How to read a PFAS lab report: ppt, PFOA, PFOS and limits

What is on a PFAS lab report?

A PFAS lab report lists the compounds the lab tested for, the result for each one, and the unit. Most reports also show the sample ID, the date and place collected, the method used, and a detection or reporting limit. Read the unit first. Almost every drinking water result is in parts per trillion.

Names will look like a code: PFOA, PFOS, PFHxS, PFNA, HFPO-DA, PFBS. These are different PFAS compounds. Our glossary defines PFAS, PFOA and PFOS. Start with PFOA and PFOS. They are the two with the tightest federal limit.

What do ppt and ng/L mean?

They are the same unit. ppt is parts per trillion, and EPA’s rule table says it is “also expressed as ng/L,” which is nanograms per liter. The glossary entry on parts per trillion explains the scale. If your report shows µg/L (micrograms per liter), that is a thousand times bigger. One µg/L equals 1,000 ng/L. Check the unit before you compare anything.

What numbers should you compare your results to?

Compare to EPA’s drinking water limits. These are legally enforceable levels, called Maximum Contaminant Levels (MCLs), for public water systems. The table below shows EPA’s final 2024 limits. The MCL entry explains the term.

These limits apply to public water systems. A result from a private well or a site sample is not a “violation” in the same sense. Use the table as a yardstick and ask a professional what the result means for you.

Are those limits still in force as of October 30, 2026?

For PFOA and PFOS, EPA’s page says it has proposed to keep them. As of October 30, 2026, EPA describes a proposed rule that would “not change” the 4.0 ppt limits while letting some systems request more time to comply, to 2031. EPA also proposes to rescind the rules for PFHxS, PFNA, HFPO-DA and the Hazard Index mixture. Both proposals are not final. Read our post on the 2031 proposal for the details, and check EPA before you rely on a number.

What does the Hazard Index line mean?

It is a way to add up several PFAS at once. EPA uses a Hazard Index, a unitless number, for mixtures of two or more of PFHxS, PFNA, HFPO-DA and PFBS. A lab report will not print a Hazard Index in most cases. A utility or consultant calculates it from the individual results. If your report lists those four compounds, ask someone to run the calculation.

Does one result tell you if you are over the limit?

No. EPA says “compliance with MCLs is determined by running annual averages at the sampling point.” One sample is a snapshot. A high result means retest and ask questions. A low result in one sample does not clear a source.

What should you ask the lab or your consultant?

Ask these questions when you get the report.

1. Which method did you use, and which compounds does it cover?
2. What is the reporting limit for each compound? Is it below 4 ppt for PFOA and PFOS?
3. Did anything flag a quality problem, such as a blank with PFAS in it?
4. How should I compare these results to EPA’s limits?
5. Do I need a second sample or a site assessment?

Our guide to PFAS testing for property owners shows what to do after a result, and our PFAS sampling page explains how samples are collected.

What can a lab report not tell you?

It cannot tell you where the PFAS came from, how far it spread, or whether your cleanup is done. A water result is one point in time at one tap or well. Soil and groundwater at a site need a plan for where and how to sample. The result also does not tell you about health effects for your household. For that, see our health effects of PFAS page and talk to your doctor.

If a result is above 4.0 ppt for PFOA or PFOS, three things are reasonable: retest, ask your water utility for its own results, and read EPA’s page on steps to reduce your risk. If the sample came from a property you own or manage and a source is possible, a PFAS assessment is the next step.

Next step

Advanced Environmental Services offers PFAS testing and PFAS assessment. Call (314) 865-2662 or use the quote form at /contact to talk about your testing needs. Our team is available 24/7 for urgent cleanup needs.