Non-linear hair-to-blood mercury partitioning in a chronically exposed Amazonian population

Human biomonitoring of mercury (Hg) exposure commonly relies on blood and hair measurements, often assuming a stable conversion ratio between these matrices. However, this assumption neglects dynamic toxicokinetic processes governing Hg distribution and elimination. Here, we characterized the distribution, determinants, and toxicokinetic implications of the hair-to-blood total Hg ratio in 453 individuals from Amazonian riverine communities chronically exposed to methylmercury (MeHg). Total Hg (THg) was determined in blood and hair, mercury speciation in hair by LC-ICP-MS, and selenium (Se) in blood was measured by ICP-MS. Model-based estimates of MeHg intake and Hg excretion were derived, and linear, non-linear, and multivariable analyses were applied. Blood and hair THg were strongly correlated (R = 0.86, p < 0.0001), confirming hair as a robust biomarker of chronic exposure. However, the hair-to-blood ratio showed high inter-individual variability (median 303; range 73.6–2132) and a significant inverse association with blood THg (β = −0.394, p < 0.001), indicating non-linear partitioning. Blood Se levels were elevated and positively associated with the ratio (β = 1.081, p < 0.001). Estimated MeHg intake exceeded the USEPA reference dose by over tenfold. These findings demonstrate that the hair-to-blood Hg ratio is a dynamic biomarker influenced by exposure intensity and potentially by selenium status.