Toxicological impacts of microplastic fibers: A review assessing risk to human and aquatic health

Microplastics (particles less than 5 mm in size) are emerging contaminants that are widely distributed in the environment. Among the various morphologies, microplastic fibers (MPFs) are one of the most prevalent in environmental matrices and at multiple levels of biological organization. Most existing literature on the toxicological implications of microplastics on organisms utilizes morphologies that are not commonly recovered from the field (i.e. spheres or beads), therefore limiting our understanding of true toxicological concerns. Thus, this scoping review aimed to summarize and critically discuss the available data on the toxicological impact of MPFs, providing recommendations for future assessments based on the current knowledge gaps in the literature. The novelty of our review lies in identifying similarities across studies to better understand how laboratory approaches can inform toxicological outcomes. Our review found that most of the literature meeting our search criteria focused on work specific to aquatic and mammalian systems, with the latter yielding very few studies. Through the analysis, multiple knowledge gaps in MPF research emerged, such as MPF toxicity can be an artifact of the fiber dimensions (i.e. length or aspect ratio), and that subsequent leaching of additives from fibers may contribute to toxicity. Additionally, we found that there are varying responses to MPFs versus natural fibers, and a limited understanding of the organismal response to MPFs in the presence of other co-contaminants.