Mercury treatment in late medieval European leprosaria? New data from human dental calculus

This study investigates the potential use of mercury-based treatments for leprosy during the late medieval period through the analysis of mercury concentrations in human dental calculus. Calculus samples were obtained from individuals buried at St Leonard's (Peterborough, England) and St Thomas's (Aizier, France) leprosaria, with comparative samples collected from non-leprosaria burial sites where individuals showed no osteological evidence of leprosy. Mercury levels were measured in soil (N = 45) from the burial grounds using atomic absorption spectrometry and in dental calculus (N = 76) using cold vapour fluorescence adsorption spectroscopy. The findings reveal significant variability in mercury concentrations within and between sites, with the dental calculus values ranging from 0.11 to 9.7 mg kg−1. Notably, individuals from leprosaria cemeteries exhibited significantly higher mercury concentrations in their calculus than those from non-leprosaria cemeteries (t(70) = 3.22, p = 0.002). Although mercury levels in the burial soils associated with the individuals sampled in leprosaria cemeteries were elevated compared to the natural geochemical background, the analysis suggests that contamination is probably due to mercury accumulated in soft tissues during life, which could substantially increase the surrounding soil's mercury content during decomposition. Thus, contamination from the surrounding soil was ruled out as the source of mercury in the dental calculus. Integrated with historical and archaeological evidence, these findings may suggest the intentional use of mercury-based treatments for leprosy. This study highlights the role of bioarchaeology in uncovering medieval therapeutic practices and provides the first evidence of mercury linked to medical treatment detected in dental calculus.