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PFAS Toxicology and Risk Assessment

This article presents an overview of current practices for human health and ecological risk assessment related to per- and poly-fluoroalkyl substances (PFAS) exposures at aqueous film-forming foam (AFFF) impacted sites.

Related Article(s):

• Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)
• PFAS Sources
• PFAS Transport and Fate

Contributors: Jennifer Arblaster, Jason Conder, Jean Zodrow and Elizabeth Nichols

Key Resource(s):

• State of the Science for Risk Assessment of PFAS at Contaminated Sites
• Interstate Technology Regulatory Council (ITRC), PFAS – Per- and Polyfluoroalkyl Substances

PFAS Exposure and Conceptual Site Models

Figure 1. Simplified Conceptual Site Model for Sites Impacted by AFFF or other PFAS Sources. Used with permission

This article provides a brief overview of the environmental toxicology and risk assessment of per- and polyfluoroalkyl substances (PFAS). The article’s main focus is on the environmental toxicology and risk assessment of PFAS derived from aqueous film-forming foam (AFFF).

The use of AFFF can release PFAS into the environment during fire training, an emergency response, or as a result of leaks or spills from AFFF systems. Following AFFF releases, perfluoroalkyl acids (PFAAs), particularly PFOS, PFOA, and PFHxS, tend to be the most commonly detected PFAS in environmental media. Due to their solubility, sorption, and bioaccumulation properties, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs) can be prevalent in a variety of environmental media, including groundwater, surface water, soil, sediment, biosolids, landfill leachate, plants, fish, invertebrates, and wildlife.

PFAS exhibit a range of physical and chemical properties, with the fate of the PFAAs, particularly the PFCAs and PFSAs, being the most studied PFAS. PFAAs are relatively water-soluble and mobile in the environment, are not volatile (i.e., they do not evaporate to the atmosphere readily) and can sorb to the organic carbon present in soil or sediment. PFAAs are more soluble and mobile compared to other persistent organic chemicals of concern documented at contaminated sites. PFAS can bioaccumulate in animals and plants, and persistent PFAS, such as PFCAs and PFSAs, do not undergo significant biodegradation or biotransformation once present in a biological system.

The current state of the science and understanding of PFAS fate and transport indicates that the human health issues associated with PFAS AFFF sites are primarily the exposure pathways associated with drinking water ingestion and dietary intake of PFAS. Incidental soil ingestion and/or dust inhalation are typically of moderate concern and are recommended for inclusion into human health risk assessments, but compared to drinking water and dietary ingestion, generally result in lower exposures for most receptors. Exposures via dermal contact with soils and water, and inhalation of vapors (due to volatilization of PFAS), are generally of even lower concern for most sites with AFFF PFAS sources. Human health conceptual site models (CSMs) for AFFF sites typically reflect common receptors including current or future residents and industrial or commercial workers, depending on the current and reasonable anticipated future land uses at the site, along with potential exposures in offsite areas. Receptors associated with recreation and fishing activities may be incorporated if water resources used for recreational purposes are located near the site. Additional considerations may need to be incorporated into the CSM, such as the source of PFAS release into the environment. Release mechanism can differ based on site uses of PFAS. For example, while AFFF use often resulted in historic releases to ground surfaces, industrial emissions can result in aerial deposition, and biosolids application can result in widespread releases to soils which result in different exposure pathways that should be considered.

Ecological CSMs generally focus on exposures in areas adjacent to or downgradient of initial AFFF releases which have habitats present which support ecological resources (Figure 1). Most areas at the point of AFFF releases (and many industrial areas where PFAS products are or were used) do not generally feature favorable ecological habitats that make these areas relevant for ecological risk assessment. However, the relatively high solubility of PFAS in water results in a high potential for offsite transport via groundwater, surface water and stormwater, or by erosion of impacted soils and sediment.

