## 1 Introduction  
In the built environment, conventional life cycle assessment (LCA) can be used to assess the environmental impacts of building materials from cradle-to-grave in order to assist in the selection of materials, components, and designs based on environmental performance indicators [1]. Conventional LCA, as applied within the built environment, was developed for the assessment of product systems following a linear economic life cycle (i.e., production, use, and disposal of material) [2]. For products and services with long service life, like buildings, and in other areas of LCA, such as co-production, LCA has been adapted to enable its use. Additionally, LCA has had to be adapted to product systems that include extensive use phases where emissions might occur [3]. More specifically, in relation to the topic of this discussion, the building industry is moving towards a circular economy [4]. This shift requires prioritization based on environmental and performance criteria to ensure that materials/components for the built environment are chosen while considering both previous and future uses for optimizing their environmental performance. This accounting for both previous and future uses of materials can be problematic from an LCA methodological perspective (see e.g., [5]). Historically, the environmental performance assessment of building materials/components, as well as entire buildings, has focused on a few selected impact indicators mainly related to impacts induced by energy conversion [6]. As a result, more complex impact categories, such as toxicological impacts have not been given the same emphasis as the other more utilized and less uncertain impact categories, such as global warming potential. This also implies that life cycle inventory (LCI) analysis and life cycle impact assessment (LCIA) tools and models, and the data facilitating impact assessments induced by the elementary flows of chemicals with toxicological effects occurring over the long service lives and temporal scopes of buildings, have lagged behind the general development within LCI, LCA, and LCIA for the built environment. However, there have certainly been examples of LCA applied to the circular economy of building materials [7; 8] as well as efforts to introduce temporal issues such as non-linearity into characterization modeling [9]. Despite this, the general lack of representative tools, models, and data means that the human toxicological impacts induced by building materials during the use phase are often not accounted for, which is potentially problematic even in the assessment of a conventional linear economy, and can be seen in an analysis of typical LCIA methods applied in the built environment [10]. In a circular economy, where materials are reused--thus inhibiting the ability for current regulations for production to provide protections--the risk of exposing users to dangerous substances increases and should be assessed methodically. As pointed out by [PERSON] et al. [5], there is a major challenge in assessing environmental impacts in a circular economy with regard to allocating impacts from the initial production of a building component across all life cycles in which the building component is used. When it comes to the assessment of the toxicological impacts induced by a building component designed for the circular economy, the complexity of the assessment escalates. This is primarily because it is necessary to account for not only the distribution of the toxicological impacts induced by the initial production and subsequent life cycle stages including reuse and recycling, but also for the toxicity accumulated by the building material/component in one life cycle that can induce impacts in subsequent life cycles. For example, a concrete building component intended for the circular economy, which in its first life cycle was used in the garage of a single-family home where it was frequently exposed to fuels and solvents because the homeowner repaired cars, would have accumulated chemical residues over time.