How to read a PFAS test result

PFAS results appear in three units that all describe the same thing. A nanogram per litre (ng/L) is identical to a part per trillion (ppt). A microgram per litre (ug/L) is a thousand times larger: 0.004 ug/L is 4 ng/L is 4 ppt.

EPA's own national testing file reports in ug/L, which is why raw federal data shows 0.0091 where a news story about the same system says 9.1 ppt. Umwelt converts everything to ng/L on display, because that is the unit the legal limits are written in.

Scale is worth pausing on. Four parts per trillion is roughly four drops in an Olympic swimming pool. That such a quantity is regulated at all tells you these compounds are unusually persistent and unusually biologically active, not that the measurement is fussy.

In April 2024 EPA set the first federal drinking water limits for PFAS: 4 ng/L each for PFOA and PFOS, 10 ng/L each for PFHxS, PFNA and HFPO-DA (GenX), plus a Hazard Index for mixtures.

In May 2026 EPA moved to rescind the limits for PFHxS, PFNA, HFPO-DA and the Hazard Index, keeping only PFOA and PFOS, and pushed the compliance deadline for those two from 2029 to 2031.

That is why Umwelt reports two different things in two different ways. A result above 4 ng/L for PFOA or PFOS is flagged as above a final federal limit, because that limit remains law. A result for one of the rescinded four is reported as a detection with its number, and nothing more, because there is no longer a federal threshold to compare it against.

Several states set their own limits and some are stricter than the federal ones. A system can be legal federally and out of compliance with its state, or the reverse. State limits are the next layer to check once you know your numbers.

A system-level result is an average of samples taken at entry points to the distribution network. It is not what comes out of one tap. Water sits in pipes, mixes between sources, and gets treated at different plants within the same utility.

Sampling is also a snapshot. UCMR5 required a small number of sampling events per system, so a system with one detection and three non-detects has a real detection and a thin picture.

And absence of data is not absence of PFAS. Every system serving more than 3,300 people was tested, but smaller systems were sampled only in part, and private wells were never in scope at all. A blank row can mean untested.

A certified laboratory test of your own tap is the only thing that answers what you are drinking. Mail-in kits from accredited labs run roughly $100 to $300 depending on how many compounds are on the panel, and the result arrives as a report with the same units discussed above.

Order a PFAS panel specifically. A standard water quality panel checks hardness, metals and bacteria and will not look for PFAS at all.

If you are on a private well, this is the only route. No system testing covers you, and wells near airports, military installations, landfills and firefighting training grounds are the ones that most often turn up results.

Two treatment technologies have solid evidence behind them for PFAS: reverse osmosis and activated carbon, particularly in cartridge form at a single tap. Look for certification to NSF/ANSI 53 or 58 with a specific PFOA/PFOS reduction claim, not a general 'filters contaminants' statement.

Standard pitcher filters, refrigerator filters and water softeners are not designed to remove PFAS and should not be assumed to.

Boiling does not help. It concentrates PFAS rather than removing them, because the compounds do not evaporate with the water.