Environmental Toxicology and Chemistry—Volume 38, Number 4—pp. 703–711, 2019 703
ET&C FOCUS Focus articles are part of a regular series intended to sharpen understanding of current and emerging topics of interest to the scientific community.
Rethinking Microplastics as a Diverse Contaminant Suite
Chelsea M. Rochman,a,*,1 Cole Brookson,a,1 Jacqueline Bikker,a,1 Natasha Djuric,a,1 Arielle Earn,a,1 Kennedy Bucci,a,1 Samantha Athey,b,1 Aimee Huntington,a,1 Hayley McIlwraith,a,1 Keenan Munno,a,1 Hannah De Frond,a,1 Anna Kolomijeca,a,1 Lisa Erdle,a,1 Jelena Grbic,a,1 Malak Bayoumi,a,1 Stephanie B. Borrelle,a,c,1 Tina Wu,a,1 Samantha Santoro,a,1 Larissa M. Werbowski,a,1 Xia Zhu,a,1 Rachel K. Giles,a,1 Bonnie M. Hamilton,a,1 Clara Thaysen,a,1 Ashima Kaura,a,1 Natasha Klasios,a,1 Lauren Ead,a,1 Joel Kim,a,1 Cassandra Sherlock,a,1 Annissa Ho,a,1 and Charlotte Hunga,1 aDepartment of Ecology and Evolutionary Biology, University of Toronto, St. George Campus, Toronto, Ontario, Canada bDepartment of Earth Sciences, University of Toronto, St. George Campus, Toronto, Ontario, Canada cDavid H. Smith Conservation Research Program, Society for Conservation Biology, Washington, DC, USA
Microplastics are not microplastics are not microplastics, just like In our Focus article, we make the case that it is necessary to pesticides are not pesticides are not pesticides. “Microplastics,” rethink microplastics (plastic particles <5 mm in size) and like other classes of chemical contaminants, is a catch-all term for a consider them a suite or class of contaminants, in the same variety of unique chemical compounds. Yet, many scientific way we do for pesticides, trace metals, or flame retardants. publications, policy reports, and media articles present micro- Microplastics are diverse; they come from many different plastics as if they are simply a single compound or type of material. product types; incorporate a broad range of sizes, colors, and Such simple communications have consequences, leading to morphologies; are composed of various polymers; and include a simplified studies and protocols that may be inadequate to inform broad array of chemical additives (Figure 1 and Textboxes 1 and us of the sources and fate of microplastics, as well as their biological 2). This diversity is important to consider, and thinking of them and ecological implications. For example, studying the fate and like we do other classes of contaminants may help us advance effects of one plastic type with a specific shape and size does not tell methods for sampling and analysis and help us better us the fate and effects of microplastics in general. Moreover, not understand the sources from which they enter the environment; recognizing the diversity of materials in a microplastics sample may their fate in water, sediment, and organisms; their toxicity; and overlook the complexity necessary to inform robust quality analysis relevant policies for mitigation. and quality control (QA/QC) needed in sampling and analytical measurement techniques. For instance, some methods are better at recovering specific sizes, shapes, or types of microplastics. RETHINKING MICROPLASTICS TO Simplifying microplastics as a single compound has also led to INCORPORATE THEIR DIVERSITY confusion around the need for new policies and strategies to reduce Just like pesticides are made of diverse molecules, have varying future emissions of microplastics. For example, some policymakers molecular structures, and can be used for a variety of applications and scientists are under the impression that banning microbeads (e.g., fungicides, herbicides), microplastics are made from diverse from rinse-off personal care products has eliminated future releases molecules, have varying molecular structures, and come from of microplastics in general to the environment. In reality, such bans products with various applications (e.g., tires, textiles, and packag- eliminate only one source of the diverse and complex emerging ing). What is unique to pesticides and other chemical contaminants is global contaminant suite that is “microplastics.” This can be that microplastics are particles, comprising different sizes, shapes, compared to banning one specific use of a pesticide (e.g., in the and colors. Microplastic particles are not simply “microplastic” but a home), leaving the market full of other applications of diverse diverse suite of contaminants that we refer to as “microplastics.” pesticides that need to continue to be assessed for environmental persistence, bioavailability, and toxicity. Diverse product types As a contaminant class, microplastics come from a large 1These authors contributed equally to this manuscript. * Address correspondence to [email protected] diversity of product types and are generally classified as either Published online in Wiley Online Library (wileyonlinelibrary.com). primary or secondary. Primary microplastics are manufactured to wileyonlinelibrary.com/ETC C 2019 SETAC 704 Environmental Toxicology and Chemistry—Volume 38, Number 4—pp. 703–711, 2019
