In Life Cycle Impact Assessment (LCIA) the emissions which occur in the life cycle of a product are translated (characterised) into their potential impacts on the environment ranging from local impacts from land use over regional impacts due to e.g. toxic substances, acidification or photochemical oxidants to global climate change. For each category of impact (like global warming or photochemical ozone formation), the impact assessment applies substance-specific characterization factors (CFs) which represent the substance’s potency.
Figure 1. Life Cycle Impact Assessment translates the inventory of emissions from a product’s life cycle into an environmental profile of the product representing its potential contributions to a wide range of environmental impacts
An emission inventory for the life cycle of a product often contains hundreds of substances. It is thus estimated that 10-20 000 different chemicals are used in the life cycle of products marketed within the EU. Many of these substances have the potential to damage humans or ecosystems when released to the environment and should thus have characterization factors for the human and ecotoxicity categories of impact.
A number of different models have been developed for this purpose around the world over the last 15 years varying in their scope, applied modeling principles and not least in terms of the characterization factors they produce. These characterisation models all cover a limited number of substances, and the current situation for the LCA practitioner who wishes to include the chemical-related impacts in the impact assessment is thus that: (a) there will probably be many substances in the life cycle inventory for which no characterization factor is available from any of the models, (b) for some substances several of the models may have published characterization factors, but these often vary substantially between the models. The chemical-related impacts are hence often excluded from the LCIA which de facto reduces it to an energy impact assessment.
This unsatisfactory situation was the background on which a Task Force on Toxic Impacts under the UNEP-SETAC Life Cycle Initiative launched a comparison and harmonization of existing characterization models in order to
- Identify which differences in the old characterization models cause the observed differences in their characterization factors
- Develop a scientific consensus about good modeling practice based on the identified influential differences
- Harmonise the old characterisation models removing unintended but influential differences
- Develop a scientific consensus model based on the learnings from the comparison of the characterization models with the following characteristics:
o parsimonious (as simple a s possible, as complex as needed) containing only the model elements which were identified as the most influential in the comparison of the existing characterisation models;
o transparent and well documented;
o falling within the range of the existing characterisation models, i.e. not differing more from the old characterisation models than these differ among themselves;
o endorsed by the modellers behind all participating models.
The result of the scientific consensus model development is the USEtox™ model
Information about the development and characteristics of the USEtox™ model
The characterization model comparison and the development of the USEtox™ model have been described in a feature article in Environmental Science & Technology (http://dx.doi.org/10.1021/es703145t)
The USEtox™ model and the characterization factors that it provides as well as recommendations on how to assess toxicity in comparative assessments have been described in a paper in the International Journal of Life Cycle Assessment (http://dx.doi.org/10.1007/s11367-008-0038-4 - free access).