For decades, a small program in the Environmental Protection Agency conducted the painstaking scientific work of assessing the toxicity of chemicals.

The calculations done by scientists at IRIS, as it was commonly known, underpin vast numbers of chemical regulations, permits and other environmental rules in the U.S. and abroad.

Now the Trump administration is suggesting that their library of more than 500 chemical assessments can’t be trusted, opening the door to weakening hundreds of efforts to protect people from harmful chemicals at the state and federal level. The second-guessing could extend even to long-settled standards, environmental scientists said, such as how much arsenic is allowed in drinking water and how much lead is acceptable in paint and soil.

In an internal memo obtained by ProPublica, David Fotouhi, the deputy administrator of the agency, sharply criticized IRIS this week and directed EPA offices that have used any of the chemical assessments the program has produced to review them. He also advised “external entities” that have used the IRIS assessments to consider undertaking similar reviews and cautioned against using them in future regulations.

The six-page memo said the EPA would be adding “disclaimer language” to the website of the program — the Integrated Risk information System — stating that its toxicity findings are not necessarily meant to be used in regulation.

“This creates the opportunity for companies that pollute to push back on rules and regulations they don’t like,” said Robert Sussman, an attorney who has worked for chemical companies and environmental groups as well as the EPA. “Anybody who wants to ignore a regulation, permit or enforcement action can now just point to this memo........

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