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Glossary

Plain-language definitions of the PFAS terms used on this site, with the source for each.

AFFF (aqueous film-forming foam)
A firefighting foam used to put out fires involving flammable liquids, in training and emergencies at places such as airports, military bases and refineries. [1]
Biomonitoring
Measuring chemicals in people’s blood or urine. CDC’s NHANES survey has measured PFAS in the blood of the U.S. population since 1999. [2]
Biosolids
Material from wastewater treatment plants that can be used as fertilizer on farmland; EPA lists it as a possible source of PFAS in soil and water. [1]
CERCLA (Superfund)
The Comprehensive Environmental Response, Compensation, and Liability Act. EPA designated PFOA and PFOS as hazardous substances under it in 2024. [3]
Fluoropolymers
Large-molecule PFAS. FDA describes them as critical components of some medical devices, including catheters, cardiovascular implants, tubing and wound dressings. [4]
Forever chemicals
A common name for PFAS, used by EPA, reflecting how slowly many of them break down. [5]
GenX chemicals
HFPO dimer acid and its ammonium salt, associated with the GenX technology for making fluoropolymers without PFOA. [6]
Hazard index
A way of setting a single limit for a mixture of chemicals. EPA’s 2024 rule used one for mixtures of PFHxS, PFNA, HFPO-DA and PFBS. [7]
IARC Group 1 and Group 2B
Categories used by the International Agency for Research on Cancer. Group 1 means carcinogenic to humans; Group 2B means possibly carcinogenic to humans. IARC placed PFOA in Group 1 and PFOS in Group 2B in 2023. [8]
Legacy PFAS
Older, well-studied PFAS, above all PFOA and PFOS, that have been phased out of production in the U.S. [9]
Long-chain and short-chain PFAS
A grouping by the number of fully fluorinated carbon atoms. Under OECD usage, long-chain carboxylic acids have seven or more and long-chain sulfonic acids six or more; definitions vary by jurisdiction. [9]
MCL (maximum contaminant level)
The highest level of a contaminant allowed in public drinking water under U.S. law. EPA set MCLs of 4.0 parts per trillion for PFOA and PFOS in 2024. [7]
ng/mL (nanograms per milliliter)
The unit used for PFAS levels in blood. The National Academies use thresholds of 2 and 20 ng/mL for the combined level of several PFAS. [10]
NHANES
The National Health and Nutrition Examination Survey, run by CDC, which measures chemicals such as PFAS in a representative sample of people in the U.S. [2]
NSF/ANSI 53 and 58
Certification standards for home water filters. EPA advises looking for certification under either standard for PFAS reduction. [11]
PFAS
Per- and polyfluoroalkyl substances: a large group of manufactured chemicals containing chains of carbon and fluorine atoms. [5]
PFOA
Perfluorooctanoic acid, a legacy PFAS. Classified by IARC as carcinogenic to humans (Group 1). [8]
PFOS
Perfluorooctane sulfonate, a legacy PFAS. Classified by IARC as possibly carcinogenic to humans (Group 2B). [8]
PFHxS, PFNA and PFBS
Three other PFAS covered by EPA’s 2024 drinking water rule, either with their own limit (PFHxS, PFNA) or as part of the hazard index mixture (PFBS). [7]
ppt (parts per trillion)
A unit of concentration used for PFAS in drinking water. One part per trillion is one part of a substance in a trillion parts of water. [7]

Sources

  1. U.S. EPA, Our Current Understanding of the Human Health and Environmental Risks of PFAS, updated 14 July 2026.
  2. ATSDR, Fast Facts: PFAS in the U.S. Population, 12 November 2024.
  3. U.S. EPA, Designation of PFOA and PFOS as CERCLA Hazardous Substances, updated 14 April 2026.
  4. FDA, PFAS in Medical Devices: What You Need to Know, current as of 1 September 2026.
  5. U.S. EPA, Proposal to List Nine PFAS Compounds as RCRA Hazardous Constituents, updated 7 May 2026; NIEHS, PFAS.
  6. U.S. EPA, Human Health Toxicity Assessments for GenX Chemicals, updated 12 November 2025.
  7. U.S. EPA, PFAS National Primary Drinking Water Regulation, updated 18 May 2026.
  8. IARC, IARC Monographs evaluate the carcinogenicity of PFOA and PFOS, 1 December 2023.
  9. OECD, Reconciling Terminology of the Universe of PFAS, 2021; ATSDR, PFAS and Your Health.
  10. National Academies, New Report Calls for Expanded PFAS Testing, 28 July 2022.
  11. U.S. EPA, Identifying Drinking Water Filters Certified to Reduce PFAS, updated 18 November 2025.

Last reviewed: 29 September 2026.

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