Mammalian reproductive and developmental toxicity of micro- and nanoplastics: a critical appraisal of evidence using the SciRAP tool.

Human exposure to micro- and nanoplastics (MNPs) is increasingly documented, yet their potential effects on reproductive and developmental health remain inadequately explored. Preliminary findings presented in scientific literature suggest that MNPs may translocate systemically and localize in reproductive tissues, prompting growing concern among both researchers and regulators. However, existing toxicological studies differ in terms of methodological adequacy and regulatory applicability. The aim of this study is to provide a systematic evaluation of current scientific evidence regarding the reproductive and developmental effects of MNPs, addressing both scientific findings and study quality for regulatory purposes. We structured our study by first conducting a systematic review focused on in vivo mammalian studies examining the effects of MNPs on reproductive and developmental endpoints. This was followed by an assessment of the quality of these studies in terms of regulatory reliability and relevance using the Science in Risk Assessment and Policy (SciRAP) tool, version 2.3, originally developed for evaluating chemical toxicity studies. Recognizing that this tool does not fully capture MNPs-specific challenges, we supplemented the evaluation with additional criteria, including polymer type, particle size and shape, preparation methods, and plastic test item characterization. Due to common gaps in reporting, particularly regarding exposure doses, we incorporated a "not reported" option in the Relevance assessment. Each study's overall Critical Appraisal of Evidence Score was then estimated based on its SciRAP score and the extent to which it addressed MNPs-specific criteria. To our knowledge, this represents the first systematic application of SciRAP to studies on MNPs toxicity. Following a tiered and targeted search strategy, the review included 102 studies, with findings grouped into three categories: Biodistribution, Reproductive System, and Fertility and Development. Our analysis found evidence of MNPs presence in reproductive organs, resulting in disruptions to testicular, ovarian, and uterine functions. Both male and female gamete quantity and quality were negatively affected. Additionally, we identified studies indicating maternal transfer of MNPs via the placenta and lactation, with consequences for fetal development and offspring fertility. The outcome of the critical appraisal of evidence evaluation was that 7 studies were classified as strong, 89 as moderate, and 6 as weak for risk assessment e.g. in a weight of evidence assessment of MNPs effects on biodistribution, the reproductive system, fertility and development. Although many studies report adverse reproductive and developmental outcomes following MNPs exposure, methodological shortcomings and insufficient reporting hinder data synthesis and integration into structured assessments, such as weight-of-evidence approaches. Critical information, such as particle characterization, administered dose, and dose-response relationships, was often missing. In addition to calling for more studies performed in accordance with SciRAP criteria, we emphasize the need to improve the design, documentation, and reporting quality of MNPs studies to enhance the overall evidence base for risk assessment. With this work, we provide valuable insights for the academic and testing communities, along with a robust overview of current critical appraisal of evidence on the potential reproductive and developmental toxicity of MNPs based on existing literature.