Hazard evaluation of polystyrene nanoplastic with nine bioassays did not show particle-specific acute toxicity

Plastic is a wide-spread pollutant and must be evaluated for potential adverse effects of its breakdown product, microplastic (≤5 mm) along with its subfraction, nanoplastic (1-100 nm). Risk assessment of pollutants cannot be conducted without their toxicity (dose-response) data. In this study, toxicity of polystyrene nanoplastics (PS-NPL) was evaluated using 8 acute and 1 subchronic toxicity assays with 10 organisms of different biological complexity (bacteria, yeast, algae, protozoans, mammalian cells in vitro, crustaceans, midge larvae). Commercial 26 and 100 nm carboxylated PS-NPL spheres were chosen as model and tested in nominal concentrations up to 100 mg/L (1.025·10¹⁶ 26 nm and 1.83·10¹⁴ 100 nm particles/L). In most of the assays, both PS-NPL proved non-toxic (L(E)C₅₀ > 100 mg/L) but three tests (V. fischeri, R. subcapitata, D. magna) flagged toxicity in 'as received' 26 nm PS-NPL and D. magna also in 100 nm PS-NPL (EC₅₀ ranging from 13 to 71 mg/L). As, according to manufacturers, both PS-NPL suspensions contained additives (surfactants and biocidal NaN₃), the three toxicity tests were repeated also on dialysed PS-NPL and on NaN_3. Non-toxicity of dialysed PS-NPL indicated that the toxicity of 'as-received' PS-NPL was not particle-specific but false positive due to water-soluble additives in the PS-NPL preparations. NaN₃ was very toxic to D. magna (48 h EC₅₀ = 0.05 ± 0.03 mg NaN₃/L), toxic to R. subcapitata (72 h EC₅₀ = 4.97 ± 3.7 mg NaN₃/L) and non-toxic to V. fischeri. Toxicity of 'as-received' PS-NPL was not fully explainable by NaN₃ but also attributable to other additives in the suspensions. Toxicity research of microplastic using commercial model particles must always consider the potential influence of additives, e.g. test the toxicity of dialysed NPL for comparison. In our study, D. magna, R. subcapitata and V. fischeri were the most sensitive to PS-NPL water-soluble additives and flagged their presence in NPL preparations.