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Persistent Pollutants

US EPA deems two GenX PFAS chemicals more toxic than PFOA

Agency will use its calculations to set drinking-water limits for the substances

by Cheryl Hogue
October 29, 2021 | A version of this story appeared in Volume 99, Issue 40


Drawing shows the chemical structure of a PFAS chemical called GenX.

Two Chemours fluoroethers are more toxic than the chemical they were developed to replace, according to a US Environmental Protection Agency assessment released Oct. 25.

Drawing shows the chemical structure of hexafluoropropylene oxide dimer acid.

The EPA’s health assessment examines GenX and hexafluoropropylene oxide dimer acid (HFPO-DA), the chemical GenX hydrolyzes into in the presence of water.

A safe daily dose of GenX or HFPO-DA is 3 ng/kg of body weight, the EPA says. In contrast, the agency’s safe daily dose for the chemical they replace, perfluorooctanoic acid (PFOA), is 20 ng/kg. The assessment notes that the agency is reevaluating toxicity information for PFOA and that the safe daily dose for that chemical could change.

“This science-based final assessment marks a critical step in the process of establishing a national drinking water health advisory for GenX chemicals,” EPA assistant administrator for water Radhika Fox says in a statement. States can also rely on the calculations if they opt to set their own drinking-water or cleanup standards for these substances.

Drawing shows the chemical structure of perfluorooctanoic acid.

The EPA’s final safe daily dose for HFPO-DA and GenX is significantly lower than the 80 ng/kg the EPA calculated in a 2018 draft assessment of the two chemicals.

GenX, an ammonium salt manufactured by E. I. du Pont de Nemours and Company starting in 2009 and later by its Chemours spin-off, was marketed as a “sustainable substitute” for PFOA. US fluoropolymer manufacturers used PFOA as a processing aid for decades before chemical makers voluntarily phased out its production in the country.

PFOA is toxic and remains a serious pollutant across the US and world, however. GenX, HFPO-DA, and PFOA—and the fluoropolymers that they are or were used to produce—are per- and polyfluoroalkyl substances (PFAS), synthetic chemicals so persistent that they are called “forever chemicals.”

In its GenX and HFPO-DA assessment, the EPA relied on data from studies with laboratory animals showing adverse effects in the liver, kidneys, immune system, and fetuses and babies. PFOA is linked to these same harmful effects as well as to thyroid problems, changes in cholesterol levels, and testicular and kidney cancers.

In an emailed statement, Chemours says it is reviewing the technical information that the EPA used in the toxicity assessment. “We are unaware of data that would support the conclusions drawn by the agency,” the company says.

HFPO-DA taints drinking-water sources, including rivers and wells, for hundreds of thousands of people living in the vicinity of a Chemours plant near Fayetteville, North Carolina. The chemical also contaminates wells in Ohio and West Virginia that are close to a Chemours plant near Parkersburg, West Virginia.

Researchers have also found the compound in air, rainwater, and river sediment.

David Andrews, a senior scientist at the Environmental Working Group, commended the EPA for finalizing its assessment of the two fluoroethers.

“This final toxicity value is an important step toward protecting those living in communities contaminated by GenX chemicals, who have been waiting for help for far too long,” Andrews says in a statement.


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