## 5 Conclusions  
The outdoor studies have made a remarkable progress toward the understanding of the importance, composition, sources, and transformation of the WSOC present in ambient PM. Nevertheless, because of the complex nature of aerosol WSOC, which reflects into both the aerosol-climate and aerosol-health interactions, there still remain many open questions to be addressed by the outdoor air chemistry community [4; 5; 10]. One of these issues concerns the relationship between the oxidative and pro-inflammatory potential of aerosol WSOM and their atmospheric concentrations and structural characteristics. In indoor air, however, much remains to be uncovered on the physicochemical characteristics, sources, dynamics, and health effects of particulate WSOC. Indeed, this water-soluble OA fraction constitutes a new challenge for indoor air quality, for which the outdoor experience would assist in delineating and prioritizing research on this indoor OA component. The water-soluble organic compounds including both the gas- and particulate-phases are clearly an important fraction of indoor environments. Based on the outdoor experience, one should expect a particularly complex chemical composition for the water-soluble OA indoors. In a similar vein, it is unlikely that a full representation of the state of all water-soluble organic species will be achieved throughout their lifetime in indoor environments. However, the progress reached in resolving the chemical complexity and impact of aerosol WSOC in outdoor scenarios will help to delineate a set of objectives for future research indoors including (but not limited to):  
1. selection of the measurement location(s), considering the spatial intra- and interbuilding heterogeneity, which is ultimately closely linked to the purpose of investigation;
2. understanding the indoor WSOC composition and its dynamics at the structural or molecular level through the application of both online (e.g., AMS) and offline (e.g., NMR spectroscopy and HR-MS) sophisticated multidimensional analytical techniques;
3. ensure the collection of an adequate and representative amount of aerosol WSOC for subsequent offline structural and molecular characterization;
4. identification and quantification of indoor SOA components in either chamber experiments or field studies through the application of powerful analytical techniques (e.g., HR-ToF-AMS);
5. elucidation of indoor SOA formation pathways and OA aging processes using laboratory or chamber experiments;
6. apportionment of the indoor water-soluble OA among different sources based on the identification of unique source and process organic markers (i.e., source fingerprinting), and application of sophisticated statistical approaches for both data mining and source apportionment; and
7. establish a relationship between exposure to indoor water-soluble OA and oxidative and inflammation induction. With the research goals previously outlined, it is expected to enhance clarity and encourage discussion within the indoor air chemistry community on the need to include the water-soluble OA as a key particle-phase component in indoor air quality studies. [PERSON] and A.C.D. contributed equally to this feature paper. All authors have read and agreed to the published version of the manuscript. Thanks are due to FCT/MCTES for the financial support to CESAM (UIDP/50017/2020+UIDB/50017/2020) through national funds. [PERSON]. [PERSON] gratefully acknowledge the support of the European Cooperation in Science and Technology (COST) through the INDoor AIR POLLution NETwork (INDAIRPOLLNET). [PERSON] also acknowledges the financial support from FCT/MCTES through an Exploratory Research Project (IF/00798/2015/CP1302/CT0015, Investigator FCT Contract IF/00798/2015). This work was also supported by project AMBIEnCE (PTDC/CTA-AMB/28582/2017) funded by FEDER, through COMPETE2020-Programa Operacional Competitividade e Internacionalizacao (POCI), and by national funds (OE) through FCT/MCTES.