Per- and polyfluoroalkyl substances (PFAS) are a large class of molecules characterized primarily by alkyl chains in which one or more hydrogen atoms have been replaced by fluorine, although this definition has recently expanded to also include substances containing at least one fully fluorinated carbon (e.g., –CF₂– or –CF₃ groups). In perfluoroalkyl substances, all hydrogen atoms are replaced with fluorine, whereas in polyfluoroalkyl substances, some hydrogen atoms remain. PFAS also often contain additional functional groups, such as carboxylic acids and sulfonic acids, which further expand their structural diversity.
Historically, PFAS have been used in a variety of commercial products, including non-stick cookware, fire-retardant textiles and furniture, and various electronic devices. Although the use of many legacy PFAS has been restricted or phased out by several major regulatory agencies over the past two decades, their widespread historical usage combined with their high environmental persistence means that they are still a concern. Although the highest concentrations of PFAS tend to be found in industrialized regions, PFAS can easily migrate to other locations due to their generally high aqueous solubility, particularly for short-chain analogs.
PFAS can enter the food chain through multiple pathways, such as plant uptake from contaminated soil and water, as well as the direct contamination of aquatic systems with industrial waste. In these environments, PFAS can bioaccumulate (particularly so for long-chain PFAS) and biomagnify, leading to elevated exposure risk for humans and other apex organisms. Despite this recognized exposure risk, how such differences in PFAS structure affect their environmental distribution and associated human exposure risk is still not fully understood.
Why do PFAS persist for so long?
Arguably, the most notable feature of PFAS is their high environmental persistence and resistance to degradation, with many types of PFAS able to persist in the human body for several years and in the environment for up to several decades. This unusually high environmental persistence of PFAS, which has led to their designation as “forever chemicals,” arises from key structural features, several of which are discussed in more detail below.
High carbon–fluorine bond strength:
A typical C–F bond has a bond energy of approximately 485 kJ/mol, which is significantly higher than the energy of a typical C–H bond (413 kJ/mol). This high bond energy, which is due to the short and strong nature of the C–F bond, means that C–F bond cleavage is rare under ambient or biological conditions. The degree of fluorination in PFAS also correlates with environmental persistence and stability: many polyfluoroalkyl substances have half-lives from hours to days, and perfluorinated alkyl substances have half-lives of up to several decades.
Fluorophilic partitioning behavior:
Due to the high concentration of fluorine atoms, PFAS are often described as “fluorophilic,” meaning that the fluorinated carbon chain interacts weakly with both aqueous and organic phases. This fluorophilic behavior is due to the high electronegativity and low polarizability of fluorine, which limits favorable interactions with both hydrophobic phases and aqueous environments. As a result, PFAS do not preferentially partition into organic phases in the same way as many conventional organic contaminants.
Chain length and functional group effects:
PFAS are often divided based on their number of carbon atoms (being short-chain or long-chain), although the exact cutoff depends on their functional group class (e.g., ≤C6 for perfluorocarboxylic acids and ≤C5 for perfluorosulfonic acids are often considered short-chain). Short-chain PFAS are more water-soluble than longer-chain analogs, with solubility decreasing by approximately one order of magnitude with each additional carbon atom. The presence of polar functional groups also affects how PFAS behave in water, particularly for sulfonic acids (which are typically fully ionized) and carboxylic acids (which are generally deprotonated under environmentally relevant pH conditions, but can exhibit pH-dependent speciation under more acidic conditions).
Taken together, these structural features explain the unusual combination of persistence and mobility that distinguishes PFAS from many other environmental contaminants. The high C–F bond strength limits chemical degradation, while weak interactions with organic phases and, in many cases, high aqueous solubility allow PFAS to remain in water and be transported over long distances.
“PFAS persistence and environmental mobility are primarily governed by their physicochemical properties, such as chain length, functional groups, and hydrophobicity, which control sorption behavior, bioavailability, and resistance to degradation,” Weilan Zhang, PhD, assistant professor in the department of environmental and sustainable engineering at the University at Albany, told Technology Networks.
How do PFAS migrate from the environment into the food chain?
PFAS are released into the environment via the use of PFAS-containing commercial products and due to leakage from various industrial processes, where they primarily accumulate in water.
Following the initial contamination event, PFAS are taken up by plants exposed to the contaminated water, then retained within plant tissues. Livestock may ingest these contaminated plants or drink contaminated water. Unlike many organic contaminants that preferentially accumulate in fatty tissues, PFAS accumulate in protein-rich tissues, including blood and muscle. Human exposure to PFAS comes predominantly through eating these contaminated animal tissues and drinking contaminated water, although additional exposure can occur from other sources, such as inhaling contaminated dust in indoor environments.
