Total and inorganic mercury in breast milk and blood in relation to fish consumption and amalgam fillings in lactating women

Total mercury concentrations (mean plus or minus standard deviation) in breast milk, blood, and hair samples collected 6 wk after delivery from 30 women who lived in the north of Sweden were 0.6 plus or minus 0.4 ng/g (3.0 plus or minus 2.0 nmol/kg), 2.3 plus or minus 1.0 ng/g (11.5 plus or minus 5.0 nmol/kg), and 0.28 plus or minus 0.16 mu g/g (1.40 plus or minus 0.80 mu mol/kg), respectively. In milk, an average of 51% of total mercury was in the form of inorganic mercury, whereas in blood an average of only 26% was present in the inorganic form. Total and inorganic mercury levels in blood (r = .55, p = .003; and r = .46, p = .016; respectively) and milk (r = .47, p = .01; and r = .45, p = .018; respectively) were correlated with the number of amalgam fillings.

The concentrations of total mercury and organic mercury (calculated by subtraction of inorganic mercury from total mercury) in blood (r = .59, p = .0006; and r = .56, p = .001; respectively) and total mercury in hair (r = .52, p = .006) were correlated with the estimated recent exposure to methylmercury via intake of fish. There was no significant between the milk levels of mercury in any chemical form and the estimated methylmercury intake. A significant correlation was found between levels of total mercury in blood and in milk (r = .66, p = .0001), with milk levels being an average of 27% of the blood levels. There was an association between inorganic mercury in blood and milk (r = .96, p < .0001); the average level of inorganic mercury in milk was 55% of the level of inorganic mercury in blood. No significant correlations were found between the levels of any form of mercury in milk and the levels of organic mercury in blood.

The results indicated that there was an efficient transfer of inorganic mercury from blood to milk and that, in this population, mercury from amalgam fillings was the main source of mercury in milk. Exposure of the infant to mercury from breast milk was calculated to range up to 0.3 mu g/kg – d, of which approximately one-half was inorganic mercury. This exposure, however, corresponds to approximately one-half the tolerable daily intake for adults recommended by the World Health Organization. We concluded that efforts should be made to decrease mercury burden in fertile women.