Boston, MA -- With methylmercury a worldwide contaminant of seafood and freshwater fish and known to produce adverse nervous system effects, especially during brain development, researchers from the Harvard School of Public Health and institutions in Japan, Denmark and the Faroe Islands undertook an assessment of possible brain function impairment in adolescent children due to prenatal exposure to mercury when the mothers’diet was high in seafood. The authors found that high levels of mercury passed from mother to child in utero produced irreversible impairment to specific brain functions in the children. The study was carried out in the Faroe Islands and appears in the February issue of The Journal of Pediatrics.
Mercury exposures among the children in the study were assessed through analyses of cord blood samples at birth and hair samples taken at ages 7 and 14. Some 1,022 mothers and their children from the Faroe Islands participated in the research. The mothers’ hair mercury levels at childbirth in most cases exceeded 1 microgram per gram, the exposure limit recommended by the National Research Council and the U.S. Environmental Protection Agency (EPA). Follow-up testing of the children showed much lower exposure levels. At the most recent follow-up, more than 850 14 year-olds participated in the study.
The Faroe Islands are located in the North Atlantic Ocean between Norway and Iceland. The islands’ economy is centered on the fishing industry and fish processing. The diet of the inhabitants includes high intake of seafood and whale meat.
To assess the impact of the exposure to mercury, brainstem auditory evoked potentials (BAEP) were recorded using surface electrodes placed on the skull. At two different sound frequencies, the researchers measured the transmission of electrical signals in the brain from the acoustic nerve, via the pons (connecting the medulla oblongata to the thalamus) to the midbrain. The latency of the electrical transmission from the acoustic nerve to the pons was significantly increased at higher intrauterine exposure to mercury. This observation was found to be true both at 7 years and at 14 years, suggesting that this effect is lasting.
Prolonged latencies of the electrical signals to the midbrain among the 14 year-olds was linked to the current mercury exposure and therefore suggested that postnatal mercury exposure may damage brain functions that are different from those that are sensitive to mercury during fetal development. Although hearing loss has been observed in severe mercury poisoning cases in adults, the mercury levels among the children in the study was not associated with detectable hearing loss. The authors did not find a link between simultaneous prenatal exposure to PCBs and electrical signal latencies.
A second paper in The Journal of Pediatrics by the same authors reports that the neurological changes are also linked to decreased nervous system control of the heart function. At higher mercury exposures, the children were less capable of maintaining the normal variability of the heart rate necessary to secure proper oxygen supply to the body.
“We found that both prenatal and postnatal mercury exposure affects brain functions and that they seem to affect different targets in the brain. The fact that the current exposure has an additional effect, despite the low mercury concentrations is worrisome, especially for communities where seafood constitutes an important part of the diet,” said Philippe Grandjean, senior author of the study and adjunct professor in the Department of Environmental Health at the Harvard School of Public Health. He added, “The current focus on protecting pregnant women against this neurotoxin should be expanded to cover children and adolescents as well. Seafood is an important part of a healthy diet, and consumers should choose species low in the food chain caught in waters without mercury pollution.” He added, “In this country we need to set uniform mercury exposure levels. The children in the study had average exposure levels that are similar to the current limit used by the U.S. Environmental Protection Agency, and 95 percent of them were below the current limit used by the Food and Drug Administration.”
The research was supported by grants from the National Institute of Environmental Health Sciences, the Danish Medical Research Council and the Nissan Science Foundation.
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