## 5 Conclusions  
This article raises several questions. Of paramount importance is the question of whether LCA is presently capable of assessing circular economic models of building materials/components. To answer this question, one must consider the four ISO defined LCA steps \"goal and scope\", \"inventory\", \"impact assessment\" and \"interpretation\". Regarding the goal and scope, it is primarily the scoping issue which is considered a major issue in relation to circular economic models for building materials, since, in modeling buildings and components of the built environment in the circular economy, impacts must be accounted for that are induced by emissions potentially taking place hundreds of years in the future. This issue poses challenges for validating LCA approaches for the circular economy in the built environment, as the service life can exceed the typical time horizon of an LCA and poses issues with regard to the lack of knowledge of the future status quo and its impact on characterization factors. The inventory step is also a cause for major concern in relation to circular economic models for building materials since models and data capable of describing how chemicals are absorbed by building materials/components and migrate in the value chain across time and space are lacking. In terms of impact assessment, it was illustrated through lack of coverage in the assessed building materials that the characterization models available today do not cover chemicals present in building materials that can be emitted during use. Thus, characterization models need to be further developed to enable the assessment of emissions from a product providing different services during its multiple use cycles. The issue of multiple use cycles with unknown provenance and fate necessitates the development of a more flexible setup than that of the exposure models applied in LCIA today. One might consider the need for developing characterization models where the user can select combinations of exposure scenarios. In addition, indoor exposure models also need to be developed for the characterization models, as the findings of this study confirm previous studies (e.g., [48]) of indoor environmental quality representing equal or greater potential impacts relative to external impacts. The interpretation step of LCA in terms of toxicological impacts also need further development to be capable of handling prospective normalization and weighting centuries into the future. Furthermore, compounding the issue of characterization, there is the inventory issue of uncertainty in provenance, where waste flows from decommissioned buildings are mixed, as is commonly done in Denmark, more extensive testing is needed to quantify the expected embedded toxicity of particular building components (e.g., concrete or wood). One other common aspect to the challenges induced by circular economic models for building materials for all ISO defined steps of LCA is that the temporal perspective that needs to be considered is so large that neither forecasting nor foresight provides a simple or easy solution. A potential solution to these challenges could be scenarios equivalent (however much more detailed and reaching even further into the future) to the commonly known Shared Socioeconomic Pathways [49], which could be introduced as a form of sensitivity analysis. The above-presented observations, questions, and challenges all lead to one very simple conclusion: If LCAs of circular economic models for building materials/components are to include toxicological impacts, which is necessary for providing a holistic picture of environmental impacts, then there is a need for continued method development. This potentially includes more focus on spatial and temporal coverage in addition to the further development of methods for quantifying and incorporating embedded toxicity into LCA.