The microplastics working group conducted a series of literature searches during 2019–2022 on any topic related to microplastics. Thousands of articles were identified. Figure 1 shows a time series of published literature from 2010–2019. Most published literature occurred after 2015. Team members were assigned a group of documents to review. A scoping review organized the literature into three broad categories: environmental, adsorption and absorption, and human exposure toxicology and health. Much of the publish literature was related to the environment. Figure 2 shows the one reviewer’s results (Webb et. al. 2021). Publications most frequently addressed questions related to microplastics in water, more than any other environmental media, consistent with all staff reviews. The higher focus on water is consistent with the early availability of sampling methodologies in water for ecologic purposes (NOAA 2013). These methods were applied to study ocean plastics and then to study plastics in fish. This left many questions regarding public health implications for fish consumption.

ATSDR applied its health assessment process (ATSDR 2005) to published environmental data to estimate human exposures (Zarus et al. 2021). This assessment included human exposure via three main routes: ingestion, inhalation, and absorption. ATSDR applied relevant human ingestion and inhalation rates to calculate exposure rates. Microplastics in seafoods could amount to thousands of microplastic particles ingested per day. Although exposures to the microplastics measured in water, air, and food additives appeared to be much less than in seafood, the sampling methods limited the size of microplastics able to be detected in those media. Packaging, as with plastic tea bags, appeared to increase microplastic exposure, but packaging studies used different analytical methods than those used for most foods. Table 1 summarizes the published environmental microplastic data to assess human exposures. The data gaps identified in the exposure media were used to inform a workshop and follow up work involving international scientists (Zarus, Casillas et al., 2021).

Reviews showed target organs and systems that microplastics can affect, including the immune system, the respiratory system, the hepatic system, and the gastrointestinal (GI) tract, (Zarus et al., 2021; Zarus, Zarate-Bermudez et al., 2020). Importantly, ATSDR found that documented clinical effects were not associated with the term “microplastics,” but rather exposure to specific synthetic substances.

Direct exposure to microplastics resulted in lung effects in animals and, because of occupational exposures, in humans. Immune system effects included polyethylene translocating in the lymph system from implants and a foreign body response. Neurologic system effects included polystyrene affecting neurologic mouse cells and polyethylene associated with human dopamine levels. Microplastics were detected in the GI tracts and feces of environmentally exposed individuals and worker studies identified GI health effects. Studies of the hepatic system included an associated health effect in workers. Table 2 summarizes data and data gaps germane to human exposures to microplastics and associated effects. While exposures to many populations are demonstrated, clear clinical effects were only observed in workers or patients with plastic implants. Many data gaps exist relating animal studies to human exposures.

Although microplastic literature heavily favored an environmental focus, data gaps remain within that arena. Very few studies examine the most bioavailable plastics — those smaller than10 microns (Table 1). Methods in the marine environment, while more standardized than other media, lack inclusion of the smaller particles that can move within cells. Other environmental media lack method standardization. Additionally, few studies define how microplastics behave in the atmosphere and in the sediment. ATSDR identified several pressing data gaps related to identifying microplastics exposure and toxicity to humans. A general list of the data gaps that need to be addressed to form a more complete picture of microplastics toxicity and exposure to humans is available in Table 2.

In December 2021, ATSDR led a session to address some of the health-related data gaps within the Asian Pacific Economic Cooperation (APEC) Workshop on Nanoplastics in Marine Debris (Zarus, Casillas et al. 2021). During this session, presenters provided new data identifying unique effects of polystyrene, polyvinyl chloride, and acrylic on lung cells, and a means to assess the effects of microplastic exposures (Sang 2021; Mahadevan and Valiyaveettil, 2021; Mumtaz and Gehle 2021). The direct cell dosing studies cannot be used to assess human health implications because they incompletely characterize human exposures. However, they do suggest a need for follow up studies. A current review at ATSDR is providing a statistical analysis of the data in Table 2 to assist in prioritizing the data needs.

The study of microplastics is relatively new, with researchers quickly responding to the published data gaps. ATSDR and NCEH are conducting scoping reviews on thousands of published research studies on microplastics. The lack of microplastic-specific information before 2015 does not exclude the important work that had been conducted prior to 2015, as prior to that date much work was conducted on specific plastic substances such as nylon, polyethylene, polyvinyl chloride, etc.

General conclusions that can be made now include the following:

Most microplastics research has been focused on the environment, specifically that of oceans, lakes, and rivers.