of this toxic metal in dental calculus samples taken from individuals buried in ancient leper hospitals. The finding allows for the reconstruction of medieval therapeutic practices and opens a new avenue to study how diseases such as leprosy and syphilis were treated.

The research focused on human remains from two European leprosariums: Peterborough Abbey in England, founded in 1125, and Saint-Thomas-d’Aizier in Normandy, France, built in the late 11th century. During the medieval period, mercury was used in ointments applied to the skin to treat diseases considered severe or stigmatizing. The presence of the metal in the analyzed remains suggests that some of these individuals received medical treatments while alive.

Dental calculus: a new window to study medieval treatments

Until now, studies on mercury exposure in human remains typically focused on bones, teeth, or hair. The novelty of this work is that the team analyzed dental calculus, a mineralized plaque that forms in the mouth during life and can preserve traces of substances incorporated by the body. This material acts as a biological capsule capable of providing complementary information about diet, environment, health, and medical treatments.

Fiorin explained the value of this methodology with a central phrase: “Dental calculus offers a new and complementary perspective.” She then detailed why this record can be so useful: “As it forms in the mouth during life, it can more directly capture substances that enter the body, including medical treatments applied in or around the mouth.” The quote highlights that dental calculus not only preserves food residues but also signals therapeutic practices.

The team also sought to rule out an alternative explanation: that mercury had entered the remains after burial. To do this, the researchers analyzed soil samples taken from the graves and compared the results with remains from individuals buried in cemeteries not associated with leprosariums in England and France. The comparison reinforced the hypothesis that mercury was incorporated during life and not from later contamination.

Medieval leprosariums: high levels of mercury and possible inequality in access to treatment

The results showed a clear difference between those buried in leprosariums and those who were not. “Individuals buried in leprosariums show significantly higher levels of mercury than those from other cemeteries, and our analyses indicate that this mercury was incorporated during life and not from the soil,” noted Fiorin. The researcher added: “Moreover, there is no evidence of local environmental sources, such as mining, that could explain these patterns.”

This point is key to interpreting the finding. If there were no local environmental sources to explain the presence of the metal, the medical hypothesis gains strength. Additionally, the fact that the mercury in dental calculus was higher than that detected in the soil suggests that the metal may have entered the ground from the contaminated bodies themselves, rather than the other way around.

The study also identified differences within the leprosariums themselves. The highest levels of mercury were found in the remains of two individuals buried in a chapel of the leprosarium. This location could indicate that they were individuals of higher status or with access to more medical treatments, although the interpretation requires caution because burial in religious spaces could also respond to other social or spiritual factors.

In other words, the finding suggests a possible medieval health inequality. Not all sick individuals would have received the same amount or intensity of treatment, and those buried in more prestigious areas may have had access to more frequent care. Mercury, then, reveals not only a medical practice but also social differences within institutions dedicated to the isolation and care of sick individuals.

Medicine, toxicity, and archaeology: what mercury reveals about the Middle Ages

The use of mercury in medieval medicine shows how treatments could combine therapeutic intent and toxic risk. In an era without modern knowledge of toxicity, the metal was applied in ointments for diseases such as leprosy and syphilis, despite the fact that it is now known to cause severe harm to the body. Chemical archaeology allows us to verify that these practices were not just recorded in medical texts but left physical traces in the bodies.

The work published in the Journal of Archaeological Science also provides a highly valuable methodological tool. Dental calculus appears as a source of information capable of complementing traditional analyses of bones, teeth, and hair. This opens new possibilities for studying treatments, environmental exposures, and health practices in ancient populations, especially when other tissues do not preserve well.