Associations of methylmercury and inorganic mercury between human cord blood and maternal blood: A meta-analysis and its application

• The cord:maternal blood ratio for methylmerucry was obtained by meta-analyses.
• The cord:maternal blood ratio for inorganic mercury was obtained by meta-analyses.
• A new model to estimate the percentage of methylmercury in blood was introduced.

Considering the different ability of placental transfer, an assessment of the cord:maternal blood ratio for both methylmercury (MeHg) and inorganic mercury (IHg) is needed especially for interpreting the low-level prenatal exposure. In this study, we conducted a Monte Carlo-based meta-analysis to comprehensively estimate that ratio for MeHg (RMeHg) and IHg (RIHg). The obtained values followed log-normal distributions, with a mean (standard deviation) of 1.89 (0.98) and 1.01 (0.55) for RMeHg and RIHg, respectively. We also estimated the percentage of MeHg in the blood by means of THg in cord and maternal blood using the RMeHg and RIHg, and obtained a value very close to the measured one (relative deviation, −0.4%). In conclusion, the fetus is exposed to approximately twice as much MeHg and to the same level of IHg as in maternal blood; the introduced model provides a rough but reasonable estimate of the percentage of MeHg in the blood.