The extent of bioaccumulation of individual PFAS compounds is strongly dependent on their molecular structure, particularly their chain length and the presence of polar functional groups. Short-chain PFAS are more readily eliminated due to their higher aqueous solubility and reduced protein binding affinity. In contrast, longer-chain PFAS exhibit greater bioaccumulation due to stronger interactions with proteins and slower clearance from biological systems. In some cases, these compounds can also exhibit surfactant-like behavior, including aggregation or interactions with lipid membranes, which may contribute to their behavior in biological systems.
Human exposure and health concerns
Human exposure to PFAS remains an ongoing public health concern. Regulators have begun to tackle this issue, but globally, there remains a patchwork of different restrictions and limitations that vary by country and in scope.
The PFAS that have been restricted or phased out—generally the longer-chain analogs, such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS)—are often replaced with other chemicals (i.e., perfluorobutanesulfonic acid (PFBS), GenX) that have less well-characterized toxicity profiles and may exhibit even greater environmental mobility. These replacements may even complicate treatment efforts via standard environmental remediation techniques.
Although PFAS exposure has declined overall in recent years, largely due to these increasing restrictions around PFAS usage, the high environmental stability and persistence of PFAS means that even if all PFAS usage were discontinued today, human exposure to PFAS would still continue for decades.
Exposure to certain PFAS has been associated with a variety of significant health concerns, including an increased risk of certain cancers, endocrine disruption and infertility, immune system suppression, metabolic disorders, and developmental effects. However, it is important to note that the bulk of toxicity data available for PFAS comes from studies of legacy PFAS compounds (i.e., PFOA and PFOS), and so the true health impacts of PFAS exposure may not be fully captured by the current literature.
PFAS detection and regulation: Progress and limitations
Efforts to understand and manage PFAS exposure are complicated by significant limitations in current detection methods.
Most methods rely on liquid chromatography–tandem mass spectrometry (LC-MS/MS) or gas chromatography–tandem mass spectrometry (GC-MS/MS). These methods are targeted analysis approaches, and therefore are limited to capturing known PFAS compounds.
A key limitation of these methods lies in the “implicit assumption that parent PFAS disappearance reflects degradation,” Dr. Chanaka Navarathna, researcher at the University of Utah, reported. This assumption ignores the fact that the disappearance of a parent PFAS peak in a mass spectrogram may instead reflect conversion into other PFAS derivatives, many of which are highly polar and difficult to detect. Thus, Dr. Navarathna explained, “what looks like ‘destruction’ is often just redistribution that falls outside the targeted method.”
This limitation is further compounded by the continual introduction of new PFAS. These compounds are often not included in targeted analytical methods and may therefore evade detection. As a result, broader approaches such as total organic fluorine (TOF) and extractable organic fluorine (EOF) analysis, typically performed using combustion ion chromatography (CIC), are used to estimate total fluorinated content. In many cases, discrepancies are seen between targeted PFAS measurements and total fluorine content. This indicates the presence of unidentified fluorinated species, suggesting that current analytical approaches do not completely capture the total PFAS content.
These analytical limitations have direct implications for PFAS legislation and regulatory approaches. Regulation has traditionally focused on individual PFAS compounds (e.g., PFOA, PFOS), but this approach has proven insufficient given the large number of structurally related PFAS and the frequent substitution of regulated compounds with less-characterized analogs. As a result, there is increasing interest in class-based regulatory strategies that address PFAS collectively.
Dr. Courtney Carignan, associate professor in the Department of Food Science and Human Nutrition at Michigan State University expounded on the critical need for regulatory strategies, “we need action to prevent and reduce exposure, especially via drinking water and certain foods (e.g., inland fish). At the same time, we also need action to limit global production and use of the hundreds of current use PFAS until we know which, if any, are safe.”
These limitations also impact remediation strategies, as the persistence and structural diversity of PFAS, combined with incomplete detection of unknown species, hinder efforts to fully remove these compounds from the environment.
While industrial water treatment technologies may remove some PFAS during ordinary operations, full PFAS mitigation and clean-up will require the development of more effective. Current mitigation strategies include advanced water treatment methods such as activated carbon adsorption, ion exchange resins, and high-pressure membrane filtration, although their effectiveness varies depending on PFAS structure. The development of improved strategies for mitigating and remediating PFAS contamination continues to be a very active area of research for materials scientists, environmental scientists, and industry leaders alike.
PFAS contamination: What does the future look like?
Despite extensive study on legacy PFAS, exactly how the differences in PFAS structure translate into environmental transport properties and human health risks remains incompletely understood, particularly with respect to the behavior and impact of individual PFAS chemicals and lesser-known PFAS substitutes. These challenges are compounded by limitations in current detection methods and regulatory approaches, which do not fully account for the diversity and transformation of PFAS in real-world systems.
Moving forward, progress will require more systematic characterization of how PFAS structure determines environmental mobility, persistence, and bioaccumulation, as well as the development of detection methods that capture both known PFAS and their transformation products in complex environments.
