Proceedings of a Workshop

Emerging Technologies to Advance Research and Decisions on the Environmental Health Effects of Microplastics

Proceedings of a Workshop—in Brief

Plastics have become incredibly important in the modern world and are used for purposes for which other materials are unsuitable. As a result, demand for plastics is expected to increase globally for the foreseeable future. However, tiny bits of plastics, known as microplastics, have been increasing in the environment—in the air, water, soil, and food. As yet, relatively little is known about the health effects of these microplastics.

On January 27–28, 2020, the Standing Committee on the Use of Emerging Science for Environmental Health Decisions of the National Academies of Sciences, Engineering, and Medicine held a workshop to explore emerging technologies to advance research on and improve decisions about the environmental health effects of microplastics. The workshop brought together a multidisciplinary group of experts in environmental health, the presence and behavior of plastics in the environment, agriculture, wastewater treatment, and science policy to discuss what is known about microplastics and environmental health, what research is needed to fill gaps in current knowledge, and how emerging science could help address these gaps to provide an improved basis for public policy. The workshop was sponsored by the National Institute of Environmental Health Sciences. This Proceedings of a Workshop—in Brief summarizes the discussions that took place at the workshop, with an emphasis on the comments from invited speakers. The workshop videos and presentations are available online.¹

THE STATE OF KNOWLEDGE ON MICROPLASTICS AND ENVIRONMENTAL HEALTH

Kara Lavender Law of the Sea Education Association gave a keynote address on what is known about microplastics and their effects on environmental health. Law noted that her talk would cover four key points: there is no standardized technical definition of microplastic; not all plastics and microplastics are the same; microplastics are abundant and widespread in the environment; and more discussion and research is needed to better understand their health impacts.

Since 1950, Law said, the world has produced approximately 8.3 billion metric tons of plastic. Only 9 percent of that total has been recycled and 1 percent incinerated, which means that 90 percent of those 8.3 billion tons still exists on the planet somewhere.² Some of it is still in use, but about 70 percent of the total is now plastic waste.

Over the decades, she said, much of that waste has ended up in the world’s oceans. In the early 2000s, there was quite a lot of publicity about the “great Pacific garbage patch,” a collection of trash—reportedly including a great deal of plastic waste—and ostensibly covering an area of the ocean’s surface the size of Texas or France. However, Law emphatically stated that although the image seen in her slide captured the public’s imagination and raised awareness, it is not accurate—”this is not what it looks like.” She told the audience that “if anybody in this room still has this image in your mind, I’m going to ask you to please purge it permanently.” Although it is true that macroplastic—boots, buckets, teapots, and kayaks—can be found in the water, there is no floating island of trash out there, Law said. We clearly know that there is contamination in the oceans and that wildlife is being impacted. However, her real concern is microplastics. She pointed out that thousands of her students have been towing plankton nets for 30 minutes twice per day, every day, over decades. When the collected material is poured through a sieve, they find small, firm, brightly colored, and irregularly shaped particles which, she said, are plastics.

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The plastics found in the ocean are of a wide variety of sizes, shapes, and chemical compositions, and come from many different sources, Law said. Various people have offered diverse definitions of microplastics—one early definition, for instance, was “any plastic less than 5 mm in size”—and the smallest size that people have studied has been a function of the collection technique they used. Early on, the minimum size studied was about 335 microns because that was the size of the mesh in the plankton nets. But as finer and finer filters are used, smaller and smaller plastic particles—potentially down into the nanoscale range—are being found. Her key point was that there is no standardized technical definition for “microplastics.”

Law noted that the plastics found in the oceans are also of many types: industrial resin pellets, cosmetic microbeads, plastic lines and films, and fragments of larger bits of plastic, some of which are produced by weathering. Law indicated that their compositions range over hundreds of types of polymers, and many chemical additives increase the variety. Thus, microplastics are a “diverse and complex category of contaminants made even more complicated because their characteristics evolve when exposed to the environment.” For example, the particles break down over time when exposed to UV radiation, changing their sizes, shape, and chemical composition.

Law stated that there is evidence that animals of all types—including humans—are now ingesting or breathing in microplastics. More than 220 species of marine animals have been found to ingest plastic particles and the impacts of these, she stated, could be as a cause of inflammation or, potentially, transferring of small enough particles across cell membranes. Microplastics have been found in seafood, bottled water, beer, tap water, tea, honey, salt, and sugar. Law cited one preliminary study³ that sampled eight people from different parts of the world with different diets and found microplastics in the stool of all eight participants.

At this point, she said, there is no clear evidence of harm to humans. But she stated that the concern is high enough that the World Health Organization (WHO) commissioned a literature search on the effects of microplastics in drinking water by Bart Koelmans, another speaker at this workshop. The WHO report stated that “Based on this limited body of evidence, firm conclusions on the risk associated with ingestion of microplastic particles through drinking-water cannot yet be determined; however at this point, no data suggests overt health concerns associated with exposure to microplastic particles through drinking-water.”⁴ However, Law added, this carefully crafted statement from the report does not say that no health effects exist, and WHO did call for more research to address the many unknowns about microplastics in the environment. Law stated “I’m sure we would all agree that really the path to solutions should be informed by the best available science. And that’s why we’re here today, to understand where we are … and how we can efficiently and productively move that understanding forward to answer those questions about risks and impacts to human health and to the environment.”

Law reiterated her earlier points about the knowledge gaps in this field. She said that little is known about how much microplastic is in the environment and where it can be found. There is not a good sense of its variability in size, shape, and chemical composition. Also, little is known about its origins, how the particles are transported, how they are transformed in the environment, and their ultimate fate. But according to Law, “the ultimate question [is] what are its impacts on wildlife and environmental and human health?”

