Australia doubled its draft PFOS drinking water limit to 8 nanograms per litre before publishing it

The National Health and Medical Research Council published revised PFAS values for the Australian Drinking Water Guidelines on 30 June 2025. The four health based guideline values are 8 nanograms per litre for perfluorooctane sulfonic acid, 200 ng/L for perfluorooctanoic acid, 30 ng/L for perfluorohexane sulfonic acid and 1000 ng/L for perfluorobutane sulfonic acid. The guidelines state that no health based guideline value can be derived for GenX chemicals at this time, citing insufficient toxicity evidence and insufficient occurrence data in Australian waters.

Most coverage treated the announcement as a tightening, which for three of the four substances it was, since the previous values dated from 2018. The document trail shows something more specific about the fourth. Three of the four final values are identical to the draft values released for public consultation in October 2024. The exception is PFOS, which was 4 ng/L in the draft and 8 ng/L on publication, a doubling that arrived after further modelling and expert review.

The derivation explains where the factor of two comes from. The PFOS value rests on a sub-chronic 28 day rat study published by the United States National Toxicology Program in 2022, with bone marrow effects, specifically extramedullary haematopoiesis and bone marrow hypocellularity, as the critical endpoint. The committee derived the point of departure using benchmark dose modelling with a gamma distribution rather than the multistage model used by the United States Environmental Protection Agency, noting substantial differences between the modelled options and the measured no observed adverse effect level, and adopting the modelled approach as statistically more robust. The resulting tolerable daily intake is 728 nanograms per kilogram of body weight per day, divided by an uncertainty factor of 300. Applying a 70 kilogram adult, two litres of water a day and an assumption that drinking water contributes 10 per cent of total intake gives 8.49 ng/L, rounded to 8.

PFOA followed a different path. The critical study is the 2023 National Toxicology Program rat carcinogenicity bioassay, with pancreatic acinar adenomas and adenocarcinomas as the critical endpoint. Because PFOA was treated as a non genotoxic carcinogen, a threshold approach was applied rather than a non threshold one, giving a tolerable daily intake of 1946 ng/kg body weight a day and an uncertainty factor of 30. The same exposure assumptions produce 227 ng/L, rounded to 200.

That choice is where the international gap opens. Under the United States rule finalised on 10 April 2024, the maximum contaminant level for both PFOA and PFOS is 4 parts per trillion, with a maximum contaminant level goal of zero for each, and 10 ppt limits for PFHxS, PFNA and HFPO-DA, plus a hazard index of 1 for mixtures. Australia's PFOA value is fifty times the American limit and its PFOS value is twice it. Professor Stuart Khan of the University of Sydney, speaking to the Australian Water Association in September 2024 while the review was still running, described the American numbers as configured around what is practical to detect rather than around a health based target, and cautioned against importing that method.

The American position has since moved again. On 18 May 2026 the EPA proposed retaining the PFOA and PFOS limits while offering water systems an optional compliance extension to 2031, and separately proposed rescinding the limits for PFHxS, PFNA, HFPO-DA and the hazard index. So the comparison Australian regulators were measured against in 2025 is not the comparison that will apply in 2027.

NHMRC chief executive Professor Steve Wesselingh said the final values had been reviewed and supported by the expert advisory committee, an independent expert review and jurisdictional public health experts. He also said that because drinking water is only one source of PFAS exposure, higher values in drinking water "for short periods is unlikely to increase health risks".

What has not been reconciled publicly is the eight month interval in between. Wesselingh noted that since the draft was released in October, many water suppliers had tested their sources to reassure the public that their water was safe. Those tests, and the community messages built on them, were assessed against 4 ng/L for PFOS. The published value is 8. Utilities that reported an exceedance against the draft were reporting against a number that did not survive to publication, and utilities that reported compliance were, for PFOS, clearing a bar half the height of the final one.

What is still unknown is how many supplies sat between 4 and 8 ng/L during that window, since no national tally of draft period results has been published, and whether a GenX value will follow. The guidelines say only that one cannot be derived at this time.