The Trump administration on Thursday finalized a decision not to impose any limits on perchlorate, a toxic chemical compound found in rocket fuel that contaminates water and has been linked to fetal and infant brain damage.

The move by the Environmental Protection Agency was widely expected, after The New York Times reported last month that Andrew Wheeler, the E.P.A. administrator, had decided to effectively defy a court order that required the agency to establish a safe drinking-water standard for the chemical by the end of June. In addition to not regulating the toxic chemical, the E.P.A. overturned the underlying scientific finding that declared perchlorate a serious health risk for five million to 16 million people in the United States.

Environmentalists said both moves showed a disregard for science, the law and public health, and they criticized the agency for claiming credit for state regulations done in the face of federal inaction.

“Today’s decision is illegal, unscientific and unconscionable,” said Erik D. Olson, the senior strategic director for health at the Natural Resources Defense Council, an advocacy group. “The Environmental Protection Agency is threatening the health of pregnant moms and young children with toxic chemicals in their drinking water at levels that literally can cause loss of I.Q. points. Is this what the Environmental Protection Agency has come to?”

Some people are now wondering if the Republican party needs to grow a population of people with lower I.Q. points in order to assure someone continues to vote them into office.

The battle over perchlorate dates back to the early 2000s, when the George W. Bush administration decided not to regulate it. The Obama administration reversed that decision.

In 2011 the Obama administration issued a finding that perchlorate posed such a serious health risk when discharged into drinking water that it required regulation, setting off a fierce lobbying effort by defense contractors to block restrictions on using the contaminant.

The Trump administration again reversed the decision and additionally overturned the health finding, saying it was “not in the public interest” to regulate the contaminant.