Relationship between Mercury Concentrations in Hair and Blood in Dene/Métis Communities of the Northwest Territories

Abstract

Background: Mercury (Hg) is a global environmental pollutant as well as a growing concern to public health. Indigenous populations in the Northwest Territories (NWT) may experience elevated Hg exposures due to higher consumption rates of fish compared to the Canadian general populations. Many biomonitoring projects are conducted to estimate human health risk caused by environmental Hg exposure; blood and hair are two of the most frequently used matrices for quantifying individual and population exposures. The association between Hg concentrations in hair and blood is often assumed to be linear with a hair-to-blood Hg concentration ratio of 250. However, this ratio varies among populations, and further research is needed to explore the relationship between Hg concentrations in hair and blood in Indigenous populations of the NWT. Objective: The objectives of this study are to: 1) examine the relationship between Hg concentrations in hair and blood and the reliability of the widely accepted hair-to-blood ratio of 250 within this study population; 2) further improve the relationship by the application of Multiple Imputation (MI) and by the inclusion of covariates in the imputation model; 3) assess the effectiveness of MI for addressing missing data and data below limit of detection (LOD) from biomonitoring studies. Methods: A community-based project was designed based on consultations that began in 2014. This contaminant biomonitoring project provided baseline reference Hg levels for the Sahtu region and the Dehcho region of the Northwest Territories. Blood and hair samples were collected for Hg exposure assessments. Participants were also asked to provide basic demographic information. Simple linear regression models were used to explore the relationship between Hg concentrations in hair and blood with complete cases across communities. MI was applied to impute the missing data and the data below LOD. Results: The association between Hg concentrations in hair and blood were found to be linear within communities. The hair-to-blood Hg concentration ratios were reported to be between 220 and 1146 for different communities from complete cases analysis (CCA). For half of the communities, these ratios from CCA were reported to be 13% to 18% less than those estimated from a simple linear regression (SLR) MI model. In comparison to the MI model based on SLR, multiple linear regression (MLR) MI models that included covariates (age, sex, and, BMI) may better address the missingness, especially for the communities yielding atypical results, e.g. the hair-to-blood ratios for one of the communities decreased from 1138 (estimated from SLR-MI model) to 994 (estimated from MLR-MI model). Conclusions: The hair-to-blood Hg concentration ratios were generally lower when relying solely on complete cases. MI may be an effective tool to achieve less biased results by addressing the incomplete data. With the well-established linear relationships between Hg concentrations in hair and blood across communities, MI can utilize observed Hg-hair values to help recover information of Hg-blood values. Also, MI methods may be able to help extrapolate the blood levels using the current hair-to-blood ratios and segmental hair mercury levels. This approach may shed light on the seasonality of Hg exposure within Dene/Métis Communities of the Northwest Territories, Canada. The results of our study can be used to better inform strategies for biological sampling for future Hg human biomonitoring programs in the Canadian subarctic.