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Environmental Factor, March 2012

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Children exposed to cadmium may be at higher risk for learning disabilities

By Carol Kelly

Robert Wright, M.D.

Wright is especially interested in the synergistic effects from exposure to psychosocial stress and one or more environmental toxicants. (Photo courtesy of Robert Wright)

Children and teens with higher cadmium levels are more likely to have learning disabilities and be placed in special education, according to a new NIEHS-funded study published by a team of researchers led by Robert Wright, M.D., from Harvard University. Research Fellow Timothy Ciesielski, M.D., Sc.D., also from Harvard, was first author on the study.

While the study also investigated a potential linkage with attention deficit hyperactivity disorder (ADHD), no statistically significant association was found. This finding may or may not be indicative of a distinction between neurocognitive and neurobehavioral dysfunction from cadmium exposure. As the authors point out, special education placement tends to be a catch-all that may include ADHD in some places.

The link between cadmium and learning disabilities may be occurring at typical exposure levels, an inference drawn from the use of a nationally representative sample of students 6-15 years old. "If these associations are replicated in other populations, then neurodevelopmental toxicity may be a sensitive endpoint to consider in future cadmium risk assessments," the researchers wrote. 

The study used data from the National Health and Nutrition Examination Survey (NHANES ), a unique study with both interviews and physical examinations, sponsored by the Centers for Disease Control and Prevention. According to the researchers, the NHANES research design has widely recognized strengths in terms of statistical power and generalizability. Additionally, because cadmium accumulates in the kidneys for extended periods, the urinary cadmium concentration data obtained from NHANES indicates long-term exposure of children.

A research focus on neurodevelopmental toxicology and children

Wright, a physician who leads the Harvard School of Public Health Superfund Research Program, conducts extensive research in neurodevelopmental toxicology, particularly genetic and environmental factor interactions that may influence brain development and function. Wright specializes in evaluating children with learning and behavioral disabilities suspected to have been exposed to environmental toxicants.

According to Wright, human movement, awareness, sensations, language, thought, and memory — the essence of humanity — are all controlled by the brain and central nervous system. Because this system is so vital, understanding how chemicals impact neurodevelopment, even at low exposure levels, is a high priority in environmental health. By focusing on real human exposures to known toxicants like cadmium, Wright’s team hopes its research will ultimately help inform public health guidelines to protect child neurodevelopment.

What and where is cadmium?

As the authors explain, cadmium, a heavy metal, occurs naturally in lead, copper, zinc, and other ores. Water sources near existing and former cadmium-emitting industries have shown a marked elevation of cadmium in sediments and aquatic organisms. Except in the vicinity of cadmium-emitting industries or incinerators, the intake of cadmium from drinking water or ambient air is of minor significance for children. The U.S. Environmental Protection Agency requires water suppliers to limit the cadmium concentration in water to a level of less than five micrograms per liter.

Cadmium can be released into the atmosphere through metal production activities, fossil fuel combustion, and waste incineration. Cadmium emissions travel through the atmosphere resulting in long-range deposition, causing varying concentrations of this metal in soil and water.

Crops grown in contaminated water or soil are the main source of cadmium exposure for the general population. Among crops, tobacco leaves naturally accumulate large amounts of cadmium. The greatest potential for above-average exposure to cadmium is from smoking, which may double the cadmium exposure of a typical individual.

Although researchers generally know where cadmium is geologically located, exposure sources are varied and can be difficult to isolate. More research is needed on the particular origins of cadmium exposures, to better understand the health risks.

Citation: Ciesielski T, Weuve J, Bellinger DC, Schwartz J, Lanphear B, Wright RO. 2012. Cadmium Exposure and Neurodevelopmental Outcomes in U.S. Children. Environ Health Perspect; doi: 10.1289/ehp.1104152 [Online 27 January 2012].

(Carol Kelly is a research and communication specialist with MDB, Inc., a contractor for the NIEHS Division of Extramural Research and Training.)

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