Previous studies suggest that ultrafine particles (UFPs), including secondary organic aerosols (SOAs), may play an important role in triggering adverse health effects. Reactive indoor air chemistry related to production of indoor SOAs (ISOAs) is an important topic. We conducted a literature review of the toxic and adverse health effects of UFPs, including ISOAs. Furthermore, we conducted the review of engineered nanoparticles (ENPs) that a particle size is similar to UFPs and more toxic studies have been reported. Considering that the toxicological effects of ISOAs are dependent on particle size, the critical effect levels may be possible determined by comparing them to ENP levels. However, such effects would be also influenced by chemical properties or components of the particles. The goal to predict critical adverse effect levels of ISOAs may be achieved through the evaluation of key parameters such as structure - activity relationship, component analysis, and comparison of solubility with known nanoparticles.