THE PREVALENCE OF MICROPLASTICS IN THE ENVIRONMENT

How many and what types of microplastics are found in the environment? That was the topic of the first workshop session, introduced and moderated by Mark Hahn of the Woods Hole Oceanographic Institution and a member of the organizing committee. One of the biggest challenges in answering this question, he said, is that microplastics are not a single contaminant but rather a diverse and complex mixture of materials. Dealing with that complexity was a recurring theme in the session’s four presentations.

Microplastics in Food and Agriculture

One of the ways that humans can be exposed to microplastics is by eating foods that contain microplastics. Hongda Chen from the National Institute of Food and Agriculture (NIFA) at the U.S. Department of Agriculture spoke about ways in which microplastics make their way into food.

Chen began by describing the various programs sponsored by NIFA that are focused on microplastics in food and agriculture. Since 2000, the private sector investment in food and agricultural research has grown rapidly. Thus, he said, “when we’re moving forward with the discussion here; let’s not forget a private–public partnership.”

Chen outlined three basic ways that microplastics can enter the human food chain: via industrial contamination, the degradation of plastics in nature, and intentional introduction. He said that food crops are exposed to a number of diverse agricultural plastics, including mulch films for weed suppression and moisture retention, drip tape for irrigation, silage film for protection and storage of forage and silage, coverings for high tunnels (unheated greenhouses to help commercial farmers extend their growing season), seed casings, plant trays and bags, and row covers for frost protection. Mulching film, for example, is spread over fields and decomposes over time. This process has been the subject of several studies examining how the plastic mulch breaks down.

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An example of intentional introduction is the use of polymeric materials to deliver agrochemicals more efficiently to crops by controlling their release. Research has shown that certain polymers have fewer adverse effects on the crops than others, Chen said.

In aquatic environments, Chen briefly described how microplastics move through shellfish, such as mussels and oysters. Research shows that shellfish reject some types of plastics altogether but ingest others, which are later egested in their feces. Generally speaking, microplastic particles tend to be removed rapidly from the bodies of shellfish, so there is little accumulation in their tissues and probably little human health risk from eating mussels and oysters.

Characterizing Microplastics in the Context of Risk Assessment

Understanding the risks posed by microplastics in the environment will require collecting a tremendous amount of data—and the right sort of data, said Bart Koelmans, a professor of aquatic ecology and water quality at Wageningen University in the Netherlands. “What strategies do we follow to make data as useful as possible for risk assessment?”

Understanding the risks of microplastics is difficult, he said, because they come in a tremendous variety of compositions, sizes, shapes, and histories. There is also complexity among the organisms and ecosystems that the microplastics could affect. He suggested that it can take dozens of descriptors to categorize plastics—more than 20 for polymer type, 5 or more for size, and 10 or more for shape, resulting in a considerable amount of data. Fortunately, he continued, there are patterns, or “habits,” in the characteristic distributions of microplastics that make it possible to reduce the number of descriptors dramatically.

For example, there are size distribution patterns, with smaller particles being significantly more common. If one maps the log of relative abundance against size for different types of plastics, one finds a similar slope for all of the plots. This is true for both environmental microplastics and those found in food.

Analogous patterns can be found for both the shape and density of microplastic particles, Koelmans said. Various shapes can be described in a continuous way by a “shape factor” that has a hydrological meaning so that it can be used to understand how plastics move in the environment. By knowing the relative abundances of these shape types in the environment, calculations can be made for an overall distribution, and data on these 10 or 15 shapes can be translated into one continuous function, he said. This function can then be used for risk assessment, modeling, and other analyses. In a similar way, it is possible to use densities of different types of polymers to create a continuous probability distribution for density.

In short, Koelmans said, the first step in understanding microplastics in the environment is to make these various measurements. Then, to make the data more useful for risk assessment, translate the measurements into probability density functions. The result, Koelmans continued, is a three-dimensional graph with size, density, and shape factors on the three axes, which can be used to get insights into the distribution of microplastic⁵ characteristics. This way of presenting the data via contour plots and functions helps create the context for what is bioavailable and what really needs to be measured. Thus, the 35 or more descriptors he mentioned earlier in his talk can be reduced to only 12 descriptors.

Koelmans offered several take-home messages: Risks to humans and other organisms are caused by only a fraction of the whole plastic continuum, dependent on concentration. It is vital to identify this fraction. The microplastic continuum has “habits” and thus can be simplified using pattern identification. Whatever remains to be measured needs high quality measurements. In addition, meeting generic quality assurance and quality control criteria is more important than precisely prescribing how a measurement needs to be made.

Innovative Technologies for Polymer Identification

To understand the prevalence of microplastics in the environment, it is necessary to be able to identify them and determine their composition, according to Jennifer Lynch of the National Institute of Standards and Technology (NIST) and co-director of the newly established Hawaii Pacific University Center for Marine Debris Research.

Looking at a collection of particles collected from the ocean or another water body, Lynch stated that it can be difficult to tell visually what is plastic and what is sand or some other material. Furthermore, thousands of different polymers are produced, with each having different chemical properties and potential for recycling. Understanding the scope of microplastic pollution requires being able to determine which types of plastics are found and where.

People now use many different polymer identification tools, Lynch said. The tools used the most are attenuated total reflection (ATR) Fourier-transform infrared spectroscopy (FT-IR), followed by other modes of FT-IR and Raman spectroscopy. ATR FT-IR is fast and cheap, she said, taking only 1 minute or less for the instrument to identify the particle.

Once a material’s spectrum has been determined, a free online source called Open Specy⁶ can be used to identify the polymer from its spectrum. Developed specifically for the microplastic resource community, Open Specy allows users to upload and view their data, clean it up with baseline corrections, identify the sample through the library search function, and then share that information with the rest of the community.

