The study released Tuesday by the Environmental Working Group comes as state and federal regulators consider setting new standards to regulate perchlorate — the explosive ingredient in missile fuel that has been linked to thyroid damage.
"Perchlorate exposure is more widespread than we have been led to believe," said Bill Walker, vice president for the West Coast office of the EWG, a research and advocacy organization based in Washington, D.C.
The EWG did not call for Californians to stop drinking milk or giving it to their children, but said it does advocate tougher standards for perchlorate.
Perchlorate has been found in drinking water in more than 20 states, including California, which has extensive ties to the military, defense industry and the space program. The chemical has been detected in the Colorado River, the major source of drinking water and irrigation in Southern California and Arizona.
Researchers are divided about the effects of perchlorate on mental development and what exposure levels are safe.
In March, California health officials concluded that perchlorate could be dangerous at levels above 6 parts per billion in drinking water — a level that could be used later this year to set the nation's first state standard.
But U.S. Environmental Protection Agency officials, and some environmental groups, say that standard would be too weak. The EPA advocates a standard of just 1 part per billion.
The new study on milk was based on laboratory tests the EWG commissioned as well as unreleased tests by the California Department of Food and Agriculture.
The EWG tests, conducted by researchers at Texas Tech University, found the chemical in 31 of 32 samples from milk purchased at grocery stores in Los Angeles and Orange counties. The average level of the chemical was 1.3 parts per billion,
The EWG said the Food and Agriculture Department tests found an average level of 5.8 parts per billion of perchlorate in 34 samples it tested from milk silos in Alameda, Sacramento and San Joaquin counties.
Department officials confirmed those results, but spokesman Steve Lyle said the findings didn't show any need for consumers to drink less milk.
"At this point, there is not enough information to suggest that eating foods with low levels of perchlorate poses a significant health concern," Lyle said.
The EWG study didn't determine how the chemical ended up in cow's milk, but perchlorate has been found in many of the state's water sources, which are used to irrigate farmland and grow crops fed to cows.
California's dairy industry will work with state and federal officials to find out how perchlorate is getting into milk and how to remove the chemical, said Michael Marsh, CEO of the Western United Dairymen, which represents the state's $4.5 billion dairy industry. But Marsh said there is a "paucity of science" showing perchlorate's harmful effects on human health.
A recent study by the University of California, Irvine, found that healthy adults were not harmed by levels as high as 100 parts per billion of perchlorate. But the study did not draw conclusions about perchlorate's impact on pregnant women, children and infants.