## 2 Methods  
### Introduction to Case Study  
This study explores two different cases of recycled building materials: wood and concrete. Both materials were randomly sampled from the building materials for recycling at the Danish recycling facility RGS Nordic A/S in Roskilde, Denmark. The embedded toxicity of the materials was experimentally assessed using a setup consisting of a temperature-induced accelerated release of emitted chemicals from the building material samples in 250 mL DURAN(r) BlueCap flasks. The setup is intended to simulate an indoor environment situation with highly elevated temperatures. Among the chemicals emitted by the materials, the volatile organic compounds (VOCs) are extracted from the head space of the DURAN(r) BlueCap flasks using the Solid Phase Micro Extraction (SPME) passive sampler, 50 \(\upmu\)m/30 \(\upmu\)m divinylbenzene/Carboxen(r) on polydimethylsiloxane (DVB/CAR/PDMS) on an adsorbent StableFlexTM fiber [16]. The experimental setup applied represents a simplification and acceleration of an already existing and standardized laboratory testing method, in which material samples are analyzed for toxic substances such as VOCs, PCBs, metal, etc. The standardized test method involves 28 days of analysis in test chambers where the emitted substances are collected using passive samplers, such as Tenax(r) TA and 2,4-Dinitrophenylhydrazin (DNPH) tubes [17]. The simplified experimental setup used in our study is assumed to provide a representative scenario of the chemicals that can be emitted from a building material sample over a prolonged time horizon (centuries). Because of the lack of representative empirical inventory data and models including emissions and embedding scenarios for building materials (including use), there is a possibility that the temperature-induced accelerated release of chemicals monitored in our study can yield emissions of toxic chemicals, which under \"average real-world\" circumstances would not have been emitted to the air compartment. It is thus considered important to keep in mind the explorative nature of this study and the fact that the results obtained from our analyzes should be seen more as a measure or test of how far LCA is from being capable of supporting circular economical related decisions rather than a quantitatively accurate evaluation of the toxicity embedded in building materials. Additionally, should generalization about embedded toxicity in the circular economy of building materials be desired, more extensive testing would be necessitated. Materials, depending on their origin and previous uses, may contain complex mixtures of chemicals, some of which are naturally occurring. Whereas, other chemicals may occur in the material as a consequence of either intentional or unintentional incorporation into the material from exposure to chemicals during previous use. For instance, untreated wood contains complex mixtures of naturally occurring organic chemicals, such as acetic acid, formaldehyde, formic acid, and a variety of terpen [18]. Preserved wood is of great environmental concern due to chemical treatment(s) prior to use [19], though in the case of Denmark, some of the impregnating agents containing creosote and arsenic have been banned by law in Denmark since 2006 [20]. Additionally, wood intended for outdoor use can be treated with waterproofing agents and fungicides, which can both contain chemical substances that are banned for indoor use in the EU [19], creating a hurdle in terms of eventual recycling and reuse of wood. Toxic additives, such as aliphatic amines, dieldrin, various salts, and epoxide are frequently used in the production of concrete. The material itself also has the potential to absorb secondary hazardous chemicals during production and use, e.g., bitumen, chlorinated paraffin, phthalates, polychlorinated biphenyl (PCB), polycyclic aromatic hydrocarbons (PAH), as well as heavy metals such as mercury [21; 22].