A toxic chemical used in rocket fuel was found in virtually every sample taken in a new study of nursing mothers' milk, but researchers said it is too early to know whether the perchlorate (search) levels are dangerous.

The multistate study by Texas Tech University researchers, published this week, found that perchlorate levels in breast milk (search) samples were on average five times higher than those detected in dairy milk pulled from grocery stores.

Perchlorate has been linked to thyroid ailments (search), and is considered particularly dangerous to children. It has been found in drinking water supplies in 35 states and also in vegetables. While the chemical occurs naturally, the National Academy of Sciences has said most of the contamination is from its use in rocket fuels, fireworks and explosives.

Contamination is especially widespread in California because of the many current and former defense and space program sites in the state.

According to public health advocates, perchlorate is in the water that supplies more than 16 million Californians. It has also been found in the Colorado River, the major source of drinking water and irrigation in Southern California and Arizona.

California Sens. Dianne Feinstein and Barbara Boxer said the study underscored concerns about the chemical. Boxer sent a letter to state and federal health officials asking them to determine whether mothers should have their breast milk tested before breast-feeding.

"We've got to come to grips with the perchlorate situation quickly," Feinstein said in a statement. "And EPA has to move quickly to set a national drinking water standard that protects the health and safety of all Americans."

However, the milk study shouldn't raise "undue alarm" because the seriousness of its findings is unclear, said Ed Urbansky, a former Environmental Protection Agency chemist who has published several papers on perchlorate. He was not involved with the study.

"It's very difficult to determine what the findings might be other than to know it might be in so many milk samples," he said. "It's important not to raise undue alarm over the significance of the finding.

"We shouldn't be running through the streets screaming and not drinking milk because of this."

For the study, conducted over a two-year period, researchers obtained milk from more than 20 women selected at random and from stores in 23 states. It was funded out of researchers' pockets and published online Tuesday in the journal Environmental Science and Technology.

The average reading in the study was 10.5 parts per billion, less than half of the EPA's newly established safe exposure level of 24.5 parts per billion in drinking water.

The highest reading among the mothers in the Tech study was 92 parts per billion. In dairy milk, all but one of 47 samples had detectable levels of the chemical. No samples were above 11 parts per billion.

Pernendu Dasgupta, a Tech chemistry professor who led the study, said it "raises more questions than answers" but hopes it helps people become more aware.