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However, weathering of plastic particles makes identification particularly difficult, according to Lynch. When she and her colleagues used differential scanning calorimetry for polymer identification, they found that while ATR FT-IR was excellent in identifying mildly weathered microplastic particles, it was not reliable when the particles were heavily weathered, which affected the ATR FT-IR spectra.

Lynch’s lab now uses a four-step process for polymer identification. They begin by making two ATR FT-IR measurements, one on each side of the plastic particle. Next the particle is pressed into a thin film using heat and put through total transmission FT-IR, which reveals whether there are additional polymers in it. They then take a cross-section of the piece and analyze it with the differential scanning calorimeter, which can identify the various polymers—and even additives—that are incorporated into the particle. Finally, a cross-section of the piece undergoes optical or FT-IR microscopy to examine the different layers.

With this sort of analysis, Lynch said, they have found that a majority of the microplastics on the Hawaiian beach where they collected their samples consist of multiple polymers, most of which would not have been revealed by ATR FT-IR alone. “So multiple methods are needed if you want that level of detail.”

In closing, Lynch also discussed some ideas for extracting and identifying nanoparticles, which pose different challenges because of their size.

Knowledge Gaps Related to Microplastics in the Environment and Food

Elke Anklam of the European Commission’s Joint Research Centre described some of the European Union’s efforts to address microplastics. The European Commission has set a goal of reaching 100% re-use, recycling, and/or recovery of all plastic packaging by 2040, Anklam said, while acknowledging that meeting this goal will be challenging. Europe has already banned some single-use plastics but meeting their broader plastic-related goals will require innovations from the plastic-production industry. Similarly, plastic recyclers will have to overcome several hurdles, including the need for recycled materials to successfully compete with virgin materials, which can be much cheaper. There are other challenges related to the recycling of mixed and multi-layered materials. Regulatory science, she added, would need to focus on understanding how much plastic ends up as litter, the process of degradation/fragmentation to microplastics/nanoplastics (NMPs), the fate, exposure, and effect values for different NMPs, and address the need for quality assured exposure and toxicological data.

Noting that a major unanswered question is what effects microplastics have on biological systems, including humans, Anklam proceeded to describe several more specific knowledge gaps. A basic issue, she said, is exactly what people mean by “microplastics.” How large or small are the particles being considered? There needs to be a clear understanding to be sure everyone is talking about the same things.

Anklam urged that understanding the levels of microplastics in water—tap water, bottled water, ground water, and wastewater—should be one of the first knowledge gaps addressed. Some studies have been done, but a much more complete picture is needed. One challenge in this area is that there are different sampling, sample preparation, detection, and characterization methods in use, some of which may not be appropriate or reliable for detecting microplastics. Thus, there is a major need for quality assurance, which is “very important in order to have data we can compare.” One way to accomplish this is to develop reference materials for testing. The European Commission’s Joint Research Centre⁷ has hosted several workshops on detecting and identifying microplastics and has created a “proficiency test” that has drawn interest from more than 100 laboratories worldwide.

In closing, Anklam said that to develop a clear understanding of the presence of microplastics in the environment and food, it will be important to develop validated and harmonized analytical methods along with “fit for purpose” proficiency tests to ensure that all laboratories are making uniform and accurate measurements.

First Panel Discussion

Following the first group of presentations, the speakers from the first session were joined by Kay Ho of the U.S. Environmental Protection Agency (EPA) and Amy Uhrin of the National Oceanic and Atmospheric Administration (NOAA) for a panel discussion. The panelists explored questions about the needs and approaches to increase knowledge about the prevalence of microplastics. “[What are] the key challenges to understanding the distribution of plastics in food and in the environment, and can we identify the specific barriers that are holding us back from meeting those challenges?” asked Mark Hahn, the panel moderator. Koelmans answered that the ability to study the rates of aging and weathering in situ is a challenge, partly because of the formation of biofilms, which affect how the microplastic particles behave and how they reach ecological receptors. These processes differ across various environmental matrices, and are, therefore, very difficult to model. Ho agreed but noted that the instrumentation to study microplastics in different matrices is developing rapidly.

The real question in Anklam’s mind is whether the smallest microplastics are the most toxicologically relevant. Anklam, Ho, and other panelists agreed that it will be necessary to determine the particle size range where any toxicity resides to guide research and policy.

Lynch stated that the inability to compare data among different studies is a major barrier. She was unable to compare her data on concentration of plastic in Hawaii with quantities in other places because research teams all use different units for quantity, making valid comparisons extremely difficult or impossible, Lynch explained.

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Hahn asked if automating microplastic analysis pipelines is a barrier. Koelmans replied that capabilities for analyzing large amounts of data using pattern recognition computing has rapidly advanced, and the processing times are decreasing. However, instrumentation to measure the filter is expensive, so it would not be for every lab.

Uhrin and Law both reinforced Koelmans’s point that polymer type may not be the most critical factor, but rather size, shape, or the way the material interacts with other particles or behaves when ingested by people or in an ecosystem. Uhrin also noted that it is important to have an understanding of environmentally-relevant concentrations to help conduct research that informs real-world assessments.

Finally, Hahn asked about the status of screening approaches that might be used to prioritize samples for more detailed analysis. Ho responded that it can take her lab 1 week just to pick the plastics out of the sediment to get it ready for analysis. At that rate, only 50 samples could be run in 1 year, which is not enough. Uhrin said that NOAA has been considering other approaches, such as not working polymer by polymer or using other methods to get non-plastics out of the way, but they are just in the early stages of this approach.

Lynch commented that even though she is from NIST, where the goal is detail and accuracy, she believes that there is a place for screening methods. Ho mentioned it would be useful to be able to quickly answer “Are plastics there?” and get a total plastic estimate. She encouraged the development of screening methods while continuing work on detailed methods for regulatory science and other areas where accuracy is required.

