Cadmium is a soft, whitish metal that occurs naturally in soil. It's perhaps best known as one half of rechargeable nickel-cadmium batteries, but also is used in pigments, electroplating and plastic.

Lab testing organized by The Associated Press shows that it also is present in children's jewelry — sometimes at eye-popping levels exceeding 90 percent of the item's total weight.

Most people get a microscopic dose of the heavy metal just by breathing and eating. Plants, including tobacco, take up cadmium through their roots and people absorb it during digestion or inhalation. Without direct exposure, however, people usually don't experience its nasty side: cancer, kidneys that leak vital protein, bones that spontaneously snap.

Cadmium is particularly dangerous for children because growing bodies readily absorb substances, and cadmium accumulates in the kidneys for decades.

"Just small amounts of chemicals may radically alter development," said Dr. Robert O. Wright, a professor at Harvard University's medical school and school of public health. "I can't even fathom why anyone would allow for even a small amount to be accessible."

Recent research by Wright found that as cadmium exposure increased, kids were more likely to report learning disabilities.

Dr. Aimin Chen of the University of Cincinnati's medical school also has studied how cadmium affects young brains. While lead is the heavy metal most associated with harming cognitive development, Chen has concluded that cadmium lowers IQ even more than lead — though cadmium isn't harming the average American child because the typical exposure is not as large as lead.

Scientists don't know how much cadmium it takes to kill a child. The only child's death attributed to cadmium that AP found was a nearly 3-year-old boy from Toronto. According to a case study published in 1994, an autopsy showed his brain had swollen; the researchers concluded his exposure came from items around his home such as paint pigments, batteries or cadmium-electroplated utensils.