## 4 Discussion
### Qualitative Results  
All chemicals detected in the concrete samples were categorized as embedded into the material at some point in the value chain of concrete, while most chemicals identified in the wood samples are naturally occurring and hence stem from the production of the wood. The non-natural chemicals embedded in wood and concrete are most likely pollutants coming from paints, lacquers, or other materials used during maintenance, indicating that a considerable fraction of the chemicals detected were emitted from other sources and absorbed into the materials. It is striking that characterization factors for quantification of human toxicological impact potentials have only been identified for 5 out of the 18 chemicals identified during the experimental analyzes developed for assessing embedded toxicity. Of the identified chemicals, characterization factors currently only exist for benzaldehyde, benzene, hexane, naphthalene, and pyridine while they are missing for 2-norpinene, 3,6,6 trimethyl-3-care,\(\alpha\)-longipinene, \(\alpha\)-pinene, \(\alpha\)-terpinene, \(\alpha\)-terpineol, bornylacetate, cyclohexene, hexanal, 1-heptene, 2,4-hexadieneoic acid, 3-heptene-2, 6-dione, and decane. Of those chemicals, benzene is classified by the IARC as group 1 (carcinogenic to humans) and naphthalene and pyridine as group 2B (possibly carcinogenic to humans) [40]. Three of these chemicals have also been frequently observed in indoor air in residences. Among the 18 identified substances, naphthalene has been frequently monitored with representative concentrations in the indoor air of 0.1-1 \(\upmu\)g/m\({}^{3}\); this is also the case for benzene and hexane with representative concentrations in the indoor air of 1-10 \(\upmu\)g/m\({}^{3}\)[41]. The subtraction of chemical analytical equipment 'contaminations' from the results could potentially have led to invalid and much larger embedded toxicological impact potentials of the building materials. This issue was especially pronounced for concrete, where toluene was found in one of the samples. Toluene can be characterized using available characterization factors, which indicate that it is a chemical capable of inducing toxicological impacts and hence adverse effects on human health upon exposure. If toluene was not subtracted from the results, the embedded toxicity impact of concrete would appear to be even higher. For concrete, 45% of the identified chemicals could be characterized, while for wood only 4% could be characterized. This is a clear demonstration that extensive development of additional characterization factors is necessary for providing a more complete and meaningful assessment of human toxicity. It also indicates that the current state of development is not satisfactory when it comes to assessing circular economic models. The low fraction of characterizable chemicals also leads to the questions: can it be anticipated that further characterization factors will be developed in the short-term, and is this task too challenging, considering the 21,551 [42] chemical substances registered for use in the EU? In the Pharos database, developed by the Healthy Building Network [43], 756 chemicals have been identified in building materials, 184 of which were present in USERox and 67 of which were associated with human toxicity effect factors [12]. Further, considering the uncertainties of inventory and exposure models needed for appropriate assessment of toxicological impacts induced via the circular economy and the need to assess toxicological impacts in a uniform manner irrespective of origin, it is considered relevant to assess (and validate) if the current characterization paradigm applied in LCA is appropriate and relevant, or if characterization (of toxicological impacts) could be made more effective. It is also considered important that, even though the embedded toxicity of only 4% of the identified chemicals in wood could be assessed, the embedded potential carcinogenic impacts of wood are greater than the embodied equivalent.