THE EFFECTS OF MICROPLASTICS ON HUMAN HEALTH

Gina Solomon of the University of California, San Francisco, and another member of the workshop organizing committee, moderated the second session. She noted that the effects of microplastics on human health—the topic of the session—depend on two main factors: the impacts microplastics may have on human health, and the degree of human exposure to microplastics. Risk = hazard x exposure is the standard framework laid out by the National Academies for risk assessment. However, as the speakers in this session made clear, very little is known about the hazards of microplastics for human health. Therefore, much of the focus in the session was on the amount of exposure rather than on its effects.

Evaluating Human Exposures to Microplastics in Air and Water

Environmental health scientists have well-developed methods for evaluating the health risks of various environmental exposures, for example, asbestos or various toxic chemicals. Based on that, Greg Zarus of the U.S. Agency for Toxic Substances and Disease Registry (ATSDR) described what such an approach would look like if applied to evaluating the health risks of microplastics in air and water. But at present, Zarus stated, there are insufficient data for ATSDR to fully evaluate human exposures to microplastics from air and water. However, seeing what sorts of data are needed for such an analysis can help guide the collection of key information.

When making risk calculations to determine the possible health effects of exposures to a particular agent, Zarus said, ATSDR maps out an exposure pathway, which includes the source of the contaminant, its transport and fate in affected media, the points at which individuals are exposed, the exposure route, and the potentially exposed population.

Zarus described the steps he would normally take to evaluate the health risks of an environmental contaminant and pointed out various places where the necessary data are not available for microplastics. Most critical foods have not yet been sampled, and there are good data on microplastic content in only a few cases. Furthermore, the data generally do not provide a way to calculate the total mass of microplastics that a person might ingest, and smaller microplastic particles are seldom analyzed. One of the most important needs here, Zarus said, is to obtain data on microplastics of a size that makes them bioactive because most studies do not differentiate between microplastics that are bioactive and those that are not.

It will also be important to collect data on exposure-associated health effects. In particular, he said, researchers should do studies of the effects of microplastics in animals at exposure levels comparable to human exposures and in a way that will offer insights into their effects in people.

Toward Reducing Uncertainty in the Human Health Risk Assessment of Microplastics

Thava Palanisami of the Global Innovative Centre for Advanced Nanomaterials at the University of Newcastle, Australia, described some of his research efforts aimed at understanding the exposure and health risks of microplastics for humans. Although there have been many recent studies on levels of microplastics in human foods and beverages, there are many types of food for which no data exist. In addition, the data that have been published to date are lacking in quality and the lack of a standard sampling method makes the problem even more complex. There is also great uncertainty about the mass, size, and shape of microplastics, so understanding of exposure is very incomplete. These factors, Palanisami said, make it impossible to get a total figure for what an average person may ingest and what hazards this might represent. Ideally, he said, one would like to get a number for the total mass of microplastics that an average person consumes.

Palanisami’s group has been studying microplastics in wastewater treatment plants as a way of understanding what treatment plants remove and what makes its way into the discharge water. Using improved detection methods, Palanisami’s group has documented the presence of a variety of unusual proteins, polymers, fibers, and fragments as well as a large amount of “glitter,” bits of mylar that are often coated with metal to produce high reflectivity. This was abundant in the sludge from wastewater treatment plants and released in the effluent.

Palanisami believes that ingestion via water is likely to be the most important exposure route for humans. He noted that microplastics will stay in the water cycle and result in repeated and continuous exposure unless they are removed from the cycle. He called for an improved methodology for sampling and processing microplastics at relatively low concentrations in wastewater. He also said it is unclear how biofilms interact with microplastics and what their potential is to change the characteristics of microplastics in the environment.

In closing, Palanisami stated that although “the available evidence suggests the health risks associated with ingesting microplastics and the chemicals associated with them are minimal, the studies so far contain significant data gaps, which need to be corrected in future research.”

Microplastics in Seafood

Garth Covernton, a Ph.D. candidate from the University of Victoria in British Columbia, Canada, discussed the question of human exposure levels to microplastics from eating seafood. He framed his talk around three commonly held beliefs for which the scientific evidence is not clear: (1) seafood is the largest source of microplastics in the human diet; (2) ocean animals are consuming the greatest quantity of microplastics; and (3) microplastics are accumulating in food webs so that top predators and humans have the greatest risks of exposure.

First, Covernton emphasized that there are many possible pathways by which microplastics could get into food, although most are unquantified (see Figure 1). The animals or plants that a person eats may have taken up microplastics from the environment, or the food could be contaminated during processing, transport, or sale. Microplastics could leach out of plastic packaging into the food, or microplastics in the atmosphere could settle into the food during meal preparation. Unfortunately, Covernton said, there are few data concerning how much microplastic gets into food via these various routes.

When Covernton studied microplastics in clams and oysters produced by shellfish farms around British Columbia, he found that, on average, there was less than one microplastic particle per shellfish. A person’s total exposure would depend on how much shellfish he or she ate, but one estimate for European countries where people eat a lot of shellfish was a total of 4,620 microplastic particles per person per year. By contrast, Covernton said, the same person would be exposed to an average of 13,371 particles per year from microplastics settling into the food from the atmosphere during preparation and consumption.

Although there is a general perception that bivalve shellfish—for example clams, oysters, and mussels—just passively accept microplastic particles from their environment, Covernton said, research has shown that they do a good job of rejecting or egesting such particles. Although there are few data on the smallest particles, bivalves do not accumulate particles more than 100 microns in size.

Finally, he said, evidence also indicates that there is there is no increasing concentration, accumulation, or “biomagnification” of microplastics as one moves up the marine food chain. “There is actually pretty much an exponential decrease with increasing trophic level,” he said, a phenomenon referred to as “trophic dilution.” The likely reason, he said, is that sea animals excrete any microplastics they ingest and do not pass them on to animals higher up the food chain.

