Research and analysis

Developing thresholds for managing PFAS in the water environment: summary

Published 26 January 2026

Applies to England

1. Chief Scientist’s Group report summary

This project developed toxicological based thresholds for water for several per- and poly fluoroalkyl substances (PFAS). The findings will inform the Environment Agency’s ongoing efforts to understand the impact and manage the risks from PFAS in surface waters.

1.1 Background

PFAS are a large and diverse group of synthetic fluorinated chemicals, widely used in industrial applications and consumer items. They are extremely persistent in the environment, with some known to bioaccumulate and be harmful to health. Many PFAS are frequently detected in our rivers, estuaries and groundwater.

Environmental Quality Standards (EQSs) are statutory standards for chemicals in the water environment. They are threshold concentrations, used to assess and manage risks from chemicals to prevent harmful effects on ecosystems and/or human health through exposure via the environment. Only one PFAS currently has an EQS: perfluorooctane sulfonate (PFOS). This project was commissioned to derive science-based threshold concentrations in surface water for more PFAS.  

1.2 Approach

Thresholds were derived for the following PFAS:

• Perfluorooctanoic acid (PFOA)
• Perfluorohexane sulfonic acid (PFHxS)
• Perfluorononanoic acid (PFNA)
• Perfluorooctane sulfonate (PFOS)

These PFAS are frequently detected in English rivers and have relevant and reliable toxicology data.

The project used the method for derivation of EQSs, modified to focus only on the most sensitive endpoint. For the PFAS in this study, this is the protection of human health via the consumption of fish. An established health-based guidance value was used to derive a biota-based threshold, using the same approach used for EQS derivation. This value is for the sum of the four PFAS listed above. Relevant and reliable bioaccumulation data, obtained from peer-reviewed sources, was used to determine equivalent water concentrations as per the EQS guidance.

1.3 Results

The biota-based threshold is 77ng/kg, measured in fish. It was derived to protect human health via the consumption of fish and therefore relates to the combined concentration of these four PFAS in whole fish (77 ng as the sum of the four PFAS in 1kg of fish, wet weight). The threshold is expressed as the sum of four PFAS because the toxicological data on which it is based considered the combined effects of these four substances.

PFOS is included, despite already having an EQS, because it is a component of the toxicology value used for derivation of the threshold. The value for PFOS here is different to the EQS because it is based on more recent toxicology and bioaccumulation data than the EQS. Water concentrations equivalent to this threshold were derived using bioaccumulation data.

The toxicological data on which these equivalent water concentrations are based considered the combined effects of these four PFAS. They should therefore be applied in a way that accounts for the combined effects of the four PFAS. The principles used in the Hazard Index approach were identified as a suitable approach for considering combined effects using the equivalent water concentrations. This involves comparison of the concentration of a substance detected with a relevant threshold to derive a Hazard Quotient (HQ). The HQs for each substance of interest are then summed to enable the combined effect of the substances to be assessed. The equivalent water concentrations to be used in calculating the HQ are:

• 0.4 ng/L for PFOA
• 0.2 ng/L for PFHxS
• 0.3 ng/L for PFNA
• 0.015 ng/L for PFOS

1.4 Conclusions

This project derived thresholds for four PFAS expressed as concentration in fish and equivalent water concentrations. These values will inform Environment Agency’s ongoing efforts to understand and manage the risks from PFAS in surface waters. The values derived for PFOS do not replace the existing statutory EQS for PFOS.

1.5 Publication details

This summary relates to information from the following output:

• Report: SC240028/R
• Title: Developing thresholds for managing PFAS in the water environment
• Project manager: Helen Wilkinson, Research, Chief Scientist’s Group

This project was delivered by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency