More Evidence of Mercury Effects in Children

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<< Environmental Health Perspectives Volume 107, Number 11, November 1999

More Evidence of Mercury Effects in Children

In research published last summer, scientists revealed that prenatal

exposure to methylmercury, an organic form of mercury that accumulates in

animal tissues, may affect the blood pressure and ability to respond to

sensory stimuli in exposed children later in life. The team is now also

confirming findings of mercury-related neurodevelopmental effects among the

Faroese subjects by studying a cohort of Madeiran children.

The reports are the latest work of Philippe Grandjean, an adjunct professor

of public health at Boston University in Massachusetts, and Pál Weihe,

medical director of the Faroese Hospital System in Tórshavn, who, with their

colleagues, have performed extensive analysis of a longitudinal study on the

effects of prenatal methylmercury exposure among the inhabitants of the

Faroe Islands. The Faroe Islands study was funded by the NIEHS along with

European grant-making bodies including the European Commission Environment

Research Programme and the Danish Medical Research Council. The Faroese were

chosen as study subjects because their diet is rich in pilot whale meat, a

prime source of methylmercury. In the study of 917 Faroese children,

prenatal exposure to methylmercury was assessed by analyzing mercury

concentrations in cord blood and maternal hair. At age 7, the children

underwent extensive neurobehavioral testing as well as a general health

examination.

A paper published in the July 1999 issue of Epidemiology describes

mercury-related cardiovascular risk factors that were identified among the

Faroese children during the general health exam. Because of earlier case

reports and experimental findings of cardiovascular effects of mercury, the

children were examined for blood pressure, heart rate, and heart rate

variability. As a whole, the children had normal blood pressure for their

age. But among children whose cord blood mercury content had been measured

at 1-10 µg/L, the scientists found that blood pressure was raised by an

average of 14 points. The effect was magnified in children with lower birth

weights, whose blood pressure was raised by as much as 21 points. No

additional increase was seen in children whose cord blood mercury

concentration had been higher than 10 µg/L.

The scientists also found that heart rate variability decreased with

increasing mercury exposures, particularly in boys in the 1-10 µg/L exposure

range. Grandjean explains, " The heart rate must vary in accordance with the

varying needs for oxygen of the peripheral tissues. This variation is

regulated through the autonomic nervous system. A decreased variability is a

sign of abnormality, as the heart then is slower in responding to the body

needs. "

The Epidemiology findings are especially interesting from a public health

perspective because childhood blood pressure has been shown to be an

important predictor for hypertension later in life. The findings also

indicate that prenatal exposure to methylmercury at concentrations below

current exposure limits can cause adverse health effects. The daily intake

reference dose of the U.S. Environmental Protection Agency is 0.1 µg/kg body

weight per day, an intake that Grandjean says would correspond to mercury

concentrations of about 5 µg/L in cord blood and about 1 µg/g in hair.

In a related study (not funded by the NIEHS), that was published in the

July/August 1999 issue of Neurotoxicology and Teratology, Grandjean and

colleagues examined 149 children on the island of Madeira, off the coast of

Morocco. The Madeiran children were exposed to methylmercury when their

mothers ate the deep-sea fish black scabbard while pregnant.

The children's hair mercury concentration at the time of the test was

measured to determine current exposure level. Hair samples were also

collected from the mothers, with the hair of those whose diet hadn't changed

over the past seven years (some 80% of the mothers) serving as an indicator

of methylmercury exposure at the time of pregnancy.

The children then underwent neuropsychological and neurophysiological

testing, including assessment of evoked potentials (electrical signals from

the brain that are evoked by sensory stimuli). The results of prenatal

exposure would logically be linked to the maternal hair mercury

concentrations, which in the Madeiran cohort were found to vary from 1.1 to

54.1 µg/g. The scientists found that certain evoked potentials tended to be

slower in children who had been exposed to higher concentrations of mercury.

According to the report, children of mothers with hair mercury

concentrations higher than 10 µg/g experienced delays of as much as 10% in

auditory and visual latencies. Says Grandjean, " [Evoked potentials]

represent an objective assessment of nervous system function, and they are

relatively independent of confounders. They therefore provide support for

the observations of neuropsychological deficits previously reported. The

clinical implications of these findings, in total, is that children with

increased prenatal exposure to methylmercury are likely to suffer delays in

neurological development. "

In Grandjean's opinion, two primary points emerge from the Faroe Islands and

Madeira findings. " First, " he says, " the evidence is accumulating that

prenatal methylmercury exposure from seafood may cause subtle neurotoxicity

even though current exposure limits are not exceeded. Second, " he continues,

" the effects on brain function should not be looked at in isolation, as the

autonomic nervous system may also be involved, thereby affecting

cardiovascular function. "

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