The complexity of hair/blood mercury concentration ratios and its implications

The World Health Organization (WHO) recommends a mercury (Hg) hair-to-blood ratio of 250 for the conversion of Hg hair levels to those in whole blood. This encouraged the selection of hair as the preferred analyte because it minimizes collection, storage, and transportation issues. In spite of these advantages, there is concern about inherent uncertainties in the use of this ratio.

To evaluate the appropriateness of the WHO ratio, we investigated total hair and total blood Hg concentrations in 1333 individuals from 9 First Nations (Aboriginal) communities in northern Québec, Canada.

We grouped participants by sex, age, and community and performed a 3-factor (M)ANOVA for total Hg in hair (0-2 cm), total Hg in blood, and their ratio. In addition, we calculated the percent error associated with the use of the WHO ratio in predicting blood Hg concentrations from hair Hg. For group comparisons, Estimated Marginal Means (EMMS) were calculated following ANOVA.

At the community level, the error in blood Hg estimated from hair Hg ranged -25% to +24%. Systematic underestimation (-8.4%) occurred for females and overestimation for males (+5.8%). At the individual level, the corresponding error range was -98.7% to 1040%, with observed hair-to-blood ratios spanning 3 to 2845.

The application of the ratio endorsed by the WHO would be unreliable for determining individual follow-up. We propose that Hg exposure be assessed by blood measurements when there are human health concerns, and that the singular use of hair and the hair-to-blood concentration conversion be discouraged in establishing individual risk.