What You Need to Know About Mercury Exposure and Child Development Outcomes

I. What is Mercury

Mercury is a ubiquitous environmental toxicant. It exists in three forms: the metallic element, inorganic salts, and organic compounds (methyl mercury, ethyl mercury, and phenyl mercury). Although volcanoes and other natural sources release some elemental mercury to the environment, anthropogenic emissions from coal-fired electric power generation facilities, chloralkali production, waste incineration, and other industrial activities now account for approximately 70% of the 5,500 metric tons of mercury that are released into the earth’s atmosphere each year.

The toxicity of methyl mercury to the developing brain was first recognized in the 1950s in Minamata, Japan, where consumption of fish with high concentrations of methyl mercury by pregnant women resulted in at least 30 cases of cerebral palsy in children; exposed women were affected minimally if at all. A similar episode followed in 1972 in Iraq when the use of a methyl mercury fungicide led to poisoning in thousands of people; again, infants and children were most profoundly affected.

Throughout the 1990s, the U.S. Environmental Protection Agency (EPA) made steady progress in reducing mercury emissions from anthropogenic sources. However, the U.S. EPA recently proposed to slow this progress, citing high costs of pollution abatement.

Since January 2003, the issue of early life exposure to methyl mercury has become the topic of intense debate after the U.S. Environmental Protection Agency (EPA) announced a proposal to reverse strict controls on emissions of mercury from coal-fired power plants, the “Clear Skies Act” However, the U.S. EPA’s technical analyses in support of “Clear Skies” failed to incorporate or quantify consideration of the health impacts resulting from increased mercury emissions. After legislative momentum for this proposal faded, the U.S. EPA proposed an almost identical Utility Mercury Reductions Rule, which again failed to examine impacts on health. The U.S. EPA issued a final rule on 15 March 2005.

II. Specific Chemical Threat

A. Economic Impact:

Between 316,588 and 637,233 children each year have cord blood mercury levels > 5.8 μg/L, a level associated with loss of IQ. The resulting loss of intelligence causes diminished economic productivity that persists over the entire lifetime of these children. This lost productivity amounts to $8.7 billion annually. Of this total, $1.3 billion each year is attributable to mercury emissions from American power plants. 

This significant toll threatens the economic health and security of the United States. Fetal blood mercury levels > 5.8 μg/L are associated with small but significant loss of IQ. This decrement in IQ appears to be permanent and irreversible, and it adversely affects a significant portion of the annual birth cohort’s economic productivity over a lifetime. Using the base-case assumptions, calculated costs for the 405,881 children who suffer IQ decrements resulting from fetal methyl mercury exposure was estimated. Under these assumptions, 89,293 children suffered a 0.76 decrement in IQ and another 113,647 experienced a 1.15 IQ point decrement.

The 5% most highly exposed children in the 2000 birth cohort suffered subclinical losses in IQ. Although the diminution in intelligence is small in comparison with the loss of cognition that can result from other genetic and environmental processes, the loss resulting from methyl mercury exposure produces a significant reduction in economic productivity over a lifetime. The estimated aggregate cost of the loss in IQ that results from exposure of American children to methyl mercury of anthropogenic origin is $8.7 billion annually.

The estimated cost of anthropogenic methyl mercury exposure ranges from $2.2 billion to $13.9 billion. Using the linear dose–response model that was selected by the National Research Council in recommending a reference dose for mercury exposure it was found that the environmentally attributable cost of methyl mercury exposure is $32.9 billion. 

After applying the 36% fraction to restrict the analysis to American anthropogenic sources, it was estimated that the attributable cost of methyl mercury exposure to the developing fetus from American anthropogenic sources is $3.1 billion annually. The sensitivity analysis suggested that the true cost of methyl mercury exposure from American emissions ranges from $0.4 to $15.8 billion annually. 

To focus specifically on the costs of fetal exposure to mercury released by American coal-fired power plants, the impact of the 41% of U.S. anthropogenic emissions of mercury attributable to these facilities was examined and it was estimated that the attributable cost of methyl mercury exposure from American electric generation facilities to the developing fetus is $1.3 billion. Applying the sensitivity analysis in the model, it was found that the true cost of methyl mercury exposure from electric generation facilities to the American birth cohort ranges from $0.1 to $6.5 billion/year. Again, the major source of these costs is loss of earnings over a lifetime.

The major findings in this analysis are:

a) that exposure to methyl mercury emitted to the atmosphere by American electric generation facilities causes lifelong loss of intelligence in hundreds of thousands of American babies born each year, and

b) that this loss of intelligence exacts a significant economic cost to American society, a cost that amounts to at least hundreds of millions of dollars each year.

           

Moreover, these costs will recur each year with each new birth cohort as long as mercury emissions are not controlled. By contrast, the cost of installing stack filters to control atmospheric mercury emissions is a one-time expense. The high costs of in utero exposure to methyl mercury are due principally to the lifelong consequences of irreversible injury to the developing brain. Similar lifelong neurobehavioral consequences have been observed after exposure of the developing brain to other environmental toxicants, including lead, polychlorinated biphenyls and ethanol.

The true economic costs of methyl mercury toxicity to the developing brain did not include the cost of exposures to the 231,352 children born to women in 2000 with blood mercury concentrations between 3.5 and 4.84 μg/L. The analysis also omits the cost of the cardiovascular impacts of mercury exposure or the costs of mercury exposure to children in the first 2 years of postnatal life. These aspects of methyl mercury toxicity in the range of estimates were not included because there is no sufficient quantitative data to permit construction of a reliable model.

B. Mercury exposure and child development outcomes

Mercury is ubiquitous in the global environment, ensuring universal exposure. Some forms of mercury are especially neurotoxic, including clinical signs at high doses. However, typical human exposures occur at low to moderate doses. The vulnerability of the developing brain to methyl mercury reflects the ability of lipophilic methyl mercury to cross the placenta and concentrates in the central nervous system. Moreover, the blood–brain barrier is not fully developed until after the first year of life, and methyl mercury can cross this incomplete barrier.

The US Environmental Protection Agency proposed a reference dose (an estimate of a daily dose that is likely to be without a risk of adverse effects over a lifetime) for methyl mercury that is substantially lower than previous guidelines from the World Health Organization, the US Agency for Toxic Substances and Disease Registry, and the US Food and Drug Administration. Some questions have been raised about the Environmental Protection Agency's guidelines, but the issue remains unresolved. 

Meanwhile, consumer groups have raised questions about the potential link between mercury exposure and autism spectrum disorders as well as other adverse neurodevelopmental outcomes.