Research and analysis

Exploration of potential GB-specific modifications to the EUSES model for UK REACH: summary

Published 16 December 2025

Applies to England, Scotland and Wales

1. Chief Scientist’s Group report summary

This project identified modifications that could be made to the European Union System for Evaluation of Substances (EUSES) to make the model more specific to Great Britain (England, Scotland and Wales). Making the model better reflect GB conditions may yield improved estimates of risk which can be used as a prioritisation tool, allowing regulators to investigate specific issues relating to risk management within GB.

1.1 Background

The UK Registration, Evaluation, Authorisation and restriction of Chemicals (REACH) Regulation applies to most chemicals that are manufactured in, or imported into, GB. The primary environmental exposure model used in UK REACH assessments is based on EUSES. It was developed more than 20 years ago to facilitate standardised environmental risk assessments within the European Union. The EUSES model contains a set of default values that define the properties of the “standard” environment being modelled. These are based on data ranges for the European Union. The purpose of this project was to investigate whether modifying parameters to better reflect GB conditions would have a significant impact on predicted environmental concentrations (PECs). Climate change may have consequences on chemical exposure, so GB specific projections were considered with the aim to improve the model’s longevity.

1.2 Approach

The EUSES default parameters were screened to identify those for which GB values might be different, based on expert input and a review of the scientific literature. A sensitivity analysis was carried out to identify which parameters had the largest impacts on the PECs, and to evaluate the effect of the proposed changes compared to default parameter values. For the first objective, the model was run 24,000 times using 47 parameters (within a set range of ±10% and sampled randomly) and the outputs analysed using Sobol analysis to identify the most influential parameters based on the magnitude of the first-order Sobol sensitivity index (values above 0.1). For the second, the model was run using a smaller subset of 38 parameters and four hypothetical substances (representing different combinations of chemical property values), and the results were compared to modelling of the same four substances using the default EUSES values.

1.3 Results

Modifications that could make model results more relevant to the GB environment include those relating to wastewater management such as: the default dilution factor (DF) for sewage effluent entering a river, the flow rate of the receiving water, size of the standard sewage treatment plant (STP), the number of inhabitants feeding one STP, and the sewage flow rate. Other changes include those affecting terrestrial systems (e.g. the characteristics of soil, the fraction of the population connected to the sewer system, application rate of sludge onto grassland, the mixing depth of agricultural soil and grassland soil), windspeed and annual precipitation, and human dietary intake values (based on the composition of a typical GB food basket).

The PECs obtained using GB-specific values are generally higher at both a local and regional scale (except for marine compartments at a regional scale). This increase becomes more pronounced for surface water compartments and for human exposure via the environment when a DF of 3 is used. The changes to rainfall and windspeed (affecting regional PECs) are expected to account for climate change scenarios at least in the short term (2020-2039) based on UKCP18 climate projections. These changes are relatively small, but updated GB-specific values are nonetheless considered to be an improvement over EUSES default values. The model’s default temperature value of 12 °C was retained as this is within the range of projected average temperatures covering 2020 to 2069, but PECs were found to be insensitive to temperature changes.

1.4 Conclusions

In general, the GB-specific parameter values result in PECs that are higher at a local scale for most substances and compartments, when compared to the PECs obtained using the default EUSES values, and are also higher in most cases, except marine compartments, at a regional scale. When a DF of 3 is used, the increase in PECs becomes more pronounced for surface water compartments and for human exposure via the environment.

1.5 Disclaimer

The proposed changes do not represent formal regulatory guidance.

1.6 Publication details

This summary relates to information reported in detail in the following output:

• Title: Exploration of potential GB-specific modifications to the EUSES model for UK REACH
• Project manager: John D. Crosse and Amber Manley, Chemicals Assessment Unit
• Research contractor: Vitis Regulatory Limited

This project was commissioned 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