Mercury in human brain, blood, muscle and toenails in relation to exposure: an autopsy study

Abstract

Background: The main forms of mercury (Hg) exposure in the general population are methylmercury (MeHg) from seafood, inorganic mercury (I-Hg) from food, and mercury vapor (Hg0) from dental amalgam restorations. While the distribution of MeHg in the body is described by a one compartment model, the distribution of I-Hg after exposure to elemental mercury is more complex, and there is no biomarker for I-Hg in the brain. The aim of this study was to elucidate the relationships between on the one hand MeHg and I-Hg in human brain and other tissues, including blood, and on the other Hg exposure via dental amalgam in a fish-eating population. In addition, the use of blood and toenails as biological indicator media for inorganic and organic mercury (MeHg) in the tissues was evaluated.

Methods: Samples of blood, brain (occipital lobe cortex), pituitary, thyroid, abdominal muscle and toenails were collected at autopsy of 30 deceased individuals, age from 47 to 91 years of age. Concentrations of total-Hg and I-Hg in blood and brain cortex were determined by cold vapor atomic fluorescence spectrometry and total-Hg in other tissues by sector field inductively coupled plasma-mass spectrometry (ICP-SFMS).

Results: The median concentrations of MeHg (total-Hg minus I-Hg) and I-Hg in blood were 2.2 and 1.0 microg/L, and in occipital lobe cortex 4 and 5 microg/kg, respectively. There was a significant correlation between MeHg in blood and occipital cortex. Also, total-Hg in toenails correlated with MeHg in both blood and occipital lobe. I-Hg in both blood and occipital cortex, as well as total-Hg in pituitary and thyroid were strongly associated with the number of dental amalgam surfaces at the time of death.

Conclusion: In a fish-eating population, intake of MeHg via the diet has a marked impact on the MeHg concentration in the brain, while exposure to dental amalgam restorations increases the I-Hg concentrations in the brain. Discrimination between mercury species is necessary to evaluate the impact on Hg in the brain of various sources of exposure, in particular, dental amalgam exposure.

Figure 1

Methylmercury in blood related to concentration of methylmercury in brain and total-Hg in toenails. Concentration of methylmercury in blood (C_{MeHg-Blood; μg Hg/L}) related to concentration of (a) methylmercury in occipital lobe cortex (C_{MeHg-Brain; μg Hg/kg}) and (b) concentration of total-Hg in toenails (C_{Hg-Toenail; μg Hg/kg}). The equations for the regression lines were (a) C_MeHg-Brain = 2.0 × C_MeHg-blood + 0.8, (correlation coefficient 0.725, n = 30, p < 0.001) and (b) C_Hg-toenail = 59 × C_MeHg-Blood + 125, (correlation coefficient 0.634, n = 29, p < 0.001).

Figure 2

Concentration of methylmercury in brain related to concentration of total-Hg in toenails. Concentration of methylmercury in occipital lobe cortex (C_{MeHg-Brain; μg Hg/kg}) related to concentration of total-Hg in toenails (C_{Hg-Toenail; μg Hg/kg}). The equation for the regression line was C_MeHg-Brain = 0.017 × C_Hg-toenail + 1.5, and the correlation coefficient was 0.586 (n = 29, p = 0.001).

Figure 3

Amalgam surfaces related to I-Hg in blood and brain, and total-Hg in pituitary and thyroid. Number of amalgam surfaces related to concentration of (a) inorganic Hg in blood (C_{I-Hg-Blood; μg Hg/L}), (b) inorganic Hg in occipital cortex (C_{I-Hg-Brain; μg Hg/kg}), (c) total-Hg in pituitary (C_{Pituitary; μg Hg/kg}) and (d) total-Hg in thyroid (C_{Thyroid; μg Hg/kg}). The equations for the regression lines were (a) C_I-Hg-Blood = 0.12 × [number of amalgam surfaces] + 0.79, (correlation coefficient 0.433, n = 29, p = 0.019), (b) C_I-Hg-Brain = 0.15 × [number of amalgam surfaces] + 4.4, (correlation coefficient 0.550, n = 29, p = 0.002), (c) C_Pituitary = 6.0 × [number of amalgam surfaces] + 25.5, (correlation coefficient 0.541, n = 29, p = 0.002), and (d) C_Thyroid = 0.49 × [number of amalgam surfaces] + 13.7, (correlation coefficient 0.530, n = 27, p = 0.004).

Figure 4

I-Hg in brain related to I-Hg blood. Concentration of inorganic Hg in occipital cortex related to concentration of inorganic Hg blood. The correlation coefficient was not significant (r = -0.026, p = 0.891, n = 30). When the dental nurse (case 28, denoted with in the Figure) was excluded, the correlation coefficient was 0.253 (p = 0.186, n = 29).

Figure 5

I-Hg in brain related to concentration of total-Hg in thyroid. Concentration of inorganic Hg in occipital cortex related to concentration of total-Hg in thyroid gland. The correlation coefficient of the log₁₀ transformed values was 0.769 (n = 28, p < 0.001).

Figure 6

I-Hg in brain related to total-Hg in pituitary. Concentration of inorganic Hg in occipital cortex related to concentration of total-Hg in pituitary. The correlation coefficient of the log₁₀ transformed values was 0.693 (n = 30, p < 0.001).

Figure 7

Determinations of total-Hg by ICP-SFMS related to determinations of total-Hg by CVAFS. Data from determinations of total-Hg by ICP-SFMS related to determinations of total-Hg by CVAFS in duplicate samples of blood (a) and occipital cortex (b). Correlation coefficients were 0.979 (n = 30) and 0.971 (n = 30), respectively.