In closing, Covernton said that human ingestion of microplastics via seafood should not be ignored but should be put into context with other sources of exposure. “We need to start doing research on all the other things that people are eating, as well as drinking water, [and] inhalation.” He stated that he believes that human exposure is probably high “because we’re already living in these highly plastic environments.” Covernton noted again that there remain many data gaps in this field.

Effects of Microplastics in Aquatic Ecosystems

The last speaker in this session took a broader perspective on the issue of microplastics in the environment, examining their effect not on individual species but on entire ecosystems. Allen Burton of the University of Michigan focused on aquatic ecosystems in human dominated watersheds, including lakes, rivers, wetlands, and coastal waters.

To put things in perspective he discussed the pollutants that affect these waterways and which of them have the most deleterious effects on their aquatic ecosystems. At the top of the list are nutrients in runoff from agricultural lands, but there are eight other factors, including pesticides and metals, with particles from tires and other microplastics in the “questionable” category at the bottom of the list. The EPA also has a list of more than a dozen factors affecting the waters in various states, including pathogens, nutrients, and metals, but does not mention microplastics. According to Burton, microplastics barely register when compared to nutrients, pesticides, and metals.

But, Burton said, the amount of plastic in the environment is increasing exponentially, and most of what is known about microplastics concerns particles from 300 to 50 microns in size, while the smaller particles may be more damaging. So, the potential risks of microplastics to aquatic ecosystems cannot be ignored.

Is there a problem now, Burton asked? He described studies of fish from two parts of the world—Lake Erie and the waters off Southeast Asia—that have some of the highest concentrations of microplastics in the water. Both studies found negligible concentrations of microplastics in the fish. He noted, however, that while the relative concentrations of microplastics in water at this time are unlikely to be impacting organisms, over time microplastics will accumulate in sediment as more continue to enter the environment.

Instead, Burton stated that it seems that the greatest threats may be from the two ends of the size spectrum—macroplastics and microplastics that are smaller than those usually measured. The threat of macroplastics, from whales tangled in nets to seabirds with stomachs full of plastics, is well known and is the primary risk from plastics in aquatic ecosystems. But, he added, it is possible that researchers are “missing the boat” on the threat of smaller microplastics, such as small flakes of anti-fouling paint from boat hulls. These paints also generally contain copper or zinc, which themselves are toxic. Burton asked why this is not being looked at and speculated that it is due to the extreme difficulty in collecting samples. He asserted that it is the 100-micron size down into the nano range that we really need to be looking at.

Second Panel Discussion

Gina Solomon then moderated a panel discussion to explore how to characterize human exposure and health risks. She first asked Nigel Walker of the National Toxicology Program (NTP) who joined the panel at this point to reflect on how he might approach risk assessment. Walker emphasized that the key issue is what to test, particularly given that microplastics are mixtures. Walker explained that NTP has worked on mixtures for such things as botanical supplements and disinfection byproducts where there is a great diversity of components. In these cases, they took a “sufficient similarity” approach where they use numbers of multiple mixture samples instead of identifying a single representative one. In this way, scientists can use distance metrics based on composition, short term bioactivity in in vitro systems, and short-term activity in in vivo systems among other techniques to examine where things are happening, and link that data to existing knowns, such as a Standard Refence Material or another common material about which there is a lot of information. Walker also noted that the peer review system developed for the National Nanotechnology Initiative⁸ (NNI) put forward valuable minimum technical criteria and specifications for funders of research and reporting.

Solomon then asked Zarus what is at the top of his wish list for evaluating hazards given the worker and animal studies he has been involved in. Zarus asserted that chemicals from occupational exposures should be prioritized to avert potential human health problems.

Solomon also asked the panelists what food sources of exposure, if not seafood, may be the most concerning. Covernton advocated for more research on unexamined foods, such as red meats and vegetables. Covernton and Chen both encouraged studying how processing and plastic packaging affects the presence of plastics in foods.

Solomon also asked whether the federal approach to nanomaterials provided a useful model as well as some cautionary tales. Walker stated that some questions about microplastics will be addressed more quickly because of NNI. He noted that 15 years earlier, scientists and federal policy makers were faced with the same type of questions about nanomaterials that are being asked now about microplastics: What are the sources of exposure? What are the rates of fate and transport? What do nanoparticles do in which cell types? Are they vectors for chemical pollutants in the environment? By instituting a national nanotechnology strategy, federal agencies were able to better coordinate research and collaborate. Within 10 years, the science community made “good headway” on addressing such questions, he said.

DAY 1 CLOSING REMARKS

Mark Hahn of the Woods Hole Oceanographic Institution wrapped up the first day of the workshop by recapping some of the key messages heard in that day’s presentations. He first noted that the day’s talks had addressed a number of misperceptions about microplastics. For example, the term “microplastics” actually refers to complex mixtures of pollutants that consist of

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particles with diverse chemical compositions, shapes, and sizes. The behavior of these pollutants in the lab far from accurately reflects what is happening in the environment. This is partly because of the complexity of the microplastics themselves, but also reflects diversity among the different environmental matrices, such as soil, freshwater, or the oceans, in which they are found.

Hahn stated that the day’s presentations had highlighted that understanding microplastics in the environment is further complicated by a lack of standard definitions, testing protocols, and quality control measures, and by limitations on the sizes of microplastics that can currently be measured. It may be the nanoparticles that are the most bioavailable and that pose the greatest risks. But these are the most difficult to measure and characterize although the technologies to do this are developing. There are also critical knowledge gaps about microplastics. Although it is known that there are multiple sources of exposure to humans and animals, the identity and relative contributions of these sources are not yet known. Not only is there a lack of information on doses, he said, but there is also very little known about dose responses.

