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Uncertainty analysis for environmental exposure assessment

August 30th, 2019

Risk assessment

Uncertainty analysis for environmental exposure assessment

Every result, whether predicted or measured, has uncertainty associated with it. But does that matter? PFA has been working on tools to address that question for properties used in environmental exposure assessment for REACH.


SimpleBox is multimedia environmental fate model that predicts concentrations of a chemical in air, water, sediment and soil. It is used to model regional, continental and global distributions in EUSES and in CHESAR, used in the REACH Chemical Safety Assessment.

The inputs to the model are the basic physicochemical properties of the substance, such as partition coefficients, water solubility and vapour pressure, and its release rates into the environmental compartments. The outputs are the steady-state concentrations in each environmental compartment.

Uncertainty module

PFA now have uncertainty analysis modules for both SimpleBox and EUSES. These allow us to efficiently explore a range of input values and study the impact on the predicted environmental concentrations.

There are various circumstances in which this might be useful. The ECHA Guidance on information requirements and chemical safety assessment chapter R19 discusses this in the context of Reach Registration. Two areas where we have used it are discussed below.

Refining the risk assessment

If a Risk Characterisation Ratio (RCR) is above 1, the assessment must be refined before a Registration can be made. The uncertainty analysis can help to identify which inputs have most impact on the RCRs. Perhaps an old unreliable study, or QSAR prediction, is used for the log Koc and you are considering making a new measurement. The uncertainty analysis can identify how much impact changing the log Koc has on the outcome of the assessment.

Exploring persistence

We have been working with a persistent substance and seeking to understand its potential to back-deposit to surface media in remote regions. The uncertainty tool allows us to explore how the physicochemical properties and the release rates to different compartments affect the degree of back deposition.

1 https://www.rivm.nl/en/soil-and-water/simplebox

2 information_requirements_r19_en.pdf