For those with the highest cadmium levels, the odds of having a learning disability were 3.21 times higher than for the children with the lowest exposures. For special education, the odds were 3 times higher. No association was found with attention deficit hyperactivity disorders.
“The three times higher risk is high for such low cadmium levels,” said Aimin Chen, an assistant professor of environmental health at University of Cincinnati’s College of Medicine. He was not associated with the study.
But Chen said the link at this point is preliminary because researchers have not yet looked for any specific, more definitive neurological effects, such as reduced IQs, memories or vocabularies -- outcomes already linked to two other toxic metals, lead and mercury.
The connection to both learning disabilities and special education could indicate cadmium has an array of effects on a child’s brain just like lead does. Those two outcomes “are actually a mixture of different brain function problems,” Chen said.
Lead has been studied and regulated for many decades, leading to a large amount of evidence that it reduces children's IQs at low concentrations and contributes to attention disorders and even violent behaviors. It interferes with the development of synopses, or connections between neurons, that allow a child to learn.
Since cadmium is also a heavy metal, it might have similar effects on the brain, Lanphear and Wright said. But unlike lead, cadmium “is relatively understudied as respect to brain toxicity,” Wright said.
One big difference turned up in the new study: No link was found between cadmium and attention deficit disorders. “It stands out because one thing we’ve found fairly consistently with lead, tobacco and others is that it seems that some of these contaminants might increase the risk for ADHD,” Lanphear said. That could be a sign that cadmium is working on a different part of the brain, not the prefrontal cortex.
Another big mystery is the source of the cadmium in the kids. Cadmium stays in the body for long periods, so the tests measured amounts the children were exposed to over years.
Cadmium is in tobacco smoke, but surprisingly, concentrations in the kids were similar whether they lived with smokers or not. That “might mean for most kids [secondhand] smoking was not a major source,” Wright said.
An abundant element in the Earth’s crust, cadmium is found naturally in soil in some parts of the country. But it also is released by battery manufacturers, smelters, electroplating plants and other industries. It is one of the top chemicals reported in Superfund sites, found in virtually all of them, according to a Centers for Disease Control and Prevention document.
Renee Gardner, a postdoctoral fellow at Sweden's Karolinska Institutet who studies heavy metals, said “the most important source of exposure is food. Green leafy vegetables and grains are the biggest sources, though most plant foods have some cadmium in them.”
Since these foods are important nutritionally, they shouldn’t be avoided. But Gardner said that iron helps prevent absorption of cadmium, so parents worried about exposure should ensure their kids have adequate iron in their foods.
Some children may have been exposed through inexpensive jewelry. In 2010, the Associated Press tested children’s jewelry manufactured in China and found cadmium, prompting recalls by stores. Cadmium was being used to replace lead.
Last fall, the Consumer Product Safety Commission considered standards, but backed off when the industry set its own voluntary testing procedures and limits for cadmium in children’s jewelry. California set its own standards.