The formation of secondary organic aerosol (SOA) produced from alpha-pinene ozonolysis was examined using a dynamic chamber system that allowed the simulation of ventilated indoor environments. Particle-bound reactive species (ROS) including peroxides, peroxy radicals and ions that could penetrate into the lungs and deliver oxidative stress to the tissue causing damage were quantitatively determined from filters collected from the chamber. ROS was determined using dichlorofluorescin such that resulting fluorescent intensities were converted to equivalent H(2)O(2) concentrations. Measured ROS concentrations at alpha-pinene and ozone concentrations relevant to prevailing indoor concentrations ranged from 1.1 to 7.2 nmol/m(3) of H(2)O(2). Particle density was also determined from scanning mobility particle sizer measurements and mass collected onto filters to obtain volume and mass concentration, respectively. Partitioning theory reveals the fact that with increased SOA mass loading, even for more volatile species, partitioning onto particle phase is favored relative to low SOA mass loadings. Other recent studies have found changes in composition of the SOA depending on the precursor VOC concentrations. This behavior was reflected in these experiments in terms of a change of density. Measured densities ranged from 1.07 to 1.69 g/cm(3). CONCLUSIONS A better understanding of the formation mechanism of secondary organic aerosol generated from indoor chemistry allows us to evaluate and predict the exposure under such environments. Measurements of particle-bound reactive oxygen species (ROS) shed light on potential adverse health effect associated upon exposure to particles.