## 4 Water-Soluble Indoor Organic Aerosols: Challenges and Opportunities
### Why Should Aerosol WSOC Be Considered in Indoor Air Chemistry Studies?
Additional reasons to include the analysis of WSOC in indoor air chemistry studies are outlined in the following paragraphs. As aforementioned, the WSOC fraction present in indoor PM is still poorly characterized, although there have been a few studies that have employed the WSOC as an indicator of SOA formation in indoor source apportionment studies [88, 89, 92, 108]. Nonetheless, there is a certain level of uncertainty that one should be aware of when using the content of aerosol WSOC to perform indoor SOA estimates in a straightforward manner. First, the multiplication factor used to convert WSOC to SOA (a OM-to-OC ratio of 2.5) and the assumption that 20% of the SOA is water-insoluble OC [92]. Second, the aerosol WSOC can also have contributions from primary indoor emissions of water-soluble organics such as those from cleaning and other consumer products, cooking, and biomass burning [88, 89, 92]. Addressing these two issues alone should prompt a comprehensive investigation on the quantity and chemical features of the WSOC fraction present in indoor PM, considering different indoor spaces, occupancy purposes, and conditions as well as locations (e.g., urban, and rural). Furthermore, considering that indoor SOA formation can be a substantial fraction of indoor aerosols [25; 102], and that indoor SOA composition is dominated by multifunctional and low-volatile highly oxygenated organic molecules [26; 102], whose characteristics may resemble those of WSOC, more measurements of both WSOC and SOA composition indoors are warranted in order to improve the current knowledge on indoor air composition. Another matter of debate is the oxidative aging of indoor OA, leading to changes in oxygen content and volatility, which may ultimately enhance the condensed-phase OA concentration [109]. Indoor chemical processes and chemical transformations have been highlighted as a research priority by the INDoor AIR POLLution NETwork (INDAIRPOLLNET), recently supported by the European Cooperation in Science and Technology (COST) [100]. Considering that for ambient air, the aging process in the atmosphere leads to the formation of increasingly oxidized, less volatile, more hygroscopic, and more water-soluble OA [7], it would be beneficial to consider the secondary formation and evolution of WSOC when predicting indoor OA concentrations. Recently, indoor WSOGs, comprising carbonyl compounds, carboxylic acids, epoxides, organic peroxides, organic nitrates, amines, and phenols, have been described as an ubiquitous and abundant component in residential indoor air, with potential implications on indoor air chemistry, air quality, and human health [27; 110; 111]. Nevertheless, little is still known about the concentration, composition, dynamics, fate, and health effects of WSOGs indoors. As it is quite possible that the indoor air composition of WSOGs is altered by aqueous chemistry reactions (in indoor surface films, skin or in wet particles) [27], one could argue that WSOGs will partition into the aqueous-phase and react further, thereby becoming less volatile and leading to the formation of condensed-phase secondary WSOC. This hypothesis is also critically needed to be assessed to predict its possible impact on indoor air composition and OA content. As highlighted in Section 3, exposure to ambient fine aerosol WSOC is a potential driver of both ROS generation and pro-inflammatory effects, being therefore considered an important and prevalent form of air pollution. In indoor environments, however, very limited information is available on the cytotoxic effects upon exposure to the water-soluble OA component. Nonetheless, if one considers that indoor and outdoor PM\({}_{2.5}\) share a similar bioactivity behavior including inflammatory reaction, oxidative stress, genetic toxicity, mutagenicity, and carcinogenicity [112], one could likely expect that exposure to healthy relevant indoor PM\({}_{2.5}\) constituents might also trigger toxicity effects. In this regard, [PERSON] et al.