Infants and children who drink milk from cows raised in some parts of California may be exposed to unsafe levels of perchlorate, a toxic chemical found in rocket fuel, according to a new study by an environmental group.

The study released Tuesday by the Environmental Working Group is the first to look for perchlorate in California supermarket milk. The group found perchlorate in almost every sample tested - 31 out of 32 samples purchased from grocery stores in Los Angeles and Orange counties. “The average level of perchlorate in the samples was 1.3 parts per billion (ppb) - just above the EPA’s currently recommended safe dose of 1 ppb,” EWG said in a statement.

Perchlorate can affect the thyroid gland’s ability to make essential hormones. For fetuses, infants and children, disruptions in thyroid hormone levels can cause lowered IQ, mental retardation, loss of hearing and speech, and motor skill deficits, EWG said.

“Our findings are not a call for California mothers to stop drinking milk or stop giving it to their children,” said Bill Walker, EWG Vice President/West Coast. “They do show that the state must set a drinking water standard that fully protects public health. Mothers should not be forced to wonder if milk is affecting their child’s growth and development.”

Currently there are no enforceable perchlorate safety standards at the state or federal level.

(via Environmental Working Group)