However, Hahn pointed out that there was also some good news. Technologies for measuring NMPs and for determining the relative contributions of particles of varying sizes, shapes, and chemical compositions are coming along. There are also lessons that can be learned from prior work in nano-pharmacology and nanotoxicology. Public–private partnerships between government agencies and industries, such as chemical manufacturing and pharmaceuticals, offer the potential to leverage what has been learned in earlier efforts. Several of the day’s speakers cited a need for the various groups and disciplines to work together and for the establishment of an overall strategy for proceeding.

REDUCING MICROPLASTICS IN THE ENVIRONMENT

The third session addressed how to reduce the level of microplastics in the environment. Each of the four speakers described different approaches: instituting new policies, biodegradation, wastewater treatment, and an improved recycling approach called “up-cycling.”

Preventing Secondary Sources of Microplastic in the Environment

Rebecca Traldi of the World Wildlife Fund (WWF) described a series of programs aimed at preventing secondary sources of microplastics in the environment. She noted the distinction between primary and secondary microplastics. Primary microplastics are those that are less than 5 mm in size when they enter the environment, while secondary microplastics are those that have degraded over time from larger sizes—plastic bottles, for example—due to environmental conditions to produce much smaller particles.

The WWF is interested in microplastics, Traldi said, because an estimated 8 million metric tons enter the oceans every year, and that number is projected to increase in the future as economies and populations continue to grow. The WWF has set an overarching goal of “no plastics in nature by 2030,” Traldi stated.

Reaching that goal will require overcoming various challenges, Traldi said. One is a lack of infrastructure for dealing with plastics. Another is a set of market issues, such as a lack of adequate incentives. Other challenges include collecting and processing wastes. She said that these factors explain why only a small portion of plastic materials are recycled, reused, and recovered.

The WWF is working to meet these challenges in several ways, Traldi said. “We’re working with cities on areas like innovation and improvements in recovery, we’re collaborating with companies on setting sustainability commitments and following through on those, we’re engaging on policy at international and national levels, and then we’re also engaging with the public through campaigns, awareness raising activities, and others.”

The Role of Biodegradation

Kathleen McDonough from Procter & Gamble Company addressed the issue of when biodegradation can be a valuable technique for removing plastics from the environment. She defined biodegradation as microplastics being broken down biologically—generally by microbes—into carbon dioxide, water, and other small molecules. “We’re not talking about something fragmenting into small pieces,” she clarified.

McDonough said the value of biodegradation will depend on several factors, such as the type and the form of the plastic, how it is used, and where it is being used and discarded. In the developed world, for example, microplastics that go down the drain—such as those in cosmetics—are routed to a wastewater treatment plant where they can be completely degraded. By contrast, a plastic water bottle in a country without a waste collection system will end up in a landfill or out in the environment where it will eventually break down and contribute to the microplastic burden.

In replacing plastics with materials that are intended to biodegrade, she said, it is important to make sure that the new materials do biodegrade. For example, she described a study⁹ that she and colleagues conducted that found that various wax materials—beeswax, jojoba beads, stearyl stearate, rice bran, and others—biodegrade well, but walnut shells did not biodegrade at all.

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Smaller microplastic particles also biodegrade faster than larger ones, she said. So, for example, it would be valuable to make exfoliates used in face washes biodegradable because these products are widely used and are disposed of down the drain so it would be very difficult to collect them, at least in countries that do not have good wastewater treatment facilities.

Another material to consider is the sort of capsule used to apply time-release fertilizers. These capsules cannot easily be collected from the fields where they have been distributed, so it would be valuable to make them biodegradable. But, she said, it will be tricky to design the capsules so that they last long enough to hold the fertilizer but then biodegrade later at the right time.

Tires are a tricky question, she said, because they must be made to last while they are on a vehicle, and “in general it is hard to get stable materials to biodegrade.” Are biodegradable tires really an option and will people want to buy them if they must treat them with antimicrobial products regularly to prevent biodegradation while they are still on the vehicle? These are the sorts of essential questions that need to be addressed, McDonough said, when considering the biodegradation option.

Removal of Microplastics from Wastewater

Steve Carr of the Sanitation Districts of Los Angeles County began his presentation by stating that his goal was to address a misconception about plastics and wastewater treatment plants. “We’re supposed to be one of the bad guys, right?” he said. “The [treatment] plants are supposed to be discharging lots of microplastics into the environment.” But the reality is very different, and he offered evidence that wastewater treatment plants are places that can help minimize the amount of microplastics in the environment.

Carr said that he and his colleagues in the Los Angeles County sanitation districts had become interested in microplastics about 5 years earlier when a group of reporters showed up at their facilities and started asking questions about how their wastewater treatment plants handled microplastics. “We had no answers,” he said. “We had no idea of how much was getting into the plants, [or] how much was leaving the plants.”

So, he and his colleagues set out to understand how microplastics were handled in their own treatment plants. They conducted a series of lab experiments on microplastics to understand how they would behave in the system. Among their discoveries was that they could not separate out particles of different sizes because they tended to clump together. Examining samples from upstream, they discovered that the microplastics tended to associate with the grease, fats, and oils that were coming into the plants. It did not matter what size they were—they all clumped together and floated to the top, where the skimmers removed them. “There is a misconception that size has something to do with the ability of plants to separate microplastics,” stated Carr. “I can say to you categorically that that’s not the case.”

With a better understanding of how microplastics behaved in the wastewater treatment plants, Carr and his colleagues examined the level of microplastics in the treated water that the plants discharged. They found that there was almost a complete absence of microplastics in the discharge water. “We did this at seven different wastewater treatment plants,” he said, “and in the secondary effluent we found one microplastic particle per 15,000 gallons. So, I’m saying that this process is a pretty efficient one.” Second, the plant’s filters played almost no role in the process. “The assumption was the filters were the point where the plastic residues or microplastic particles were being removed, and that [this] was not an efficient process, but that was completely wrong.”

