Frequently Asked Questions

USEtox is a model based on scientific consensus providing midpoint characterization factors for human and freshwater ecotoxicological impacts of chemicals in life cycle impact assessment, developed under the United Nations Environment Program (UNEP) and the Society for Environmental Toxicology and Chemistry, (SETAC) Life Cycle Initiative. USEtox does not represent the latest scientific state of the art, but a scientific consensus defining best application practice as an interface between ever advancing science and a need for stability, parsimony, transparency, and reliability.

The USEtox model operates on two scales; the continental scale and the global scale. The continental scale consists of six compartments, namely urban air, rural air, agricultural soil, natural soil, freshwater, and coastal marine water. The global scale has the same compartmental structure, but without urban air (see figure).

The main output of USEtox are interim and recommended characterization factors, which should always be used together. Excluding interim characterization factors is in principle only meaninful for sensitivity analysis in an life cycle assessment study. The model and database include environmental fate, exposure, and effect parameters for human toxicity and freshwater ecotoxicity. Characterization factors are calculated by following three steps:

1. Environmental fate, where the distribution and degradation of each substance is modelled,
2. Exposure, where the exposure of humans, animals and plants is modelled, and
3. Effects, where the inherent damage of the substance is modelled.

USEtox will be constantly improved (by including new chemicals, etc.) and is recommended for comparative chemical toxicity assessment by the European Union, the United States - Environmental Protection Agency, and other organizations. USEtox is increasingly used in a wide range of national and international projects as well as in industrial life cycle assessment studies. For details see Publications.

If you refer to USEtox and/or its characterization factors in general, please cite the following publications:
Rosenbaum, R.K., Bachmann, T.M., Gold, L.S., Huijbregts, M.A.J., Jolliet, O., Juraske, R., Koehler, A., Larsen, H.F., MacLeod, M., Margni, M.D., McKone, T.E., Payet, J., Schuhmacher, M., van de Meent, D., Hauschild, M.Z., 2008. USEtox - The UNEP-SETAC toxicity model: Recommended characterisation factors for human toxicity and freshwater ecotoxicity in life cycle impact assessment. The International Journal of Life Cycle Assessment 13, 532-546.

Hauschild, M.Z., Huijbregts, M.A.J., Jolliet, O., Macleod, M., Margni, M.D., van de Meent, D., Rosenbaum, R.K., McKone, T.E., 2008. Building a Model Based on Scientific Consensus for Life Cycle Impact Assessment of Chemicals: The Search for Harmony and Parsimony. Environmental Science and Technology 42, 7032-7037.

If you refer to USEtox human exposure and/or toxicity modeling, please cite in addition the following publication:
Rosenbaum, R.K., Huijbregts, M.A.J., Henderson, A.D., Margni, M., McKone, T.E., van de Meent, D., Hauschild, M.Z., Shaked, S., Li, D.S., Gold, L.S., Jolliet, O., 2011. USEtox human exposure and toxicity factors for comparative assessment of toxic emissions in life cycle analysis: sensitivity to key chemical properties. The International Journal of Life Cycle Assessment 16, 710-727.

If you refer to USEtox fate and/or freshwater ecotoxicity modeling, please cite in addition the following publication:
Henderson, A.D., Hauschild, M.Z., van de Meent, D., Huijbregts, M.A.J., Larsen, H.F., Margni, M., McKone, T.E., Payet, J., Rosenbaum, R.K., Jolliet, O., 2011. USEtox fate and ecotoxicity factors for comparative assessment of toxic emissions in life cycle analysis: sensitivity to key chemical properties. The International Journal of Life Cycle Assessment 16, 701-709.

The USEtox team intends to regularly publish updated versions of the software as well as updates to model inputs and outputs. Corrections affecting existing characterization factors that are found to be potentially erroneous will be made once per year (corrective updates). Updates based on data, scientific and technical progress that affect existing characterization factors will be made at the maximum once a year, while updates adding new characterization factors, but not affecting existing/published characterization factors can be made anytime (progress-based updates). Further information is given in the full Update procedure document.

The USEtox model supplies a stable method and the version number can be found on the sheet “Version” of the USEtox model file. The latest available USEtox version will always be available in our USEtox Download section.

Substances:
 - Metals
 - Dissociating chemicals
 - Organometallics
 - Amphiphilics

Other aspects:
 - Indoor compartment
 - Pesticide chemical property data
 - Human exposure to pesticides

The official USEtox model documentation can be found on this website under Model Documentation.

The official version of USEtox does not include particulate matter (PM) at the moment. Other atmospheric chemical fate models can better describe the fate of particulate matter in the air, including the formation of secondary particles from NH₃, NO_X, and SO₂ emissions. The International Reference Life Cycle Data System (ILCD) Handbook recommends using the characterization factors computed in Humbert (2009). These characterization factors have their intake fractions based on an adaptation of USEtox for primary PM and CO, Greco et al. (2007) for secondary PM from SO₂ and NO_x, Van Zelm et al. (2008) for secondary PM from NH₃ and RiskPoll (Spadaro and Rabl 2004) to differentiate among high-stack, low-stack, and ground-level emissions of primary PM for urban and rural conditions, respectively, and their effect factors based on a combination of Van Zelm et al. (2008), Kuenzli et al. (2000), and Hofstetter et al. (1998).

The current version of USEtox only models impacts in the freshwater environment. The reason only freshwater impacts are investigated is that mainly toxicity tests on freshwater species are available.

The damage matrix links the number of incidences (i.e. cancer or non-cancer cases) to the potential consequences of a chronic toxicological effect. It accounts for severity of the diseases/incidents considered. As there is a high uncertainty related to the severity and a lack of evidence for significant differences when combining dose-response slope and severity of disability, the damage matrix and differences between cancer and non-cancer have been left out in the current version of the USEtox model.

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
CTUe : comparative toxic unit for aquatic ecotoxicity impacts
PAF    : potentially affected fraction of species

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 “interim” 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 “interim” means 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 interim 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, but are all considered "interim", i.e. these factors can be used, but should be interpreted with care, since they have much 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.