How to use USEtox® characterization factors in SimaPro

USEtox®, available in SimaPro, is a well-known model for characterizing the toxic impacts of chemical emissions in a life cycle assessment (LCA). But what is it exactly and how do you use it? This article gives a short overview.

What is USEtox?

To assess the impact of chemical emissions in a product’s life cycle, researchers need to follow a systematic modeling procedure through the fate, exposure, and effects of the chemical. However, there are multiple methods to do this. USEtox is a multimedia box model for characterizing the human and ecotoxicological impacts of chemicals in a life cycle impact assessment. The model represents a scientific consensus, taking into account advancing science as well as the need for stability, parsimony, transparency, and reliability. USEtox is endorsed by the Life Cycle Initiative of the United Nations Environment Program (UNEP) and the Society for Environmental Toxicology and Chemistry (SETAC).

How to interpret the characterization factors?

In contrast to other impact categories such as global warming, the impact categories in USEtox do not use a reference substance. Instead, the characterization factors are expressed in terms of comparative toxic units (CTU) per kg of emission. With USEtox, characterization factors for human toxicity and freshwater ecotoxicity can be calculated at the midpoint level.

Human toxicity impacts: The comparative toxic unit for human toxicity impacts (CTUh) expresses the estimated increase in morbidity (the number of disease cases) in the total human population per unit of mass of the chemical emitted. In practice: disease cases per kg emitted = CTUh per kg emitted. In SimaPro, the characterization factors are reported as CTUh. USEtox takes into account both toxicity related to ingestion exposure and inhalation exposure. Characterization factors are available for carcinogenic as well as non-carcinogenic human toxicity impacts. Freshwater toxicity impacts: The comparative toxic unit for aquatic ecotoxicity impacts (CTUe) expresses the estimated potentially affected fraction of species (PAF) integrated over time and the volume of the freshwater compartment, per unit of mass of the chemical emitted. In practice: PAF × m³ × day per kg emitted = CTUe per kg emitted. In SimaPro, the characterization factors are reported as CTUe.

Choosing the right subset of characterization factors in SimaPro

The USEtox characterization factors for human toxicity and freshwater ecotoxicity are implemented in SimaPro and ready to use. When using USEtox in SimaPro, you can pick between two sets of USEtox characterization factors: one with so-called ‘recommended’ + ‘interim’ characterization factors, and one with only the ‘recommended’ characterization factors. The difference between recommended and interim characterization factors is the level of reliability of the calculations.

Interim characterization factors: For metals, dissociating chemicals, and amphiphilics (e.g. detergents), the uncertainty related to fate and exposure is high. A recommendation cannot be supported because the available data are insufficient or because the model considered is too uncertain. Therefore, these characterization factors are classified as interim to indicate that not all the minimum requirements are met for the calculation.

Recommended characterization factors: For the remaining chemicals, a scientific consensus has been reached on how to model them. For example, aquatic toxicity characterization factors must be based on data of the chemical’s effects on at least three different species from at least three different trophic levels or taxa in order to ensure a minimum variability of biological responses.

Using the characterization factors set with only the recommended characterization factors means that characterization factors for substances like metals and detergents are missing. The USEtox team advises to ALWAYS choose the combined set of recommended and interim characterization factors. Otherwise, no characterization factor is applied to the emissions of substances with high uncertainty in their fate and exposure, and consequently, these substances would be characterized with zero impact! Obviously, the environmental impacts of a product are underestimated this way.


If the emission of a substance with interim characterization factors dominates the overall impact, the LCA results are highly uncertain and should be interpreted with great care. An example can be found in metals. They are all modeled with interim factors and tend to dominate the effects of organic substances with several orders of magnitude in most LCAs. In cases where metal impacts dominate, it makes sense to apply a sensitivity analysis based on only the recommended characterization factors to see how the results, and possibly the conclusions, change. Doing a sensitivity analysis is the only meaningful reason to exclude interim characterization factors. I recommend LCA practitioners to be careful in communicating results that are dominated by interim characterization factors, even though using interim characterization factors is more useful than leaving them out.

Ongoing research

USEtox will be improved constantly by including new chemicals, new impact routes, etc. Ongoing research is being done, for instance, within the EU-funded project TOX-TRAIN. If improved data become available, or the model itself is updated, current interim factors could eventually be recalculated and become recommended factors if the new information allows them to fulfill the criteria.

Laura Golsteijn

Senior Consultant

I am eager to increase the environmental awareness of our society, and I believe that everyone can contribute to a more sustainable world, every day. At PRé we provide companies with both the knowledge and the tools to improve their products and services. I am excited to work for an organisation that is involved in developing sustainable initiatives.

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