Toxicological Effects of PFAS

The characterization of toxicological effects in human health risk assessments is based on toxicological studies of mammalian exposures to per- and polyfluoroalkyl substances (PFAS), primarily studies involving perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). The most sensitive noncancer adverse effects involve the liver and kidney, immune system, and various developmental and reproductive endpoints. A select number of PFAS have been evaluated for carcinogenicity, primarily using epidemiological data. Only PFOS and PFOA (and their derivatives) have sufficient data for USEPA to characterize as Likely to Be Carcinogenic to Humans via the oral route of exposure. Epidemiological studies provided evidence of bladder, prostate, liver, kidney, and breast cancers in humans related to PFOS exposure, as well as kidney and testicular cancer in humans and limited evidence of breast cancer related to PFOA exposure.

USEPA’s Integrated Risk Management System (IRIS) Program is developing Toxicological Reviews to improve understanding of the toxicity of several additional PFAS (i.e., not solely PFOA and PFOS). Toxicological Reviews provide an overview of cancer and noncancer health effects based on current literature and, where data are sufficient, derive human health toxicity criteria (i.e., human health oral reference doses and cancer slope factors) that form the basis for risk-based decision making. For risk assessors, these documents provide USEPA reference doses and cancer slope factors that can be used with exposure information and other considerations to assess human health risk. Final Toxicological Reviews have been completed for the following PFAS:

• Perfluorooctanesulfonic acid (PFOS)
• Perfluorooctanoic acid (PFOA)
• Perfluorobutanoic acid (PFBA)
• Perfluorohexanoic acid (PFHxA)
• Perfluorobutane sulfonic acid (PFBS)
• Perfluoropropionic acid (PFPrA)
• Perfluorohexane sulfonic acid (PFHxS)
• Lithium bis[(trifluoromethyl)sulfonyl]azanide (HQ-115)
• Hexafluoropropylene oxide dimer acid (HFPO DA) and its Ammonium Salt

Toxicity assessments are ongoing for the following PFAS:

• Perfluorononanoic acid (PFNA)
• Perfluorodecanoic acid (PFDA)

It is important to note human health toxicity criteria for inhalation of PFAS are not included in the Final Toxicological Reviews and are not currently available. In addition to IRIS, state agencies have developed peer-reviewed provisional toxicity values that have been incorporated into USEPA’s RSLs, which are updated biannually. These values have not been reviewed by or incorporated into IRIS.

With respect to ecological toxicity, effects on reproduction, growth, and development of avian and mammalian wildlife have been documented in controlled laboratory studies of exposures of standard toxicological test species (e.g., mice, quail) to PFAS. Many of these studies have been reviewed to derive ecological Toxicity Reference Values (TRVs). TRVs can be used alongside exposure information and other considerations to assess ecological risk. Avian and mammalian wildlife receptors are generally expected to have the highest risks due to PFAS exposure. Direct toxicity to aquatic life, such as fish and invertebrates, from exposure to sediment and surface water also occurs, though concentrations in water associated with adverse effects to aquatic life are generally higher than those that could result in adverse effects to aquatic-dependent wildlife. Soil invertebrates and plants are less sensitive to PFAS when compared to terrestrial wildlife, with risk-based PFAS concentrations in soil being much higher than those associated with potential effects to terrestrial wildlife.

PFAS Screening Levels for Human Health and Ecological Risk Assessments

Human Health Screening Levels

Human health screening levels for PFAS have been modified multiple times over the last decade and, in the United States, are currently available for drinking water and soil exposures as Maximum Contaminant Levels (MCLs) and USEPA Regional Screening Levels (RSLs). USEPA finalized a National Primary Drinking Water Regulation (NPDWR) for six PFAS:

• Perfluorooctanoic acid (PFOA)
• Perfluorooctane sulfonic acid (PFOS)
• Perfluorohexane sulfonic acid (PFHxS)
• Perfluorononanoic acid (PFNA)
• Hexafluoropropylene oxide dimer acid (HFPO-DA, commonly known as GenX chemicals)
• Perfluorobutane sulfonic acid (PFBS)

MCLs are enforceable drinking water standards based on the most recently available toxicity information that consider available treatment technologies and costs. The MCLs for PFAS include a Hazard Index of 1 for combined exposures to four PFAS. RSLs are developed for use in risk assessments and include soil and tap water screening levels for multiple PFAS. Soil RSLs are based on residential/unrestricted and commercial/industrial land uses, and calculations of site-specific RSLs are available.

