Core Concepts
This article introduces the basic structure and chemistry of common per and polyfluoroalkyl substances (PFAS). PFAS are a family of organic chemicals made up of strong carbon-fluorine bonds.
Introduction
Polyfluoroalkyl substances (PFAS) are a diverse family of chemicals, over 7 million have been identified so far. Simply put, PFAS are heavily fluorinated hydrocarbons. They must have at least one fully fluorinated methyl (-CF3) or methylene carbon atom (-CF2-). Their heavy fluorination gives them unique hydrophobic properties that means they are applicable in a wide range of industries. For example, they are used as coatings for non-stick pans and waterproof clothing items. PFAS are extremely stable in the environment and are resistant to degradation. However, they have been linked to significant negative health effects such as cancer and thyroid disease.
Long-Chain versus Short-Chain PFAS
PFAS are typically categorized as “long-chain” or “short-chain” depending on the number of carbons in the fluorinated carbon chain. Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) are the two most widely studied compounds today. They are both considered “long-chain” compounds, because they each have 8 carbons in their structure.
Perfluorooctanoic acid (PFOA)
Perfluorooctanesulfonic acid (PFOS)
Short-chain PFAS emerged as replacements for long-chain PFAS as evidence of negative health impacts of long-chain compounds developed. An example of a short-chain compound is perfluorobutanesulfonic acid (PFBS). PFBS has the same functional group as PFOS (a sulfonic acid). The difference between the two is the number of fluorinated carbons in the carbon chain. The number of fluorinated carbons in the carbon chain affects how the compound interacts in the environment and changes the compound’s solubility.
Perfluorobutanesulfonic acid (PFBS)
Both short and long chain compounds can have a variety of end groups on their carbon-fluorine chain such as acids, sulfates, and amines. These end groups affect the charge of the compound. Some PFAS are cationic (positively charged), anionic (negatively charged), zwitterionic (both + and -), or neutral (net charge of zero). Their intended uses and applications differ due to the way the charge influences their chemical properties. For example, cationic PFAS are used in antimicrobial agents because their positive charge allows them to disrupt cells by binding with the negative charge of the membrane. Anionic PFAS are commonly used as stain resistant coatings as well as surfactants. Zwitterionic PFAS are used as coatings for ships to prevent fouling. Neutral PFAS are typically used as water/oil repellants.
However, some PFAS can actually be precursors for other PFAS. In the environment, PFAS can transform into other PFAS through processes like oxidation and hydrolysis. For example, neutral PFAS often transform to anionic PFAS. These PFAS are more stable than others, so they are called the “terminal end products”. Other PFAS do not transform at all such as zwitterionic PFAS. Cationic PFAS can transform but specific conditions are required, so this doesn’t happen readily in environmental matrices. Due to the large number of substances classified as PFAS and the wide variation between them, this article primarily focuses on anionic PFAS. They are the most studied in literature today. Anionic PFAS are typically referred to as organic perfluoroalkyl acids (PFAAs) because they have negatively charged terminal functional groups at the end of their fluorinated carbon chain. The PFAAs typically have either a carboxylic acid functional group or a sulfonic acid functional group. PFOS and PFOA are great examples of PFAAs. While they both have the same number of carbon atoms, the functional group is different. PFOA has a carboxylic acid functional group and PFOS has a sulfonic acid functional group. This functional group changes how the compound moves and interacts with the environment.
