Our understanding of the potential risks of microplastics to human health and the environment remains incomplete.
A fundamental concept in chemical risk assessment is that the risk of toxicity from a chemical is related to both hazard (i.e., adverse effects caused by the chemical) and exposure (i.e., the levels at which one would be exposed to the chemical). While there is evidence that humans and wildlife are routinely exposed to microplastics, the potential human health and ecological risks attributable to such exposure remain unclear. Here, we summarize recent scientific findings, and corresponding data gaps, regarding the potential impacts of microplastics on humans and wildlife.
The potential effects of microplastics on biological organisms are generally understood to be related to their physical (e.g., size and shape) and chemical properties (Brander et al., 2021). At the organismal level, physical effects including entanglement; blockage of the digestive tract and reduced feeding due to a false feeling of satiation; and abrasion and irritation of tissue mucosa have been demonstrated (Brander et al., 2021). Based on evidence in aquatic invertebrates, these effects are believed to negatively affect the growth and reproductive capacity of organisms, and potentially lead to adverse effects at the population level (Brander et al., 2021).
Studies that delineate the potential adverse effects caused by microplastics themselves versus those caused by adsorbed chemicals or biological agents are needed.”
Due to their hydrophobic nature, researchers have also expressed concern that microplastics may act as vectors for hydrophobic chemicals used as additives in plastic, such as plasticizers (e.g., bisphenol A and phthalates), flame retardants, and heavy metals (Yong et al., 2020). In addition to plastic additives, there are also concerns about organic contaminants that may adsorb to the surfaces of microplastics. Some of these chemicals include polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), which are associated with a wide range of potential adverse health effects in humans and wildlife, and originate from numerous sources other than microplastics. In addition to chemicals, there is also concern that microplastics may act as vectors for biofilm-associated microbial organisms that colonize the particle surface (Yong et al., 2020). It is currently unclear whether any potential chemical-related effects of microplastics may be due to the polymeric materials themselves or to any potentially sorbed co-contaminants.
Identifying biological effects caused by microplastics is challenging due to their diversity with respect to physical and chemical characteristics, coupled with the lack of a harmonized method for quantifying exposure levels. For example, applying an adverse outcome pathway (AOP) analysis to assess the ecological effects of microplastics is difficult because microplastics do not cause a well-defined molecular initiating event across species, but rather appear to induce a more general oxidative stress response (Yong et al., 2020).
Human Health Impacts
The uptake, distribution, and excretion of microplastics in humans are active areas of research. Humans are exposed to microplastics through inhalation (e.g., particles associated with dust in indoor or outdoor air) and ingestion of contaminated food (e.g., commercial seafood) or drinking water. Total human exposure to microplastics is estimated to be on the order of several milligrams per day (Kannan and Vimalkumar, 2021). Although an estimated 90% of ingested microplastics are removed from the body by normal excretory processes (i.e., feces), accumulation of microplastics has been reported in the tissues of adults and children (e.g., placenta) (Kannan and Vimalkumar, 2021). Laboratory studies using human cells indicate that microplastic particles <10 µm in size can penetrate cell membranes (Yong et al., 2020). Hence, it is suggested that the environmental levels of microplastics that become systemically available to tissues throughout the body could be relevant to potential health effects in humans (Vethaak and Legler, 2021), but there are currently no reliable data to support this hypothesis. Studies of human cells exposed to high concentrations of microplastics indicate low to moderate oxidative stress or inflammatory responses, but some studies have demonstrated no such effects (Yong et al., 2020). A limitation of these studies is that the human cells were exposed to high concentrations of laboratory-generated microplastic particles with homogenous shapes (e.g., spheres), whereas microplastics in the environment are heterogeneous in terms of both shape and chemical composition (Vethaak and Legler, 2021).
Similar to ecological concerns, it has been suggested that microplastics may act as vectors for hydrophobic chemicals (such as plastic additives) and microbial pathogens; however, to date, there is insufficient evidence to support this hypothesis with regard to human exposures (Vethaak and Legler, 2021; Kannan and Vimalkumar, 2021). As a result, studies that delineate the potential adverse effects caused by microplastic themselves versus those caused by adsorbed chemicals or biological agents are needed.
Although there is a high level of public concern about potential human health effects caused by microplastics exposure, the current scientific consensus is that, due to existing major data gaps, there is insufficient evidence to assess the risk of any potential adverse health effects of microplastics in humans. Currently, regulatory and scientific agencies in the US (particularly in California) and internationally are identifying and prioritizing data gaps that should be addressed by future research in order to establish a framework for assessing potential health risks posed by microplastics to humans and wildlife (Brander et al., 2021).
Brander, SM; Hoh, E; Unice, KM; Bibby, KR; Cook, AM; Holleman, RC; Kone, DV; Rochman, CM; Thayer, JA. 2021. “Microplastic Pollution in California: A Precautionary Framework and Scientific Guidance to Assess and Address Risk to the Marine Environment.” California Ocean Science Trust (OST). 68p., April.
Kannan, K; Vimalkumar, K. 2021. “A review of human exposure to microplastics and insights into microplastics as obesogens.” Front. Endocrinol. 12:724989. doi: 10.3389/fendo.2021.724989.
Vethaak AD; Legler J. 2021. “Microplastics and human health.” Science 371(6530):672-674. doi: 10.1126/science.abe5041.
Yong, CQY; Valiyaveettil, S; Tang, BL. 2020. “Toxicity of microplastics and nanoplastics in mammalian systems.” Int. J. Environ. Res. Public Health 17(5):1509. doi: 10.3390/ijerph17051509.