The Trump administration will not impose any limits on perchlorate, a toxic chemical compound that contaminates water and has been linked to fetal and infant brain damage.
The decision by Andrew Wheeler, the administrator of the E.P.A., appears to defy a court order that required the agency to establish a safe drinking-water standard for the chemical by the end of June.
The policy, which acknowledges that exposure to high levels of perchlorate can cause I.Q. damage but opts nevertheless not to limit it, could also set a precedent for the regulation of other chemicals, people familiar with the matter said.
The chemical — which is used in rocket fuel, among other applications — has been under study for more than a decade, but because contamination is widespread, regulations have been difficult.
In 2011, the Obama administration announced that it planned to regulate perchlorate for the first time, reversing a decision by the George W. Bush administration not to control it.
But the Defense Department and military contractors such as Lockheed Martin and Northrop Grumman have waged aggressive efforts to block controls, and the fight has dragged on.
According to the staff members, who asked not to be identified because they were not authorized to speak about agency decisions, the E.P.A. intends in the coming days to send a federal register notice to the White House for review that will declare it is “not in the public interest” to regulate the chemical.
Andrea Woods, a spokeswoman for the E.P.A., said in a statement that the agency had not yet made a final decision on perchlorate.
Perchlorate can occur naturally, but high concentrations have been found in at least 26 states, often near military installations where it has been used as an additive in rocket fuel, making propellants more reliable.
Research has shown that by interfering with the thyroid gland’s iodine uptake, perchlorate can stunt the production of hormones essential to the development of fetuses, infants and children.
The new policy will revoke the 2011 E.P.A. finding that perchlorate presents serious health risks to between 5 million and 16 million people and should be regulated.
To justify doing so, the Trump administration will cite more recent analyses claiming concentrations of the chemical in water must be at higher levels than previously thought in order to be considered unsafe.
In addition, because states like California and Massachusetts regulated the chemical in the absence of federal action, the E.P.A. will say few public water systems now contain perchlorate at high levels, so the costs of nationwide monitoring would outweigh the benefits, the people who have viewed the rule said.
The decision is the latest in a string of Trump administration regulatory actions that weaken toxic chemical regulations, often against the advice of E.P.A.’s own experts, in ways favored by the chemical industry.
Last year the administration announced it would not ban chlorpyrifos, a widely used pesticide that its own experts linked to serious health problems in children.
It also opted to restrict, rather than ban, asbestos, a known carcinogen, despite urging by E.P.A. scientists and lawyers to ban it outright like most other industrialized nations.
“This is all of a piece,” said Rena Steinzor, a law professor at the University of Maryland. “You can draw a line between denial of science on climate change, denial of science on coronavirus, and denial of science in the drinking water context. It’s all the same issue. They’re saying ‘We don’t care what the research says.’”
Even an exposure of 18 micrograms per liter, slightly above the current federal recommendation, would amount to an average I.Q. loss of one point.
Critics of the policy said the E.P.A. was implicitly accepting that those health outcomes are not considered adverse health effects, and that the decision could affect the future regulation of other chemicals.
“Not only is E.P.A. acting in defiance of a court order and the law, it’s setting a terrible precedent by ignoring much of the science and allowing such a high level of perchlorate in tap water that it acknowledges is associated with an average 2-point I.Q. loss in exposed kids,” said Erik Olson, senior strategic director of health and food at the Natural Resources Defense Council.