Reactive Oxygen Species (ROS) are an important class of air pollutants generated from photochemical and ozone-initiated reactions in indoor and outdoor environments. Despite the fact that people spend most of their time inside buildings, indoor pollutants have received much less attention than outdoor pollutants. I led some of the first research studies on particulate ROS in indoor environments using methods involving UV-Vis spectrometry and electron paramagnetic resonance spectrometry. A significant fraction of indoor particulate ROS was found to exist on PM2.5 which is important from a health perspective since PM2.5 can carry ROS deep into the lungs. Indoor concentrations of ROS on PM2.5 were not found to be significantly different from outdoor ROS concentrations (on a sampling volume basis). However, the oxidative potential per particle mass was about 3.5 times higher indoors than outdoors for total suspended particles (TSP) which indicates that reactions taking place indoors likely increase the oxidative potential of indoor particles. I have also evaluated the potential health effects of exposure to ROS by exposing lung cells to products of limonene ozonolysis (which include ROS). The inflammatory response of the lung cells was greater when the cells were exposed to the products of limonene ozonolysis than when they were exposed to either ozone or limonene. My research highlights the need to include biologically relevant pollutants, such as ROS, in air quality studies.