PFAS (“forever chemicals”) in drinking water, EPA's 4.0 parts-per-trillion limits for PFOA and PFOS, how compliance is measured, and the 2026 proposals.
Sources: EPA and CDC, linked inline. Last checked 2026-10-09. This guide is general information, not medical, legal or plumbing advice.
What PFAS are
EPA: “PFAS are widely used, long lasting chemicals, components of which break down very slowly over time. Because of their widespread use and their persistence in the environment, many PFAS are found in the blood of people and animals all over the world.” “There are thousands of PFAS chemicals.” (EPA PFAS Explained)
What 4 ppt means
On April 10, 2024 EPA announced final enforceable limits (maximum contaminant levels) of 4.0 parts per trillion (ppt) for PFOA and PFOS, “also expressed as ng/L”, with health-based goals of zero (EPA). One part per trillion is one nanogram per liter. EPA also set limits of 10 ppt for PFHxS, PFNA and HFPO-DA (GenX) and a Hazard Index for mixtures.
A single result above 4 ppt is not a violation. EPA: “Compliance with MCLs is determined by running annual averages at the sampling point.” Water systems had until 2027 to complete initial monitoring and “five years (by 2029)” to fix exceedances under the 2024 rule (EPA).
2026 proposals
On May 18, 2026 EPA announced two proposed rules: one keeps the PFOA and PFOS limits but would let water systems request two more years, to 2031, to comply; the other would rescind the limits for PFHxS, PFNA, HFPO-DA and the Hazard Index mixture (EPA). These are proposals, not final rules.
PFAS results on this site
Our PFAS pages show EPA's UCMR 5 monitoring results (2023–2025) for each tested water system. These results describe the water system, not your tap, and are not compliance determinations.
Check your address: look up your service line in your utility's published inventory.
Frequently asked questions
What does 4 ppt PFAS mean?
4.0 parts per trillion (4.0 nanograms per liter) is EPA's 2024 enforceable limit for PFOA and for PFOS in drinking water. Compliance is based on running annual averages, so one higher sample is not by itself a violation.
Did EPA change the PFAS drinking water limits?
On May 18, 2026 EPA proposed keeping the 4.0 ppt PFOA and PFOS limits with an option to request until 2031 to comply, and rescinding the limits for PFHxS, PFNA, HFPO-DA and the Hazard Index. These are proposals.