Small sizes of TiO2-NPs exhibit adverse effects at predicted environmental relevant concentrations on nematodes in a modified chronic toxicity assay system

► Chronic toxicity of TiO2-NPs was assessed in nematodes with a modified chronic toxicity assay system. ► Chronic toxicities of large sizes (60 nm and 90 nm) of TiO2-NPs in the range of μg/L were detected. ► Chronic toxicities of small sizes (4 nm and 10 nm) of TiO2-NPs in the range of ng/L were observed. ► We used locomotion behavior and ROS production to assess chronic toxicities of small sizes of TiO2-NPs. ► Chronic toxicities of small sizes of TiO2-NPs may be largely due to the induction of oxidative stress.