The environmental and health impacts of nanomaterials are becoming important topics of research in recent years. The unique advantages offered by these nanomaterials in wide range of applications cannot be realized until these concerns are resolved. Among all the nanomaterials, Ag nanoparticles, due to their existing extensive uses in commercial products, demand immediate attention. Since the nanoparticle suspensions will be exposed to environmental conditions different from a research lab setting, many factors, including light, temperature, salinity, etc., are suspected to affect the stability of the nanoparticle and also their toxicity. In this study, we examined the effect of sunlight on the stability and toxicity of 6 and 25 nm Ag nanoparticles coated with gum arabic (GA) and polyvinylpyrrolidone (PVP). Under sunlight irradiation, all of these nanoparticles irreversibly aggregated to different degrees depending on the surface coating. The UV content of the sunlight is identified to be the driving force of nanoparticle aggregation, and the strong oscillating dipole dipole interaction is believed to be the origin of the destabilization. Toxicity examinations of the nanoparticles to a wetland plant, Lolium multiflorum, indicate that their toxicity is greatly reduced after sunlight irradiation.