Titanium oxide (TiO2) is a known photocatalyst, able to produce reactive oxygen species (ROS) when exposed to UV light. TiO2 phototoxicity has been abundantly demonstrated in aqueous solutions. Little is known; however, about its phototoxicity as an aerosol, particularly in the nano-size particle range. An experimental setting was developed to measure the oxidation capacity of TiO2 NP with or without exposure to UV light. TiO2 NP were generated using a Collison nebulizer, carried through a diffusion dryer and exposed to UV using the collimated beam of a solar light simulator. Their oxidation capacity was measured on-line via a photonic sensor based on multiscattering absorbance enhancement (MAE) strategy. The oxidative potential of aerosolized TiO2 nanoparticles is exacerbated by exposure to UV light. The oxidative response is affected by humidity and reaches its maximum in the 70%-90% relative humidity range. Gaseous hydrogen peroxide (H2O2) was found to be the predominant oxidative specie. Our results suggest that the use of TiO2 nanoparticles in outdoor environments or close to artificial UV sources lead to an involuntary exposure to ROS, to an extent greater than previously known. It raises also concerns about other environmental pollutants known to be UV photosensitive.