Transport in the Environment
The carbon-fluorine chain is hydrophobic (water-repelling), while the functional group at the end is hydrophilic (water-loving). Hydrophobicity is measured by a compound’s Kow value. Kow is defined as the octanol-water partition coefficient. It is an indicator of the distribution of how much of a substance is in the lipid phase versus the water phase. Usually, a Kow value >1 indicates a substance is more soluble in octanol (this is called a “lipophilic compound”). A Kow value <1 indivates a substance that is more soluble in water (this is called a “hydrophilic compound”). Short-chain PFAS are viewed as more water soluble. They have lower Kow and pKa values. Because of these hydrophobic and hydrophilic properties, PFAS tend to concentrate at the surface of the water. This is called the air-water interface (AWI). The “long-chain” compounds are considered more hydrophobic because of the additional carbon-fluorine bonds in the structure. Due to their increased hydrophobicity, long-chain compounds are more likely to sorb to sediments and soil particles than short-chain compounds. The soil partition coefficient, Koc or Kd, measures how much a substance sorbs to the soil versus the water. In soils, PFAS interact with the organic matter and carbon and create strong hydrophobic interactions. Soil organic carbon has hydrophobic parts that play a strong role in PFAS retention in soils. Long-chain compounds are looked at as more toxic and have a higher potential for accumulation in mammals. They can have half-lives of multiple years (2-10+ years). Short-chain compounds are viewed as less accumulative in mammals because of their lower hydrophobicity and higher water solubility.
Implications of PFAS Presence in the Environment
PFAS presence in the environment can have negative impacts for humans, animals, and surrounding ecosystems. So, what about PFAS makes them so toxic for us? The answer lies in the chemistry of the compound. They look like fats don’t they? Well, this causes a lot of problems.
PFAS have that long hydrophobic tail which gives them similar properties to fats/lipids. This enables them to “mimic” cells that occur naturally in our body such as fatty acids. Once PFAS enter our body, transporter proteins, receptors, and enzymes bind to the PFAS but can’t be metabolized leading to their accumulation. In the environment, PFAS are found virtually everywhere even in places with no known PFAS exposure. However, regions that are close to potential point sources can have elevated levels in the air, soil, and water. Potential point sources of PFAS can include airports that use PFAS-containing fire-fighting foam, landfills, military training facilities, and wastewater treatment plants.
One well-known example of PFAS contamination in the environment is in the Ohio Valley where about 70,000 residents were exposed to elevated levels of PFAS in drinking water. The exposure was specifically of the chemical PFOA which is a long-chain compound. There was a lawsuit that led to the funding of research that linked elevated PFOA exposure to certain diseases such as cancer and decreased immune function. However, PFAS are not just harmful for humans. They can be harmful for other living organisms like birds, fish, aquatic insects, and livestock. Beyond animals, PFAS may also be harmful for plants. Although, PFAS is generally toxic to plants at higher concentrations than animals because plants can tolerate PFAS exposure better.
Research is growing on PFAS in drinking water and food. As of April 2024, the United States Environmental Protection Agency (EPA) released drinking water standards for five individual compounds: PFOA, PFOS, Perfluorohexanesulfonic acid (PFHxS), perfluorononanoic acid (PFNA), and hexafluoropropylene oxide dimer acid (HFPO-DA). They are regulated as maximum contaminant levels (MCLs) and are based on the chronic health effects PFAS are associated with over long-term exposure from drinking water. A maximum contaminant level is used to establish an enforceable, allowable limit of a chemical in drinking water. They help ensure public health by protecting individuals from potentially harmful substances. The units for the maximum contaminant levels are fairly small and are reported in nanogram per liter (ng/L) which is also referred to as parts per trillion (ppt).
| Compound | EPA Drinking Water Maximum Contaminant Level (MCL) (ng/L) |
| Perfluorooctanoic acid PFOA | 4 |
| Perfluorooctanesulfonic acid PFOS | 4 |
| Perfluoroctanehexanesulfonic acid PFHxS | 10 |
| Perfluorononanoic acid PFNA | 10 |
| Hexafluoropropylene oxide dimer acid HFPO-DA | 10 |
PFOA and PFOS are regulated at the lowest concentrations (4 ng/L); however, the EPA has set a goal of zero ng/L for PFOS and PFOA implying that there is no safe level of exposure. This goal is not feasible because current analytical techniques can not detect concentrations extremely small and treatment technologies are not 100% effective at removing PFAS from drinking water. As our understanding of PFAS develops, it is important to continue to raise awareness of PFAS chemistry, fate, and transport.