## 1 Introduction
However, the results obtained applying each method to the same soil can be significantly different in terms of the estimated bioaccessible fractions due to the diverse experimental conditions adopted by each procedure [5]. Two of the most used methods are the UBM and SBET, which present huge differences in terms of extraction juices, time, and complexity. The UBM method was developed by the Bioaccessibility Research Group of Europe (BARGE), which was looking for a joint decision on a harmonized bioaccessibility method. Hence, it was chosen to adopt the procedure previously elaborated by RIVM. The UBM procedure was tested in an inter-laboratory trial [23]) and was validated in vivo for arsenic, cadmium, and lead. The UBM procedure is physiologically based and is aimed at mimicking mouth, stomach, and small intestinal cavities. Therefore, this method includes the preparation of four digestive fluids (i.e., saliva, gastric, duodenal, and simulated bile solutions) using both chemical reagents and enzymes that are sequentially put in contact at 37 \({}^{\circ}\)C with a soil sample in two extraction phases. Although this procedure effectively simulates at the laboratory scale the process that the soil undergoes once ingested, the preparation of the digestive solution is complex and the procedure can be labor intensive. The SBET represents a simplified form of the PBET procedure developed in response to some requests from USEPA (United States Environmental Protection Agency) regional offices [15]. Unlike the UBM method, the SBET is not physiologically based and has no digestive enzyme in the gastric fluid. Namely, the procedure consists in a single extraction step with a 0.4 M glycine (pH = 1.5) solution at 37 \({}^{\circ}\)C for 1 h simulating the stomach acidic environment. This method was initially validated against swine in vivo assay for lead [24; 25]. However, afterwards the SBET procedure was also applied for measuring the bioaccessible fraction of cadmium and arsenic [5] and more recently was also applied to other metals or metalloids (e.g., [26; 27]). Currently it is adopted by the USEPA as a standard procedure for evaluating the bioaccessibility of lead and arsenic [17]. The availability of standard in vitro procedures allowed for the incorporation of oral bioaccessibility into forward HHRA on soils [28] and toxic waste [29] for estimatingrisks for exposed receptors and into backward HHRA for the definition of soil cleanup goals [30]. However, the final outcome of the HHRA procedure depends on the bioaccessibility method used but also on how bioaccessibility is incorporated into HHRA. First, one aspect to consider is that despite the characterization of soil contamination usually being carried out on the particle size fraction below 2 mm, the different in vitro procedures are typically performed on finer size fractions, i.e., the <250 \(\upmu\)m fraction for the UBM and the <150 \(\upmu\)m for the SBET. Hence, it is crucial to decide to which total metal content to refer the bioaccessible concentration. According to [PERSON] et al. [28], considering the total inorganic content of the fine fraction would provide a more appropriate and conservative estimate of the concentration in ingested soil since the fine fraction tends to have higher inorganic concentrations in general and is more likely to be ingested by children. More recently, [PERSON] et al. [29] also performed both the bioaccessibility and the risk calculations on the fine fraction, for consistency, as they carried out the bioaccessibility tests on this fraction using the UBM. Bioaccessibility was found to depend on the soil particle size considered [31], but [PERSON] et al. [32] concluded that although a general trend of higher potentially toxic element (PTE) bioaccessibility in finer fraction is found, this does not mean that the highest PTE bioaccessibility is always found in the finest size fraction.