The compounds can form PFAS, also known as “forever chemicals,” which have been linked to cancer and birth defects. The E.P.A. approvals came despite the agency’s own concerns about toxicity

For much of the past decade, oil companies engaged in drilling and fracking have been allowed to pump into the ground chemicals that, over time, can break down into toxic substances known as PFAS — a class of long-lasting compounds known to pose a threat to people and wildlife — according to internal documents from the Environmental Protection Agency.

The E.P.A. in 2011 approved the use of these chemicals, used to ease the flow of oil from the ground, despite the agency’s own grave concerns about their toxicity, according to the documents, which were reviewed by The New York Times. The E.P.A.’s approval of the three chemicals wasn’t previously publicly known.