## 4 Discussion
### Qualitative Results
Since the embedded toxicity also includes accidental additions to the toxicological footprint through contaminants absorbed during the use phase by a building material or component, cases could occur where excessive contaminations would yield embedded toxicological impacts exceeding embodied toxicological impacts. However, especially in the case of the analyzed wood samples for which only naphthalene was considered not from natural origin, this observation can also indicate the lack of precision of the current LCA methodologies and models for assessing human toxicity. The models needed for accounting for toxicological impacts in relation to a circular economy are very complex and rely on the (further) development of (1) Inventory models and data capable of accounting for the migration of chemical compounds in (circular) value chains (i.e., chemical migration inside the technosphere), (2) fate models (for estimating the dissipation of a chemical substance in the (indoor and outdoor environment) supporting both the inventory and characterization models), and (3) exposure/characterization models (to estimate the population exposure to a chemical substance under various building use situations) [30]. Each of these models is associated with considerable uncertainties and the final result of the estimation of the human toxicity of a building material/component used in the circular economy is inherently also accompanied by an even higher uncertainty [5]. Considering that the human indoor exposure during the use phase of a building material/component is only rarely taken into account no matter what LCA methodology applied, our results indicate that this omission should be a priority with regard to future research, as the use phase has the potential to contribute significantly to the total human toxicological impacts of these building materials. Furthermore, while this study is limited to a small sample size and number of materials, additional study of other materials, as well as the study of additional samples of materials that were assessed here could provide a strong indication for useful development pathways for impact assessment models and the development of new characterization factors. One other potential that this discussion illustrates is that there could be an argument for additions to fate and exposure modeling in LCA. In particular, the further development of accounting for embedded toxicity could help to capture some of the exposure discrepancies between the impacts expected in indoor environmental quality and the more global emission compartments (e.g., urban or rural air) traditionally included in LCA fate and exposure modeling, such as that found in e.g., USEtox [29] or USES-LCA 2.0 [44]. Additionally, the issues raised in this article with regard to temporal scope echo some of the issues presented in the literature on dynamic life cycle assessment [45; 46; 47] with regard to the handling of future emissions. This is due to the long (potentially centuries) service life of buildings and building components, especially when considering the circular economy. Furthermore, there is some potential for temporal differentiation methods that have been developed within dynamic life cycle assessment to aid in the development of models for approximating embedded toxicity and the need for temporal resolution in the assessment of exposure in indoor environmental quality. Lastly, the characterization factors are converted from CTUh to DALY using weighting factors, however, these weighting factors appear to not consider age weighting. This may pose a concern and add uncertainties to buildings with specific uses (i.e., different uses in different use cycles, e.g., single-family house, kindergarten, or office building).