A review of the toxic effects of microplastics based on studies on mammals and mammalian cell lines†

Microplastics have raised global alarm because of their pervasiveness, potential human toxicity, and ecotoxicity. This paper reviews studies conducted on mammals and mammalian cell lines to illustrate the toxic effects of MPs and the MP levels causing or not causing an observable negative response. Most current studies in this area have been conducted on polystyrene with few studies dedicated to polyethylene and polypropylene. In vivo studies commonly use mice or rats as the experimental subjects and ingestion as the exposure mode, while in vitro studies use different types of cell lines, with intestinal cell models being the most common. The toxic effects of microplastics are size- and biomarker-dependent, with polystyrene microplastics at 1.49 × 10⁶ to 4.55 × 10⁷ particles per mouse not leading to observable negative effects but 0.01 mg day⁻¹ to 0.15 mg day⁻¹ per mouse yielding negative responses. For cell lines, polystyrene microplastics at 10 μg mL⁻¹–20000 particles mL⁻¹ did not induce negative effects but a level of 0.01 μg mL⁻¹–5000 particles mL⁻¹ caused negative effects, depending on the types of cells used. Polyethylene microplastics at 0.125 mg day⁻¹ generally could cause mice to respond negatively, whereas polypropylene microplastics at 5000 particles mL⁻¹ were observed to cause a negative response in THP-1 macrophages. The different units for the toxic doses used make comparison of the doses challenging. It is, therefore, recommended that a common unit is used in reporting the toxic levels of microplastics, particularly mg kg⁻¹–bw day⁻¹ for in vivo studies and μg mL⁻¹ or mg L⁻¹ for in vivo studies. Standardized biomarkers and bioindicators could also be used to facilitate comparison.