Judging by the Los Angeles County system, Carr argued, state-of-the-art wastewater treatment does a good job of removing microplastics from water.

Chemical Up-Cycling of Polymers

Bruce Garrett, the director of the chemical sciences, geosciences, and biosciences division at the Office of Basic Energy Sciences at the U.S. Department of Energy (DOE), observed that a major problem with all of the plastics being produced today is that they are not being recycled effectively. “Most of them are being discarded,” he said. “We incinerate about 25 percent of those . . . and recycle a small amount globally and even less in the United States.” And it is not just an environmental issue but an energy issue as well, he said. “About 4 percent of the fossil feedstocks that we dig out of the ground, either oil or gas, is converted into the precursors that go into making plastics. About another 4 to 6 percent of those fossil feedstocks are used to generate the energy to make those feedstocks—the beads, the resin beads, and things like that. So about 10 percent of the global energy generated each year goes into plastic production.”

Recycling plastics saves energy, Garrett said. “It is less energy to take plastics that exist and convert them into new products, [then] reform them into other plastic products, than it is to go all the way from fossil feedstock.” The main problem is that current approaches to reusing plastics have major limitations. Mechanical recycling, for example, involves heating up plastics so that they can be extruded into new forms, but the process breaks some of the chemical bonds in the polymers and degrades them. It could be called “down-cycling” rather than recycling, he said.

Thus, the Office of Basic Energy Sciences is interested in “up-cycling” polymers—that is, recycling polymers in a way that does not degrade them but rather maintains or even increases their value, which will require a different approach, Garrett said. To get started on a path to up-cycling, DOE held a workshop in spring 2019. The goal that came out of the workshop, he said, was to provide the foundational knowledge to “shift this paradigm of discarded plastics from being wastes that end up in dumps or the environment to actually a resource that you can use to make new products.”

His group has identified some major research directions to explore. First, the mechanisms of polymer deconstruction and reconstruction need to be mastered. Chemists have spent very little time on the question of how to take polymers apart in such a way that the pieces can be reconstructed into something useful. There has been some recent intriguing work in the area, but much remains to be done.

Next, because today’s plastics are not simple polymers—they are mixtures of many components—it will be necessary to develop processes that can up-cycle mixed plastics. Again, there have been some preliminary successes in this area, according to Garrett.

Ultimately, Garrett said, the goal will be to develop a new generation of polymers suitable for “chemical circularity’’—polymers that can go through recycling many times by being broken apart and put back together. A major challenge, he said, will be developing new polymers that have properties equivalent to today’s polymers.

Third Panel Discussion

Anil Patri of the U.S. Food and Drug Administration moderated a panel discussion focused on evaluating options and tradeoffs to reduce microplastics in the environment. Patri began by asking the panelists how the complex mixtures found in microplastics might affect cost-effective solutions to isolating and recycling them. From the basic energy science perspective, Garrett responded that he tends to start with simple problems, in this case maybe simple polymers. Today it is primarily polyethylene terephthalate (PET) plastic and polyethylene that are recycled because of their melting points, explained Garrett. The melting point is a key property, and the fact that some other plastics require much higher temperatures to reform them is a real issue. The constituents in a polymer also pose issues; for example, polyvinyl chloride is difficult because it has chlorine in it, which can create hazardous byproducts. Garrett underscored that designing polymers requires thinking through the thermodynamics.

Carr commented that consumer trash bags or shopping bags are designed to break down once they get into the environment. This poses a double-edged sword: break down of these items generates more microplastics in a much shorter cycle, which may be either beneficial or harmful. McDonough said that she was very interested in up-cycling because in developing country markets where there is little collection infrastructure, added value from up-cycling could provide reasons to put infrastructure in place.

Patri asked what are some of the societal challenges in reducing the introduction of plastics into the environment, what are potential solutions, and how might these vary regionally. Traldi underscored the importance of consumer culture and behavior. Societal factors combined with environmental conditions, such as weather patterns, complicate the pathway from consumption to where the plastics end up after they are used. Solutions require robustly understanding these pathways, and behavioral scientists will be a crucial part of this discussion.

NEW APPROACHES TO INFORMING PUBLIC HEALTH AND POLICY DECISIONS

The workshop’s final session, which was moderated by Kevin Elliott of Michigan State University, a member of the workshop planning committee, was devoted to examining how the emerging science and technology discussed in the earlier sessions could be applied to policy making and decision making.

Mary Ellen Ternes, a partner in the law firm Earth & Water Law, LLC, began the session with an overview of the U.S. legal landscape related to microplastics in the environment. Anyone interested in using the legal system to address microplastics has two basic approaches to work with, she said: relying on laws and regulations or relying on the civil justice system.

Ternes reviewed several existing U.S. environmental laws and discussed their relevance to microplastics. In particular, she mentioned the U.S. Safe Water Drinking Act, the U.S. Solid Waste Disposal Act, the U.S. Clean Water Act, the Comprehensive Environmental Response, Compensation, and Liability Act, or Superfund, and the Clean Air Act. She concluded that “everything we are looking at doesn’t quite get us there.” It is possible, she said, that existing laws could be applied to microplastics “with better interpretation using newly developed methods with more certainty.” The alternative would be to develop new laws and regulations, she said, “but that is always time-consuming.”

Addressing microplastics in the environment using civil courts requires proving a harm, Ternes said, and this is not easy under the current system. The problem, she said, is that the current model of environmental harm was developed to deal with chemical toxicity. “We are just not used to this alien stuff that stays around forever,” she said. “We haven’t figured out a way to characterize it.” Ultimately, she concluded, it will be necessary to find a way to talk about the harms due to plastics in the environment “in order to make progress on policy and regulatory development.”

