Plastics, including microplastics, have generally been regarded as harmful to organisms because of their physical characteristics. There has recently been a call to understand and regard them as persistent, bioaccumulative, and toxic. This review elaborates on the reasons that microplastics in particular should be considered as “toxic pollutants”. This view is supported by research demonstrating that they contain toxic chemicals within their structure and also adsorb additional chemicals, including polychlorinated biphenyls (PCBs), pesticides, metals, and polycyclic aromatic hydrocarbons (PAHs), from the environment. Furthermore, these chemicals can be released into tissues of animals that consume microplastics and can be responsible for the harmful effects observed on biological processes such as development, physiology, gene expression, and behavior. Leachates, weathering, and biofilm play important roles in the interactions between microplastics and biota. Global policy efforts by the United Nations Environmental Assembly via the international legally binding treaty to address global plastic pollution should consider the designation of harmful plastics (e.g., microplastics) with associated hazardous chemicals as toxic pollutants.

Environmental microplastics are gaining interest due to their ubiquity and the threat they pose to environmental and human health. Critical studies have revealed the abundance of microplastics in nature, while others have tested the impacts of these small plastics on organismal health in the laboratory. Yet, there is often a mismatch between these two areas of research, resulting in major discrepancies and an inability to interpret certain findings. Here, we focus on several main lines of inquiry. First, even though the majority of environmental microplastics are plastic microfibers from textiles, laboratory studies still largely use spherical microbeads. There are also inconsistencies between the measurements of microplastics in the environment as compared to the concentrations that tend to be used in experimental studies. Likewise, the period of exposure occurring in experimental studies and in the environment are vastly different. Lastly, although experimental studies often focus on a particular subset of toxic chemicals present on microplastics, textile microfibers carry other dyes and chemicals that are understudied. They also cause types of physical damage not associated with microspheres. This review will analyze the literature pertaining to these mismatches, focusing on aquatic organisms and model systems, and seek to inform a path forward for this burgeoning area of research.

Microplastics have become a topic of considerable concern and intensive study over the past decade. They have been found everywhere in the oceans, including the deepest trenches and remotest parts of the Arctic. They are ingested by many animals and some are incorporated into tissues. There is considerable effort in studying what effects they have on marine life. It has become clear that when water samples are collected in ways that prevent most long thin particles from escaping through pores of a net, the most abundant type of microplastics found in water and sediments are microfibers (fibers with dimensions less than 5 mm). The major source of these pollutants is synthetic textiles, such as polyester or polyamides, which shed microfibers during their entire life cycle. Microfibers are released during textile manufacturing, everyday activities (e.g., washing, drying, wearing) and final disposal. The complexity of microfiber release mechanisms and of the factors involved make the identification and application of ways to reduce the inputs of microfibers very challenging. A comprehensive approach is strongly needed, taking into account solutions at a number of levels, such as re-engineering textiles to minimize shedding, applying washing machine filters, developing advanced wastewater treatment plants and improving the management of textile wastes. To harmonize and make mandatory the solutions identified, a variety of potential government policies and regulations is also needed.

Microfibers are the most prevalent microplastics in most terrestrial, freshwater, and marine biota as well as in human tissues and have been collected from environmental compartments across most ecosystems and species sampled worldwide. These materials, made of diverse compound types, range from semi-synthetic and treated natural fibers to synthetic microfibers. Microfibers expose organisms across diverse taxa to an array of chemicals, both from the manufacturing process and from environmental adsorption, with effects on organisms at subcellular to population levels. Untangling the physical versus chemical effects of these compounds on organisms is challenging and requires further investigations that tease apart these mechanisms. Understanding how physical and chemical exposures affect organisms is essential to improving strategies to minimize harm.

While there are numerous papers on microplastics (mps) being published every week, there is a need for improvement for the field to mature. The papers reporting numbers found in water bodies cannot be compared because there are no standard methods for collection and analysis. It is clear that using nets for sampling misses most of the microfibers, which are the most abundant form when whole water samples are analyzed, and that microscopic identification has a very high error rate compared to chemical analytical equipment which can also identify the polymers. It is clear that most animals studied eat mps; we should learn what attracts the animals to the mps and what proportion pass right through and are defecated vs those that move into the tissues. It is considered that mps are a vector for transfer of toxic chemicals into the food chain. Let us investigate to what degree what proportion of contaminants are removed in the digestive system vs. staying bound tightly to the mps. Experimental studies should also use environmentally relevant doses and the shapes and sizes of mps that are most abundant in the environment.