Aged TiO₂‑Based Nanocomposite Used in Sunscreens Produces Singlet Oxygen under Long-Wave UV and Sensitizes <i>Escherichia coli</i> to Cadmium

TiO₂-based nanocomposite (NC) are widely used as invisible UV protectant in cosmetics. These nanomaterials (NMs) end in the environment as altered materials. We have investigated the properties of T-Lite SF, a TiO₂–NC used as sunscreen, after weathering in water and under light. We have examined the formation of ROS and their consequences on cell physiology of <i>Escherichia coli</i>. Our results show that aged-T-Lite SF produced singlet oxygen under low intensity long wave UV and formed hydroxyl radicals at high intensity. Despite the production of these ROS, T-Lite SF had neither effect on the viability of <i>E. coli</i> nor on mutant impaired in oxidative stress, did not induce mutagenesis and did not impair the integrity of membrane lipids, thus seemed safe to bacteria. However, when pre-exposed to T-Lite SF under low intensity UV, cells turned out to be more sensitive to cadmium, a priority pollutant widely disseminated in soil and surface waters. This effect was not a Trojan horse: sensitization of cells was dependent on the formation of singlet oxygen. These results provide a basis for caution, especially on NMs that have no straight environmental toxicity. It is crucial to anticipate indirect and combined effects of environmental pollutants and NMs