The flame retardants polybrominated diphenyl ethers (PBDEs) and their hydroxylated compounds (OH-PBDEs) are ubiquitously found in the environment. Human exposure to PBDEs occurs primarily via contaminated house dust. PBDEs are photolytically, chemically or metabolically transformed to more bioavailable and toxic products such as OH-PBDEs. OH-PBDEs may be converted to polybrominated dibenzo-p-dioxins (PBDDs) by solar or artificial UV light. We hypothesize that light irradiation of PBDEs and OH-PBDEs generates cytotoxic, genotoxic and cancer-initiating reactive products and that co-exposure to TiO2-nanomaterials enhances this reaction. To study this hypothesis we propose to 1) measure cytotoxicity, oxidative stress, genotoxicity, and cytochrome P450 (CYP) enzyme induction (as an indicator of the production of AhR agonist formation) in human HaCaT keratinocytes after exposure to irradiatiated OH-PBDEs, 2) determine the toxic effects of irradiation-products of PBDEs + nano-sized titanium dioxide (TiO2) or PBDEs alone and 3) investigate physico-chemical changes of OH-PBDEs and PBDEs + the toxicological impacts of PBDEs and OHPBDEs.