The EPA has published draft reference doses for two PFAS substances: 'GenX chemicals' and perfluorobutane sulfonic acid (PFBS), which estimate the amount of a chemical a person can ingest daily over a lifetime (chronic RfD) or less (subchronic RfD) without suffering adverse health effects.
The development comes as part of the EPA’s broader effort to address per- and polyfluoroalkyl substances (PFASs), a class of substances which have been the subject of heightened scrutiny among concerns as to their potential toxicity and their prevalence in humans and the environment.
For PFBS, candidate values were developed based on both thyroid and kidney effects. They are as follows:
- PFBS, draft subchronic RfD:
- 0.04 mg/kg-day (based on thyroid effects);
- 0.1 mg/kg-day (based on kidney effects);
- PFBS, draft chronic RfD:
- 0.01 mg/kg-day (based on thyroid effects);
- 0.01 mg/kg-day (based on kidney effects);
- GenX chemicals, draft subchronic RfD: 0.0002 mg/kg-day; and
- GenX chemicals, draft chronic RfD: 0.00008 mg/kg-day.
Comparing these values to those of legacy substances PFOA and PFOS (which both have a chronic RfD of 0.00002 mg/kg-day), PFBS is about 500 times less toxic. Meanwhile, GenX chemicals (hexafluoropropylene oxide (HFPO) dimer acid and its ammonium salt), are about four times less toxic.
These values can be used by government and other stakeholders in combination with specific exposure information to characterise potential public health risks associated with exposure, according to a Federal Register notice announcing their release.
The EPA added that the assessments, once finalised, may be used by states, communities and other federal agencies to "to determine, under the appropriate regulations and statutes, if and when it is necessary to take action to address potential risk associated with human exposures to these PFAS chemicals".
GenX chemicals and PFBS are both "later generation" short-chain PFAS substances that have been used as surfactants and repellents to replace long-chain PFASs like PFOA and PFOS. While the short-chain PFAS are considered safer than their long-chain predecessors, their use is controversial. North Carolina has expressed concern over GenX chemicials contamination in drinking water, and PFBS has been recommended for phase out by Norway’s EPA.
According to the EPA's acting administrator Andrew Wheeler, the assessments "are critical to [the EPA’s] efforts to help communities impacted by PFAS".
He added that they are part of the agency’s forthcoming management plan for addressing PFASs, but that the EPA is "releasing the draft assessments now to provide this information – and give the public the opportunity to provide input – as soon as possible".
The EPA says it developed the draft assessments in consultation with independent peer reviewers, along with federal and state partners, using "the best available science".
Comments on the drafts will be accepted for 60 days following their official publication in the Federal Register.
The agency said in May that it plans to release its broader PFAS management plan "later this year".
According to the EPA, GenX chemicals are associated with health effects in the kidney, blood, immune system, developing fetuses, and especially in the liver following oral exposure; studies on animals are suggestive of cancer.
PFBS is associated with health effects on the thyroid, reproductive organs and tissues, developing fetuses and the kidney following oral exposure. The thyroid and kidney are particularly sensitive, though the studies on animals are inadequate to evaluate cancer, it says.
The toxicity assessments for GenX chemicals and PFBS include toxicity values associated with "potential noncancer health effects following oral exposure".
By Kelly Franklin and Lisa Martine Jenkins