Formaldehyde, the chemical of choice for undertakers and embalmers, is also used in products like furniture and clothes. But it can also cause cancer and severe respiratory problems. So, in 2021, the Environmental Protection Agency began a new effort to regulate it.

The chemicals industry fought back with an intensity that astonished even seasoned agency officials. Its campaign was led by Lynn Dekleva, then a lobbyist at the American Chemistry Council, an industry group that spends millions of dollars on government lobbying.

Dr. Dekleva is now at the E.P.A. in a crucial job: She runs an office that has the authority to approve new chemicals for use. Earlier she spent 32 years at Dupont, the chemical maker, before joining the E.P.A. in the first Trump administration.

Her most recent employer, the chemicals lobbying group, has made reversing the Environmental Protection Agency’s course on formaldehyde a priority and is pushing to abolish a program under which the agency assess the risks of chemicals to human health. In recent weeks it has urged the agency to discard its work on formaldehyde entirely and start from scratch in assessing the risks.

The American Chemistry Council is also seeking to change the agency’s approval process for new chemicals and speed up E.P.A.’s safety reviews. That review process is a key part of Dr. Dekelva’s purview at the agency.

Another former chemistry council lobbyist, Nancy Beck, is back alongside Dr. Dekleva at the E.P.A. in a role regulating existing chemicals. The council’s president, Chris Jahn, told a Senate hearing shortly after the Trump inauguration that his group intended to tackle the “unnecessary regulation” of chemicals in the United States. “A healthy nation, a secure nation, an economically vibrant nation relies on chemistry,” he said.