## 3 Discussion  
This is the first study, to our knowledge, that aims to identify changes in human exhaled breath due to exposure to indoor dampness. The analysis of VOCs in exhaled breath is an emerging strategy for monitoring human metabolism [43; 44]. For this reason, it has been proposed as a promising approach to assess the effects caused by environmental contaminant exposure [21]. Our results clearly demonstrate that the exhaled breath from mothers from the NELA cohort [45] was affected by the presence of damp stains in their homes. Thus, the levels of 2-ethyl-1-hexanol in the exhaled breath were influenced by the residential dampness. In this regard, it was previously reported that this compound increases its concentration owing to the rise in relative humidity [46]. 2-Ethyl-1-hexanol is an indoor-air contaminant derived from di-(2-ethylhexyl) phthalate (DEHP), which is the most common plasticizer of poly(vinyl chloride) (PVC) [33] (Figure 7). Plasticizers provide PVC with new properties, such as flexibility. However, plasticizer migration is frequent when plastic products suffer from deterioration, since plasticizers are not strongly joined to polymer chains [47]. 2-Ethyl-1-hexanol is emitted after the hydrolysis (KEGG reaction number: R04202) of DEHP, and this reaction is increased in the presence of humidity [33]. In this sense, it has been shown that several microorganisms, such as bacteria, fungi and yeasts degrade the plasticizers, DEHP or di-2-ethylhexyl adipate (DEHA), producing 2-ethyl-1-hexanol from the hydrolysis of their ester bonds [48]. Further, previous studies indicated that the concentration of 2-ethyl-1-hexanol in indoor air was higher in the presence of damp stains [14; 40; 49; 50]. In this sense, our results show, for the first time, that levels of 2-ethyl-1-hexanol in human exhaled breath also fluctuate with the exposure to moisture in the home. Furthermore, exposure to this compound has been determined as the main cause of SBS. In addition, 2-Ethyl-1-hexanol is an endocrine disruptor associated with the development of several respiratory diseases, such as asthma [46; 49; 50; 51; 52; 53]. Indeed, high levels of 2-ethyl-1-hexanol in exhaled breath have recently been linked to asthma with coexisting atopic diseases in women [27]. By contrast, in the present study, a significant relationship was not observed between dampness exposure and human-exhaled-breath levels of aromatic compounds, which are traditionally reported to be linked with SBS development [32]. Figure 7: Production of 2-ethyl-1-hexanol detected in human exhaled breath in presence of residential humidity. Additionally, the eosinophil blood counts of the mothers in this study who lived in homes with humidity were higher than in those not exposed to -moisture. This result is in line with previous research outcomes [40; 41]. Indeed, eosinophil count in both blood and nasal lavage is a usual strategy for inflammation detection and has been suggested as a tool for the diagnosis of asthma and other respiratory diseases [22; 54; 55; 56]. Hence, the high eosinophil blood counts in mothers exposed to indoor dampness are evidence that prolonged exposure could cause inflammation. In this sense, although no significant association was found between humidity exposure and asthma in the adults, there was a significant association between allergic rhinitis and the presence of home humidity. In fact, a significant relationship between rhinitis in adults and the presence of humidity in houses was also reported by [PERSON] et al. [57]. In addition, a relationship between home dampness exposure and allergic rhinitis was also previously observed in a pediatric population [58]. Despite the fact that the association between the presence of humidity stains in houses and the appearance of respiratory or atopic diseases in children was not significant, the children who were exposed to them suffered more respiratory or atopic symptoms than the others.