New Study Links Methyl Mercury with Drop in IQ in American Children

By January 21, 2012Got Mercury?

Three common environmental chemicals– lead, organophosphate pesticides and methylmercury– may have effects on children’s IQ in the overall population to the impacts of major medical conditions such as preterm birth or ADHD – two of the most prevalent in U.S. children. The finding from this reanalysis of published data hints that the societal toll of exposures to these invisible yet widespread contaminants – lead, organophosphate pesticides and methyl mercury – may be more severe than what previous studies of individual risk would suggest.

Lead had the most severe calculated impact, with an estimated drop in IQ of 23 million points in the population. In contrast, the predicted loss due to preterm birth was 13 million points. Any drop in IQ is considered detrimental to people’s success and society’s needs.

In one of the first studies of its kind, the author analyzed published data from 25.5 million children and compared estimates of population-wide IQ loss from exposures to those of several well-known childhood medical conditions. This new approach to categorizing population risk could help researchers identify the most harmful exposures and evaluate the success of intervention programs.


Exposure to chemicals in the environment can significantly affect a child’s developing brain and nervous system. The exposures can lower IQ, alter behavior and influence social relationships.

Lead, organophosphates and methylmercury are three contaminants known to harm a growing child’s nervous system. Lead exposure occurs mainly through paint in older buidlings and lead-contaminated soil and dust. Organophosphate pesticides are used on crops and in homes, so children can eat or breathe the chemicals. In the United States, methylmercury exposure is mostly from eating fish.

All three chemical types can be passed from pregnant women to the fetus and nursing children. Exposures can be prevented or minimized in communities through education, clean ups and other programs.

Prior studies mostly examine one chemical at a time and often only in a few hundred children. Therefore, the larger public health impact from exposure to a collection of environmental chemicals is not known in the general population.

Medical conditions can also affect thinking and learning abilities. For instance, major health problems, including birth defects, preterm birth, ADHD, autism and brain injuries also affect cognitive and social challenges in school, family and life.

Although environmental exposures and medical conditions can both harm children’s intelligence, no studies have quantified the public health impact on children’s IQ at a populational level or compared the potential harm from the environment versus medical conditions.

Comparing the potential impact on children’s intelligence from these sources would help determine if preventing exposures would reduce cognitive deficits in U.S. children.

What did they do?

To quantify the impact on children’s IQ at a societal – not an individual – level, the researcher analyzed and compared published data from studies that evaluated the impact of environmental exposures and medical conditions on children’s intelligence. He generated loss estimates based on 25.5 million children under age 5 in the United States.

He examined birth defects, preterm birth, ADHD, autism, brain injuries, and other common children’s health problems. On the environment side, he investigated lead, organophosphates and methylmercury. All have sufficient data to produce a reliable population-based estimate on effect size. The effect size is a measure of how the children’s IQ will change with increases in environmental exposure or by having a medical condition.

He limited the analysis to IQ because it can be reliably measured even before school age and is commonly measured and reported in published studies.

He estimated the IQ loss in children based on the concentrations of environmental exposures in the general population and on the prevalence of medical conditions in the population. He compared the magnitude of IQ loss by each exposure or condition.

What does it mean?

The potential public health impacts of environmental exposures on children’s nervous systems is significant, according to this study that compares the estimated drops in IQ related to exposures with those related to medical conditions. According to the study, lead and organophosphates together would have more impact than preterm birth or ADHD, two disorders that have considerable societal burden.

The results are important because they are the first to estimate in a population the extent of cognitive losses from environmental exposures. Previous studies have evaluated IQ losses in individuals, which can vary widely in children. Examining IQ effects in populations gives a better indication of impacts on society.

Environmental exposures can be prevented and efforts to reduce toxic chemicals have been made. But the study’s findings suggest they don’t go far enough. Further reductions would help avoid even more public health effects associated with a downward shift of IQ.

Even small reductions in a person’s IQ can increase the need for extra help in school or at work. At a societal level, declining IQ scores results in overall lower intellectual ability, competitiveness, career achievement and increased costs in school and health care.

Even though the analysis focuses on chemical exposures, the approach could apply to non-chemical impacts – such as radition or noise – that may also impact children’s intelligence. Prevention of both chemical and non-chemical threats to child development should be considered when trying to reduce the effects on cognitive function.

This study gathered information on IQ. Other effects  – including social adaptation, behavior and executive functions (memory, reasoning, attention) – from environmental exposures are also possible. Preventing children’s exposure to the toxic chemicals identified here may also address these effects as well.

The study suggests parents – and even pre-pregnant women – should be aware of common environmental exposures and how to prevent or reduce them. The author recommends that researchers work to identify historical, existing and new toxic chemicals that could be hazardous to a child’s developing nervous system.

Source   Bellinger, DC. 2012. A strategy for comparing the contributions of environmental chemical and other risk factors to children’s neurodevelopment.