Descripción general del proceso de incubación

This study has shown that PFOS and PFOS precursors are still detectable in a high percentage of samples from indoor dust samples in Norwegian population (100 % positive samples for PFOS) , even though their use was restricted in 2002 and PFOS was included as a POP under the Stockholm convention in 2009 (UNEP, 2010). After more than 15 years of restriction, they remain present at measurable concentrations in indoor dust samples (average concentration ΣPFAS reported in this study of 77.2 ng/g). This fact indicates that indoor environments are still a source, or a reservoir, for PFASs.

Since the early 2000s, PFOS has been measured in indoor dust samples by many authors (Moriwaki, Takata and Arakawa, 2003; Björklund, Thuresson and De Wit, 2009; Kato, Calafat and Needham, 2009; Ericson Jogsten et al., 2012; Fraser et al., 2013; Xu et al., 2013; Shoeib et al., 2016) in different indoor microenvironments like homes, offices and cars. Since then, trends in average values have been decreasing from the average value of 443.7 ng/g detected by Kubwabo et al. (Kubwabo et al., 2005) for Canadian home dust, 640.7 ng/g by Goosey & Harrad (Goosey and Harrad, 2011) for English classrooms, or 175.2 ng/g by Bjorklund et al. (Björklund, Thuresson and De Wit, 2009) for Swedish homes, but all of them reporting different orders of magnitude data depending on the location of the study (see Table 03).

More recent studies reported lower concentrations for these indoor micro environments, as Tian et al. (Tian et al., 2016) who reported average values of 13.7 ng/g for Korean houses or Shoeib et al. (Shoeib et al., 2016) who reported median values of 0.29 ng/g for Egyptian homes, but still relevant in terms of both, concentrations and detection frequencies. Average values found in this study for

PFOS (average = 8.95 ng/g), are in line with the decreasing trends, much lower than the reported by Goosey and Harrad (Goosey and Harrad, 2011) for the UK (average = 144.7 ng/g), and slightly lower than the value reported by Haug et al.

(Haug, Huber, Schlabach, et al., 2011) for the Norwegian population (average = 10.9 ng/g), even though this last ones corresponded to settled dust (sampled in 2010) – usually presenting lower concentrations of PFASs – versus the analysed vacuum cleaner bags dust (sampled in 2014) reported in this thesis.

Interestingly, as Figure 18 and Figure 19 revealed, the overall indoor exposure to PFOS and PFOS precursors in this study is not dominated by PFOS, although this is the most frequently determined PFASs from those included in this study. Instead, the profile was: EtFOSA (40 %), followed by MeFOSE (32 %), EtFOSE (15 %) and PFOS (12 %). It is well known that perfluorinated substances like EtFOSA – among other FOSAs – were employed in grease and water repellent coatings (Tittlemier, Pepper and Edwards, 2006). As a consequence of it, its presence in indoor microenvironments is feasible due to its release from coatings and later deposition in dust particles. On the other hand, FOSA was present in most of the samples (DF = 96 %) but in much lower concentrations, contributing around 1 % to the overall PFASs contribution. These findings are in agreement with recently reported papers where the underestimation of PFOS precursors was suggested when evaluating the overall exposure to PFOS, and its consequent body burdens (Vestergren et al., 2008; D’Eon J and Mabury, 2011; Gebbink, Berger and Cousins, 2015). Moreover, statistical regression analysis showed significant positive correlations between PFOS and FOSA, EtFOSE and MeFOSE, as well as EtFOSA and PFOS, suggesting common sources of indoor contamination with these PFASs, in

agreement with previous published studies (Strynar and Lindstrom, 2008;

Björklund, Thuresson and De Wit, 2009; Shoeib et al., 2016). This information is also in agreement with the need to further investigate the sources of contamination by PFOS and its precursors.

Concentrations for FOSAs and FOSEs have been reported by Goosey & Harrad (Goosey and Harrad, 2011), Haug et al. (Haug, Huber, Schlabach, et al., 2011), Shoeib et al. (Shoeib et al., 2016) and Fraser et al. (Fraser et al., 2013). Different patterns of exposure were identified when comparing this study with the English and Norwegian cohorts, which could be partially attributed to differences between the participants, seasonal differences leading to air/dust distribution changes and ventilation habits, and the origin of the collected dust. Still, all of the studies revealed appreciable concentrations of EtFOSA, MeFOSA, and MeFOSE.

The sources of PFOS and PFOS precursors could not be determined from correlations with personal data, indoor questionnaires and room contents, besides the age of the participants, the proximity to industrial areas and the renovation of the house. Some other parameters like age and origin of the furniture, carpets or waterproof clothing were not included in the indoor questionnaires, some of which could contribute to a deeper knowledge of indoor exposure origin.

Average estimate daily intake of ΣPFAS for the investigated cohort was estimated to be 4.51 pg/kg bw/day in mean scenario, and 59.7 pg/kg bw/day for the highest one.

In case of children population, worst case scenario (high intake scenario, 95^th percentile) showed an estimated daily intake of 2.6 ng/kg bw/day, two orders of magnitude higher than the values reported for adult population. Still, these values

are significantly lower than the tolerable daily intakes of 150 ng/kg bw/day (EFSA, 2008), and 100 ng/kg bw/day (BfR) promulgated by European food authorities.

Moreover, indirect exposure pathways and their certain contribution to the overall body burdens of PFOS remain partially unknown. Metabolic pathways and conversion ratios, discussed later in this thesis, need further research to better model, and understand the link between external and internal exposure to PFOS precursors, which nowadays are present in Norwegian indoor environments in higher levels (88 %) than the historically reported PFOS (12 %).