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Frequently Asked Questions - How to use USEtox characterization factors

USEtox calculates characterization factors for human toxicity and freshwater ecotoxicity at midpoint level. 

The characterization factor for human toxicity impacts (human toxicity potential) is expressed in comparative toxic units (CTUh), the estimated increase in morbidity in the total human population, per unit mass of a chemical emitted, assuming equal weighting between cancer and non-cancer due to a lack of more precise insights into this issue.
Unit: [CTUh per kg emitted] = [disease cases per kg emitted]

The characterization factor for aquatic ecotoxicity impacts (ecotoxicity potential) is expressed in comparative toxic units (CTUe), an estimate of the potentially affected fraction of species (PAF) integrated over time and volume, per unit mass of a chemical emitted.
Unit: [CTUe per kg emitted] = [PAF × m³ × day per kg emitted]

SimaPro has implemented USEtox characterization factors for human toxicity and freshwater ecotoxicity.

The characterization factor for human toxicity impacts has the following units:
CTUh per kg emitted (unit in SimaPro) = disease cases per kg emitted (unit in USEtox)

The characterization factor for aquatic ecotoxicity impacts has the following units:
CTUe per kg emitted (unit in SimaPro) = PAF × m³ × day per kg emitted (unit in USEtox)

where
CTUh : comparative toxic unit for human toxicity impacts
CTUe : comparative toxic unit for aquatic ecotoxicity impacts
PAF    : potentially affected fraction of species

GaBi has implemented USEtox characterization factors for human toxicity and freshwater ecotoxicity.

The characterization factor for human toxicity impacts has the following units:
CTUh per kg emitted (unit in GaBi) = disease cases per kg emitted (unit in USEtox)

The characterization factor for aquatic ecotoxicity impacts has the following units:
CTUe per kg emitted (unit in GaBi) = PAF × m³ × day per kg emitted (unit in USEtox)

where
CTUh : comparative toxic unit for human toxicity impacts
CTU: comparative toxic unit for aquatic ecotoxicity impacts
PAF    : potentially affected fraction of species

For considering consumer exposure, a near-field/far-field modeling framework was developed and is available (Fantke et al. 2016) that was recommended in the global UNEP-SETAC GLAM effort to be implemented in USEtox’ next update (see Pellston report 2019). This framework has already been applied in various case studies, is fully mass balance based and aligned with USEtox, which allows to combine impacts from product constituents with impacts from emissions along product life cycles.

USEtox currently applies a fixed acute-to-chronic ratio of 2 as a statistical average across several chemicals (see Jolliet et al. 2003) and is confirmed by current, much broader studies (Posthuma et al. 2019, Aurisano et al. 2019). This fixed ratio does not account for variability across chemicals and species.

For human toxicity, cancer and non-cancer health impacts are considered separately as indicators of disability in USEtox. Cancer effects rely to a large extent on TD50 (the dose eliciting a 50% population response) values that indicate potency to develop a tumor. Cancer is one type of tumor-related disease outcome and strongly associated with mutagenic properties of a chemical. As for reproductive toxic effects, these are captured in non-cancer effect factors whenever underlying reproductive toxic test data are available. Since all underlying available test data per chemical should be used to derive USEtox effect factors from the lowest converted human lifetime dose per exposure route, all underlying effects are considered in USEtox.

In USEtox, persistent and bioaccumulative chemicals are considered and included in the chemical database. These compounds typically have large characterization factors due to the significant potential for exposure through multiple pathways. If such substances are moderately soluble, this will yield lower eco-exposure factors. However, due to the generally large fate factors (i.e. long persistence in the environment) and higher bioaccumulation potential (reflected in high effect factors), overall characterization factors for such substances are typically large as compared to other chemicals.

All characterization factors of the current version of the USEtox model have been implemented in GaBi, SimaPro, OpenLCA, and the Quantis SUITE 2.0. We have no complete overview of all LCA software and their status of implementation for USEtox characterization factors. Hence, there might be other software that also has the factors included.

Yes, the so-called “indicative” USEtox characterization factors should always be used together with the “recommended” factors, as otherwise the substances concerned would be characterized with zero impact as no characterization factor is applied to their emissions. The flag “indicative” reflects a higher uncertainty of the characterization factor compared to the flag “recommended”, because not all the minima requirements are met for the calculation. Therefore, when an emission characterized with indicative characterization factors is dominating the overall impact, it implies that the associated results have to be interpreted as having a lower level of confidence. A sensitivity study might by performed by applying only the recommended characterization factors to see if and how the results (and the conclusions) change.

Emissions to air
- Urban air: Use Urban air characterization factors to assess emission to "high population density".
- Continental rural air: Use Continental rural air characterization factors to assess emissions to "low population density", "lower stratosphere/upper troposphere".
- Calculate a new weighted characterization factor based on 50% Urban air + 50% Continental rural air characterization factors to assess "unspecified" emissions.

Emissions to water
- Continental freshwater: Use Continental freshwater characterization factors for the water emission subcategories "lake", "river", "ground", and "unspecified".
- Coastal water: Use Coastal Water CFs to assess emissions into ocean.

Emissions to soil
- Continental agricultural soil: Use Continental agricultural soil characterization factors to asses emissions to "agriculture".
- Natural soil: Use Natural soil characterization factors to asses emissions to "forestry", "unspecified" or "industrial".

USEtox does not make use of a reference substance, but expresses the characterization factors in terms of comparative toxic units (CTU) per kg emitted.

The characterization factor for human toxicity impacts (human toxicity potential) is expressed in comparative toxic units (CTUh), the estimated increase in morbidity in the total human population, per unit mass of a chemical emitted, assuming equal weighting between cancer and non-cancer due to a lack of more precise insights into this issue.
Unit: [CTUh per kg emitted] = [disease cases per kg emitted]

The characterization factor for aquatic ecotoxicity impacts (ecotoxicity potential) is expressed in comparative toxic units (CTUe), an estimate of the potentially affected fraction of species (PAF) integrated over time and volume, per unit mass of a chemical emitted.
Unit: [CTUe per kg emitted] = [PAF × m³ × day per kg emitted]

Characterization factors for metals are available as part of the current version of USEtox and have been updated to consider e.g. speciation since USEtox 2.0. USEtox factors for metals, however, are considered "indicative", i.e. these factors should be used, but should be interpreted with care, since they have higher uncertainty than recommended characterization factors.

A set of 3 characterization factors can be reported, namely "carcinogenic", "non-carcinogenic" and "total", of which the latter is the sum of carcinogenic and non-carcinogenic effects.

Yes, the continental scale is the box where all the emission shall occur. The global scale is the box that is only there to capture the potential impact generated by transboundary emissions of persistent and long range chemicals at a global scale. Characterization factors for an emission at continental scale already include the cumulative impacts generated within the continental and the global scale. The global scale box should therefore not be used as an emission box.