"The deficit of thyroid hormone is particularly delicate if it's at the beginning of life because the central nervous system has not completely matured," said study author Dr. Nancy Carrasco, a professor of molecular pharmacology at Albert Einstein College of Medicine, in New York City.

Thyroid hormones are "absolutely critical" for the development and maturation of the central nervous system, skeletal muscles and lungs, she explained.

In laboratory and rat research, Carrasco's team found that perchlorate limited the amount of iodide transported to a mother's mammary glands. The only source of iodide a baby typically has is mother's milk, she explained.

Her team discovered that perchlorate accumulates in mother's milk, but before this study, "we didn't know it would be passed on as actively to the baby," she said.

Carrasco and her colleagues at Einstein and at Johns Hopkins University reached this conclusion after experimental studies on how sodium iodide carries perchlorate to, and concentrates it in, mammary glands.

The next steps in this research will include animal studies looking at the effects of perchlorate exposure during pregnancy, she said.

The debate continues on how much perchlorate is a high and harmful concentration, Carrasco said. But scientists have long known that iodide deficiency contributes to lowered IQ.

The new finding is relevant to the Environmental Protection Agency's standards for acceptable perchlorate levels, added R. Thomas Zoeller, a professor of biology at the University of Massachusetts-Amherst who has served on the EPA's peer review panels for the assessment of perchlorate.

At the time the current safety standards were established, the EPA was not thinking about how perchlorate is concentrated in breast milk, he said.

Zoeller said the study's discovery of how perchlorate is transported to breast milk is important to setting safety standards because perchlorate has a half-life of about eight hours and doesn't accumulate in the body. But because of the new findings, "we no longer have to debate whether perchlorate is being concentrated in milk," he added. "We have enough data to know that this is a very dangerous thing."

Large studies need to be done to confirm the findings, Zoeller added.

It's now "enormously important to find out if perchlorate in [breast] milk is affecting thyroid hormones in infants," he said. Such a study would be difficult to conduct because it would involve drawing blood from 1- and 2-week old infants, Zoeller said.

Tyrone Hayes, a professor of integrative biology at the University of California at Berkeley, said the discovery of a mechanism by which perchlorate can be transmitted to nursing infants is important.

"I think probably the most obvious significance is that we have a very common contaminant in the environment that has a profound negative impact, and that the most profound impact is on humans that don't have a choice at a critical development stage that can impact the rest of their lives," he said.

More information

The Environmental Protection Agency has more on perchlorate.