A rocket fuel chemical called perchlorate was found in samples of powdered baby formula tested by U.S. Centers for Disease Control and Prevention scientists, but it's not clear how much of a health threat it poses to infants.

The largest amounts of perchlorate were found in baby formula derived from cow's milk, said the researchers, who wouldn't disclose the brands of formula they analyzed, the Associated Press reported.

Scientists have said significant amounts of perchlorate can affect the function of the thyroid, which helps set the body's metabolism. Fetal and infant brain development can be affected by thyroid problems.

But the level of risk posed by the trace amounts of perchlorate in baby formula is difficult to assess, the AP reported. Formula sold in the United States must contain iodine, which counteracts perchlorate's effects. Other factors that influence risk include the infant's size and how much formula they consume.

Potential health risks weren't examined in the CDC study, which was published last month. The Environmental Working Group issued a press release Thursday to draw public attention to the study.