The US Environmental Protection Agency updated a controversial toxicity assessment for perfluorobutanesulfonic acid(PFBS) on April 8. The compound is a member of the per- and polyfluoroalkyl substances (PFAS) family of highly persistent synthetic chemicals, many of which are toxic.
That initial version set a range of values for safe long-term exposure to PFBS: between 0.0003 and 0.001 mg per kilogram of body weight per day. That broke with EPA precedent of setting a single value for safe daily doses. Environmental advocates said the range would put federal and state regulators, who use such numbers for pollution cleanup decisions, under pressure to accept higher exposures as safe.
In February, EPA officials said the initial assessment was altered at the direction of political appointees—after the document was peer-reviewed—to provide the range of values.
The revised document “fixes the errors in the version issued earlier this year, was developed by EPA career scientists, and upholds the values of scientific integrity,” says Jennifer Orme-Zavaleta, acting EPA assistant administrator for the Office of Research and Development and the agency’s science advisor.
The updated assessment retains the initial version’s two numbers, which are derived from detailed scientific calculations. But rather than setting a range, it applies the two numbers in the way EPA scientists originally intended.
For lifetime exposure to PFBS, a contaminant in some drinking water sources, the safe dose is 0.0003 mg/kg-day, the revised assessment says. For comparison, the lifetime exposure safe dose for perfluorooctanesulfonic acid (PFOS), a first-generation PFAS that PFBS replaced, is 0.00002 mg/kg-day.
Meanwhile, the revised assessment says the safe dose for PFBS at subchronic levels—defined as exposure during less than a lifetime—is 0.001 mg/kg-day.
3M introduced PFBS as a replacement for PFOS for use as a surfactant and to make water- and stain-resistant coatings.