Twenty years of microplastic pollution research—what have we learned?

Twenty years after the first publication that used the term microplastic, we review current understanding, refine definitions, and consider future prospects. Microplastics arise from multiple sources, including tires, textiles, cosmetics, paint, and the fragmentation of larger items. They are widely distributed throughout the natural environment, with evidence of harm at multiple levels of biological organization. They are pervasive in food and drink and have been detected throughout the human body, with emerging evidence of negative effects. Environmental contamination could double by 2040, and wide-scale harm has been predicted. Public concern is increasing, and diverse measures to address microplastic pollution are being considered in international negotiations. Clear evidence on the efficacy of potential solutions is now needed to address the issue and to minimize the risks of unintended consequences. Editor’s summary Twenty years after the term “microplastics” was first used, what do we know about their presence in the environment? Thompson et al. review what we have learned over that interval, including what microplastics are, their sources and sinks, their ecological impacts and risks, the dangers they pose to human health, advances in detection and identification, and prospects for managing and regulating them. The environmental burden of microplastics continues to grow, so a combination of scientific, economic, and social interventions will be necessary to curb that growth. —Jesse Smith BACKGROUND The term microplastic was first used to describe microscopic fragments of plastic debris (~20 µm in diameter) in a publication in 2004. On the basis of this paper and earlier work, it was evident that small fragments of various common plastics—including acrylic, polyamine (nylon), polypropylene, polyester, polyethylene, and polystyrene—were present in coastal environments around the United Kingdom and along the eastern seaboard of the United States and that their abundance had increased substantially since the 1960s. There was evidence that microplastics were bioavailable to invertebrates and fish but only speculation on the key sources and the potential for harmful effects. ADVANCES Microplastics, now widely defined as pieces ≤5 mm in size, are recognized as a highly diverse set of globally important contaminants. Multiple sources are now confirmed, including primary microplastics in cosmetics and paint as well as the pellets and flakes used to make plastic products, along with secondary microplastics generated by the abrasion of larger items during use, including textiles and tires, and the fragmentation of larger debris in the environment. Microplastics can be redistributed by wind and water and have since been reported in diverse locations, from the sea surface to deep-sea sediments, from farmland to our highest mountains, and in sea ice, lakes, and rivers. They have been detected in 1300 aquatic and terrestrial species, from invertebrates at the base of the food web to apex predators, with evidence of impacts at all levels of biological organization, from cellular to ecosystem. Microplastics are pervasive in the food we eat, the water we drink, and the air we breathe. They have been detected in multiple tissues and organs of the human body, with emerging evidence of potential effects. This rapidly unfolding scientific evidence, together with individual, social, and societal drivers of change, is leading to policy outcomes that include national-level regulations, such as the prohibition of microplastics in cosmetics by multiple countries and a mandate in France requiring that filters be installed in washing machines to intercept microfibers, as well as multinational policies, including the EU Marine Strategy Framework Directive and the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) legislation on intentionally added microplastics. OUTLOOK Emissions of microplastics to the environment are estimated to be between 10 and 40 million tonnes per year, and under business-as-usual scenarios, this amount could double by 2040. Even if it were possible to immediately halt emissions, quantities would continue to increase because of the fragmentation of legacy items. Modeling predictions indicate the potential for wide-scale environmental harm within 70 to 100 years, but detailed risk assessments are limited because exposure and effect data are incomplete. This is especially true for human health effects. Although we anticipate greater clarity over the next few years, public risk perception is also a key driver of actions and is often influenced by a wider range of factors than objective risk assessment; for example, German consumers recently rated microplastics in food as being their top environmental health concern. Can we afford the externalized costs of microplastics that are already understood, and if not, which criteria should guide interventions and what is essential, in the context of societal needs and desires? A whole-system approach from extraction to remediation will be key to creating material flows that satisfy human needs with minimal environmental impact. Twenty years of science defining microplastic pollution now brings a tangible opportunity for international action as part of the United Nations Environment Programme draft global plastics treaty. Together with reductions in primary polymer production, measures will be needed to reduce emissions and pollution along the entire life cycle of plastics, including dedicated provisions on microplastics. However, there is a high risk of unintended consequences if interventions are implemented without appropriate evaluation. Microplastic pollution: Sources, impacts, and actions. Twenty years of research focused on microplastic pollution has identified their multiple sources, wide-scale environmental distribution, bioavailability, and impacts. This evidence, together with the associated sociopolitical dynamics, has started to drive actions on a global scale. NGOs, nongovernmental organizations. ILLUSTRATION: J. BEADON