Photo-active nanomaterials absorb sunlight and directly convert it into more useful form of energy (electricity). They are being increasingly used in commercial products, indicating that they may be one of the ways to address environmental concerns. However, exposure of these nanomaterials to humans, other terrestrial life, and aquatic life could have short-term and long-term adverse impacts on them. Transformation of these nanomaterials in the environment would play a vital role in the toxicity, transport, and fate of these nanomaterials in the environment. Human exposure to nanomaterials can occur during manufacturing, transportation, storage, and use of the consumer goods. As nanomaterials are taken up by plants and animals they may enter the food chain and lead to biomagnification, which can be potentially harmful for animals in the higher trophic levels and for humans. Here, we discuss toxicity studies of some photo-active nanomaterials conducted in the past, possible toxicity mechanisms, means of exposure, transformation, and transport to some extent. Factors that need to be considered to assess the environmental impacts of these nanomaterials are also discussed in detail.