Perfluorohexanoic acid toxicity, part II: Application of human health toxicity value for risk characterization

Perfluorohexanoic acid (PFHxA) is a short-chain, six-carbon PFAA and is a primary impurity, degradant, and metabolite associated with the short-chain fluorotelomer-based chemistry used in the United States, Europe and Japan today. With the shift towards short-chain PFAA chemistry, uncertainties remain regarding human health risks associated with current exposure levels. Here, we present a critical review and assessment of data relevant to human health risk assessment to today's short-chain PFAA chemistry. Human biomonitoring surveys indicate that PFHxA is infrequently detected in the environment as well as in human serum and urine; however, human health concerns may persist in locations where PFHxA is detected. In a companion paper (Luz et al., 2019) we comprehensively evaluate the available toxicity data for PFHxA, and derive a chronic human health-based reference dose (RfD) for PFHxA of 0.25 mg/kg-day based on benchmark dose modeling of renal papillary necrosis in chronically exposed female rats. In this paper, we apply this RfD in human health-based screening levels calculations, and derive a drinking water lifetime health advisory of 1400 μg/L and a residential groundwater screening level for children of 4000 μg/L. Compared to environmental concentration data, even sites with more elevated concentrations of PFHxA in the environment are at least an order of magnitude lower than these screening levels. Available PFHxA human serum and urine biomonitoring data, used as a biomarker for general population exposure, demonstrates that the general human population exposures to PFHxA are low. Previous estimates of daily intake rates for infants exposed to PFHxA through breast milk, formula, and baby foods (Lorenzo et al., 2016) combined with the most conservative PFHxA peer-reviewed toxicity value (Luz et al., 2019) demonstrate that the margin of safety for PFHxA is high. Therefore, PFHxA and related fluorotelomer precursors currently appear to present negligible human health risk to the general population and are not likely to drive or substantially contribute to risk at sites contaminated with PFAS mixtures. PFHxA may also represent a suitable marker for the safety of fluorotelomer replacement chemistry used today.