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Freshwater algae show low-to-moderate susceptibility to TiO₂ exposure, with more pronounced toxic effects in the presence of UV irradiation. It has also been shown that nano-sized TiO₂ is significantly more toxic to algae Pseudokirchneriella sub-capitata than submicron-sized TiO₂. Hund-Rinke and Simon  reported that UV irradiated 25 nm TiO₂ NPs are more toxic to green freshwater algae Desmodesmus subspicatus than UV irradiated 50 nm particles, which is in agreement with Hartmann et al. UV irradiated TiO₂ NPs also inactivated other algae species such as Anabaena, Microcystis, Melsoira and Chroococcus. It was demonstrated that smaller particles have a greater potential to penetrate the cell interior than submicron-sized particles and larger aggregates. Studies have shown that the amount of TiO₂ adsorbed on algal cells can be up to 2.3 times their own weight.

When used in an ultrafine-grade formulation, titanium dioxide becomes transparent to light, effectively making it an absorber of UV light. And because its particles are so small in this form, titanium dioxide creates a transparent barrier that absorbs UV light.