FIGURE 1: Microplastics are made with a variety of polymers, augmented with an array of additives that can be manufactured into a multitude of products. Sources of microplastics can be either primary or secondary, and microplastics may be any size less than 5 mm. Microplastics are described with at least 7 morphologies and are found in many different colors. When in the environment, microplastics can sorb numerous chemical contaminants, including heavy metals and persistent organic pollutants. This is not an exhaustive list. PP ¼ polypropylene; LDPE ¼ low density polyethylene; HDPE ¼ high-density polyethylene; PVC ¼ polyvinyl chloride; PU ¼ polyurethane; PET ¼ polyethylene terephthalate; PS ¼ polystyrene; ABS ¼ acrylonitrile butadiene styrene; PMMA ¼ polymethyl methacrylate; POM ¼ polyoxymethylene; PBT ¼ polybutylene terephthalate; PC ¼ polycarbonate; PA ¼ polyamides; SAN ¼ styrene-acrylonitrile; PEEK ¼ polyether ether ketone; PSU ¼ polyarylsulfone; PAH ¼ polycyclic aromatic hydrocarbon; PCB ¼ polychlorinated biphenyl; DDT ¼ dichlorodiphenyltrichloroethane; PBDE ¼ polybrominated diphenyl ethers. be <5 mm in size. They include preproduction pellets used to make using manta trawls with a 333-mm mesh net, but methods are plastic products and microbeads used as abrasives for industrial evolving toward using smaller mesh sizes or collecting bulk water purposes or in personal care products. Secondary microplastics are (Barrows et al. 2017). In addition, researchers are beginning to small pieces of plastic which are not produced intentionally but expand their analytical techniques to incorporate those that can instead are the result of the breakup and fragmentation of larger detect and identify smaller and smaller microparticles. As a result, plastic items via biological, physical, and chemical processes. the sizes of microplastics reported in the literature are becoming Secondary microplastics can form during product use (e.g., more diverse, incorporating a broader range that is dictated by the microfibers shed from clothing during washing or tire wear lower limit defined by our sampling or analytical methodologies. particles) or once released into the environment (e.g., via fragmentation). Fragmentation is mediated by the polymer type and environmental conditions, which can be highly variable (Sivan Diverse shapes and colors 2011; Gewert et al. 2015). Microplastics can be a by-product of Microplastics come in many shapes and colors. The shape of a many plastic products, including construction materials, agricul- microplastic is often used to assign it to a common category, which tural materials, furniture, clothing, and food packaging (Figure 1). helps inform the source (Helm 2017). Generally, researchers use somewhere between 4 and 7 different categories defined by shape or morphology, which include fiber, fiber bundle, fragment, sphere Diverse sizes (or bead), pellet, film, and foam (Figure 1). To help with source Microplastics encompass a broad range of sizes. Most often, apportionment, we know that certain shapes are generally shed they are defined as any plastic particle <5 mm in one dimension as from different products. This provides clues related to where defined by the National Oceanographic and Atmospheric microplastics in nature may originate. For example, fibers and fiber Association (Figure 1). Others argue for size categorization that bundles tend to shed from clothing, upholstery, or carpet; pellets matches the metric system (e.g., 1–999 mm are microplastics). are generally associated with industrial feedstock; spheres may be However, there is generally no lower limit, and nano-sized plastics microbeads from personal care products or industrial scrubbers; (<0.1 mm) are often included in this definition. The way that discrete andfoamoftencomesfromexpanded polystyrene foam products sizes and ranges of size are reported among researchers varies. For such as insulation or food packaging. Detailed descriptions and example, researchers may use multiple grades of sieves to size- images of each of these shapes can be found in Textbox 1. fractionate samples, and thus their categorization is defined by the sizes of their sieves used during sample preparation. Defining how microplastics are described in relation to size is a Diverse polymer types prevalent topic of discussion among researchers in the field. Microplastics are composed of a diverse suite of polymer Historically, researchers predominantly sampled microplastics types just like pesticides are composed of a diverse suite of