Internationally, Canada and the European Union have also promulgated drinking water standards for select PFAS. However, large discrepancies exist among the various regulatory organizations, largely due to the different effect endpoints and exposure doses being used to calculate risk-based levels. The PFAS guidance from the Interstate Technology and Regulatory Council (ITRC) in the US includes a regularly updated compilation of screening values for PFAS and is available on their PFAS website: https://pfas-1.itrcweb.org.

Ecological Screening Levels

Most peer-reviewed literature and regulatory-based environmental quality benchmarks have been developed using data for PFOS and PFOA; however, other select PFAAs have been evaluated for potential effects to aquatic receptors. USEPA has developed water quality criteria for aquatic life for PFOA and PFOS. Following extensive reviews of the peer-reviewed literature, Zodrow et al. used the USEPA Great Lakes Initiative methodology to calculate acute and chronic screening levels for aquatic life for 23 PFAS. The Argonne National Laboratory has also developed Ecological Screening Levels for multiple PFAS. In contrast to surface water aquatic life benchmarks, sediment benchmark values are limited. For terrestrial systems, screening levels for direct exposure of soil plants and invertebrates to PFAS in soils have been developed for multiple AFFF-related PFAS, and the Canadian Council of Ministers of Environment developed several draft thresholds protective of direct toxicity of PFOS in soil.

Wildlife screening levels for abiotic media are back-calculated from food web models developed for representative receptors. Both Zodrow et al. and Grippo et al. include the development of risk-based screening levels for wildlife. The Michigan Department of Community Health derived a provisional PFOS surface water value for avian and mammalian wildlife. In California, the San Francisco Bay Regional Water Quality Control Board developed terrestrial habitat soil ecological screening levels based on values developed in Zodrow et al.. For PFOS only, a dietary screening level (i.e. applicable to the concentration of PFAS measured in dietary items) has been developed for mammals at 4.6 micrograms per kilogram (μg/kg) wet weight (ww), and for avians at 8.2 μg/kg ww.

Exposures and Effects Evaluation Approaches for AFFF Site Risk Assessment: Human Health

Exposure pathways and effects for select PFAS are well understood, such that standard human health risk assessment approaches can be used to quantify risks for populations relevant to a site. Human health exposures via drinking water have been the focus in risk assessments and investigations at PFAS sites

Advantages and Limitations of the Technology for PFAS Removal

Advantages:

• Robust, high throughput treatment
• Mature technology with well documented solute separation performance
• High rejection of PFAS and other contaminants
• Removes solutes at the molecular scale

Limitations:

• Complex and often expensive pretreatment requirements for certain waters
• Energy intensive
• High capital costs
• Membrane fouling requiring high chemical usage for cleaning
• Concentrated waste stream requiring disposal or destruction
• Permeate quality depends on feed water concentration
• Greater operation complexity than most water treatment processes
• Water loss due to membrane separation

Summary

High-pressure membranes including NF and RO are well established technologies used in a variety of water treatment fields for the purification of water resources and industrial process waste streams. Research conducted over the past decade has demonstrated that various available membrane products can achieve high rejection of PFAS, enabling compliance with state and federal PFAS regulations. As opposed to adsorbent based PFAS removal technologies (e.g., activated carbon, ion exchange), high-pressure membranes do not have a finite capacity for PFAS removal and do not exhibit breakthrough. High-recovery membrane systems are being implemented into ex situ treatment trains to simultaneously treat PFAS impacted water resources and concentrate PFAS into the retentate stream to enable more effective and efficient PFAS destruction.