In a review article published on November 24, 2021, in the Journal of Hazardous Materials, Evangelos Danopoulos from the University of Hull, United Kingdom, and co-authors evaluated microplastic impacts on human cells to inform a human risk assessment. For their rapid review, Danopoulos et al. searched for studies in the Web of Science and MEDLINE databases published until March 2021 and found 17 matching their eligibility criteria for inclusion such as experimental studies with human cell models and plastic particles between 100 nm to 5 mm. Eight of the studies were used for a meta-regression analysis to explore potential associations between specific characteristics (e.g., particle size) and health outcomes.
The scientists report that the reviewed studies found microplastics to affect cytotoxicity, oxidative stress, immune response, and barrier attributes while for genotoxicity no microplastic-associated effect was reported. Cells differed in sensitivity with the human adenocarcinoma cell line Caco-2 being most susceptible to cytotoxicity. Of the microplastic characteristics explored, the authors only found the shape to significantly influence cytotoxicity. The lowest dose to induce cytotoxicity was 10 g/mL of 5–200 µm sized particles while 20 µg/ml of 0.4 µm sized particles was the minimal concentration to induce an immune response. The scientists concluded that the effect thresholds of microplastics are much lower than previously predicted. Danopoulos and co-authors also point out that the available studies included in their review had some limitations (e.g., did not make their full data available) leading them to grade the “overall certainty of the body of evidence” as low. Since all the studies were performed under controlled laboratory conditions, they recommend subsequent analyses approach environmental conditions by including variables such as different particle shapes, synergistic toxicological effects of the mix of polymers, additives, and chemicals absorbed to microplastics in the environment.
Yi-Chun Chen from the National Formosa University, Yunlin, Taiwan, and co-authors evaluated microplastics’ effect on the kidney using environmentally-relevant concentrations of polystyrene (PS). As reported in their research article published on November 25, 2021, in the Journal of Hazardous Materials, Chen et al. exposed human embryonic kidney cells (HEK 293) to 3–300 ng/mL spheric PS particles with a size of 3.54 µm. They found that non-cytotoxic microplastic concentrations (3 ng/mL) cause oxidative stress via antioxidant inhibition while 300 ng/mL was cytotoxic and induced autophagy. In addition, the cytotoxic concentration was reported to be able to “impair kidney barrier integrity and increase the probability of developing acute kidney injury.” The authors conclude that “environmental exposure to polystyrene microplastics may lead to an increased risk of renal disease.”
In an article published on December 2, 2021, in the journal Toxicology, Zhaolan Wei and colleagues from Central China Normal University, Wuhan, China, did not use human cells but male and female mice to study PS microplastic effects on reproduction and fertility. The scientists used 5.0–5.9 µm spheres and exposed the mice for 33 or 44 days to 0.1 mg PS per day. Generally, microplastics exposure resulted in a change in hormone levels compared to the unexposed control groups, but the effects were different for female and male mice. The authors reported that “female mice appear to be more susceptible to microplastics in reproduction and fertility than male mice.” However, the study showed that the PS particles damaged both testes and ovaries and also induced oxidative stress. Furthermore, “the mice exposed to PS-microplastics had a reduced pregnancy rate and produced fewer embryos.”
Previous studies have reported that PS microplastics induce reproductive toxicity in male mice (FPF reported) and lead to fetal losses in pregnant mice (FPF reported). Small plastic particles have also been detected in the human placenta (FPF reported and here). Five Horizon 2020 projects are working to better understand the impacts of microplastic exposure to humans with one of the projects, AURORA, focusing on the effects of micro- and nanoplastics on early life health (FPF reported).
Danopoulos, E. (2021). “A rapid review and meta-regression analyses of the toxicological impacts of microplastic exposure in human cells.” Journal of Hazardous Materials. DOI: 10.1016/j.jhazmat.2021.127861
Chen, Y.-C. (2021). “The nephrotoxic potential of polystyrene microplastics at realistic environmental concentrations.” Journal of Hazardous Materials. DOI: 10.1016/j.jhazmat.2021.127871
Wei, Z. (2021). “Comparing the effects of polystyrene microplastics exposure on reproduction and fertility in male and female mice.” Toxicology. DOI: 10.1016/j.tox.2021.153059