Final Panel Discussion

Following Ternes’s presentation, Kevin Elliott moderated a final panel discussion that included Ternes, Elke Anklam, Brett Howard of the American Chemistry Council, Suzanne van Drunick of EPA, and Kimberly Warner, a senior scientist at Oceana. Elliott asked the panelists which knowledge gaps need to be filled to inform public health and policy decisions about microplastics. The panelists gave a wide range of answers and there were several points contributed by members of the audience.

Anklam commented that the first step should be to understand the problem better because policies should be based on evidence and scientific understanding of the issue, and others agreed. Warner said that there is already evidence that “macroplastics are having a devastating effect on many organisms” in the oceans. But a better understanding of how microplastics interact with microbial communities, both in the oceans and in human microbiomes, is needed.

Van Drunick and Howard cited the need to understand human exposure and risk. Van Drunick said she would begin with an evaluation of the relative contribution from food, air, soil, dust, and both tap and bottled water. The best place to start would be with a national public drinking water assessment. Once it is clear what is in drinking water and what its health effects are, one could begin discussing which policies are appropriate.

Gary Ginsberg of the New York State Department of Health and a member of the parent Standing Committee on Use of Emerging Science for Environmental Health Decisions suggested that it would be valuable to have life-cycle economic analyses of plastics, considering both their value and their cost to society. Ternes emphasized that there is a great opportunity here for a partnership with industry to study the situation.

Howard added that there are important questions about the fate of microplastics in the environment that need answers. As an example, he highlighted evidence¹⁰ that some polymers can be directly mineralized in the presence of UV light, particularly polystyrene. He also mentioned that it would be great to have more information about chemical recycling. He thinks several companies have spent decades upon decades on making these plastics, but deconstruction technologies are still nascent.

CLOSING THOUGHTS

In closing remarks, Kathleen McDonough listed some of the major take-home messages she had heard from the workshop. On the topic of understanding the amounts and types of microplastics in the environment, she said, one theme was the importance of harmonizing approaches so that data will be comparable. Quality assurance and quality control are also important, so that the data can be trusted. Because there are so many things that could be measured, researchers need to figure out which characteristics are most important and focus on those. Many speakers emphasized the importance of learning more about smaller particles, which are sometimes referred to as nanoplastics.

Concerning human exposures to microplastics, McDonough referred to the observations that seafood does not seem to be a major source of exposure and suggested that perhaps researchers should be focusing on air as a pathway for microplastic exposure, and again, there is concern that microplastics that are too small to be easily detected may be an important human health risk.

On the topic of reducing microplastics in the environment, McDonough said a main message is that there will not be a “one size fits all” answer. Rather, different situations will require different approaches, including reducing the use of plastics, greater reliance on biodegradable plastics, and the development of new types of polymers that can be up-cycled and recycled over and over again.

Before policy makers can decide the best approach to dealing with microplastics, much more data will be needed, McDonough said. It will be important to gather this data using standardized methods and approaches and to pay attention to those areas with large data gaps to be filled.

Finally, McDonough emphasized the importance of taking a cross-disciplinary approach, collaborating across different areas of expertise from environmental sciences and human health to polymer chemistry and policy. “If we’re not all talking in this space, we may come up with regrettable substitutions or a bad choice in how we move forward,” she said. “I think it is going to be really important to continue cross-fertilizing and having these discussions.”

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DISCLAIMER: This Proceedings of a Workshop—in Brief was prepared by Robert Poole, Fran Sharples, and Keegan Sawyer as a factual summary of what occurred at the workshop. The planning committee’s role was limited to planning the workshop. The statements made are those of the rapporteurs or individual meeting participants and do not necessarily represent the views of all meeting participants, the planning committee, or the National Academies of Sciences, Engineering, and Medicine.

ORGANIZING COMMITTEE ON EMERGING TECHNOLOGIES TO ADVANCE RESEARCH AND DECISIONS ON THE ENVIRONMENTAL HEALTH EFFECTS OF MICROPLASTICS: This workshop was organized by the following experts: Kevin Elliott, Michigan State University; Mark Hahn, Woods Hole Oceanographic Institution; Kathleen McDonough, Procter & Gamble Company; Anil Patri, U.S. Food and Drug Administration; Gina Solomon, Public Health Institute and University of California, San Francisco.

Reviewers: The Proceedings of a Workshop—in Brief was reviewed in draft form by Kathleen McDonough, Procter & Gamble; Gary Minsavage, Exxon Mobil Corporation; and Anne Styka to ensure that it meets institutional standards for quality and objectivity. The review comments and draft manuscript remain confidential to protect the integrity of the process.

Sponsor: This workshop was supported by the National Institute of Environmental Health Sciences.

For more information, contact the Board on Life Sciences at (202) 334-3947 or visit https://www.nationalacademies.org/bls/board-on-life-sciences.

About the Standing Committee on Emerging Science for Environmental Health Decisions

The Standing Committee on Emerging Science for Environmental Health Decisions is sponsored by the National Institute of Environmental Health Sciences to examine, explore, and consider issues on the use of emerging science for environmental health decisions. The Standing Committee’s workshops provide a public venue for communication among government, industry, environmental groups, and the academic community about scientific advances in methods and approaches that can be used in the identification, quantification, and control of environmental impacts on human health. Presentations and proceedings such as this one are made broadly available, including at https://www.nationalacademies.org/our-work/standing-committee-on-the-use-of-emerging-science-for-environmental-health-decisions.

Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2020. Emerging Technologies to Advance Research and Decisions on the Environmental Health Effects of Microplastics: Proceedings of a Workshop—in Brief. Washington, DC: The National Academies Press. https://doi.org/10.17226/25862.

Division on Earth and Life Studies

Copyright 2020 by the National Academy of Sciences. All rights reserved.