Development and application of a health-based framework for informing regulatory action in relation to exposure of microplastic particles in California drinking water

Abstract

Microplastics have been documented in drinking water, but their effects on human health from ingestion, or the concentrations at which those effects begin to manifest, are not established. Here, we report on the outcome of a virtual expert workshop conducted between October 2020 and October 2021 in which a comprehensive review of mammalian hazard studies was conducted. A key objective of this assessment was to evaluate the feasibility and confidence in deriving a human health-based threshold value to inform development of the State of California’s monitoring and management strategy for microplastics in drinking water. A tiered approach was adopted to evaluate the quality and reliability of studies identified from a review of the peer-reviewed scientific literature. A total of 41 in vitro and 31 in vivo studies using mammals were identified and subjected to a Tier 1 screening and prioritization exercise, which was based on an evaluation of how each of the studies addressed various quality criteria. Prioritized studies were identified largely based on their application and reporting of dose–response relationships. Given that methods for extrapolating between in vitro and in vivo systems are currently lacking, only oral exposure in vivo studies were identified as fit-for-purpose within the context of this workshop. Twelve mammalian toxicity studies were prioritized and subjected to a Tier 2 qualitative evaluation by external experts. Of the 12 studies, 7 report adverse effects on male and female reproductive systems, while 5 reported effects on various other physiological endpoints. It is notable that the majority of studies (83%) subjected to Tier 2 evaluation report results from exposure to a single polymer type (polystyrene spheres), representing a size range of 0.040 to 20 µm. No single study met all desired quality criteria, but collectively toxicological effects with respect to biomarkers of inflammation and oxidative stress represented a consistent trend. While it was possible to derive a conservative screening level to inform monitoring activities, it was not possible to extrapolate a human–health-based threshold value for microplastics, which is largely due to concerns regarding the relative quality and reliability of current data, but also due to the inability to extrapolate data from studies using monodisperse plastic particles, such as polystyrene spheres to an environmentally relevant exposure of microplastics. Nevertheless, a conservative screening level value was used to estimate a volume of drinking water (1000 L) that could be used to support monitoring activities and improve our overall understanding of exposure in California’s drinking water. In order to increase confidence in our ability to derive a human–health-based threshold value in the future, several research recommendations are provided, with an emphasis towards strengthening how toxicity studies should be conducted in the future and an improved understanding of human exposure to microplastics, insights critically important to better inform future risk assessments. Graphical abstract

Early enteric and hepatic responses to ingestion of polystyrene nanospheres from water in C57BL/6 mice

“…Because microplastic and nanoplastic disposal and formation in the environment are increasing, it follows that human exposure from drinking water, food, as well as indoor and outdoor air pollution are also increasing (Wright and Kelly, 2017;Revel et al, 2018;Brahney et al, 2021;Mohamed Nor et al, 2021). Only limited data in mammalian experimental animals on toxicity of nanoplastics are available (Deng et al, 2017;Yong et al, 2020), and while assessments of risk to humans from common exposure routes such as drinking water have been conducted, they are limited due to the current shortage of data (Coffin et al, 2022). A review of 305 field and laboratory studies on the potential for bioaccumulation of microplastics after ingestion indicates little biomagnification or trophic transfer (Gouin, 2020), but other findings suggest there is cause for concern and that further research is needed before ruling out the potential impacts of micro-and nanoplastics particularly given that they are now detected in human blood (Coffin et al, 2022;Leslie et al, 2022).…”

“…Only limited data in mammalian experimental animals on toxicity of nanoplastics are available (Deng et al, 2017;Yong et al, 2020), and while assessments of risk to humans from common exposure routes such as drinking water have been conducted, they are limited due to the current shortage of data (Coffin et al, 2022). A review of 305 field and laboratory studies on the potential for bioaccumulation of microplastics after ingestion indicates little biomagnification or trophic transfer (Gouin, 2020), but other findings suggest there is cause for concern and that further research is needed before ruling out the potential impacts of micro-and nanoplastics particularly given that they are now detected in human blood (Coffin et al, 2022;Leslie et al, 2022). There is negligible intestinal absorption of larger microplastics (>50 µm), but a paucity of analytical techniques for particles below this size (Brander et al, 2020;Cowger et al, 2020), and the fact that one cannot infer the intestine does not absorb smaller particles under 10 µm, necessitate additional data for these smaller particles.…”

A living tool for the continued exploration of microplastic toxicity

“…Thus, a subset of 14 selected criteria was designated as essential "red criteria. " These criteria are intended to reflect the minimum requirements for a study to be considered for a preliminary assessment, with the understanding of the significant limitations of the findings [21,23]. It should be noted that these "red criteria" are not intended to be an acceptable replacement for the full set of criteria presented by de Ruijter et al [16] or Gouin et al [22].…”

“…Here, it was applied to inform the legislative mandates of California's Senate Bills 1422 and 1263. More specifically, workshop participants were tasked with identifying the primary pathways by which microplastics cause adverse effects, prioritizing the characteristics that are of greatest biological concern (Hampton et al, 2022 [33]), and identifying critical thresholds at which adverse effects become pronounced [21,23]. Critical thresholds for the aquatic environment were derived using a combination of tools provided by ToMEx [21], and for assessing impacts to human health additional tools were used (e.g., benchmark dose modelling software; Coffin et al, [23]).…”

“…More specifically, workshop participants were tasked with identifying the primary pathways by which microplastics cause adverse effects, prioritizing the characteristics that are of greatest biological concern (Hampton et al, 2022 [33]), and identifying critical thresholds at which adverse effects become pronounced [21,23]. Critical thresholds for the aquatic environment were derived using a combination of tools provided by ToMEx [21], and for assessing impacts to human health additional tools were used (e.g., benchmark dose modelling software; Coffin et al, [23]). Data most appropriate for threshold development were selected based on a variety of factors, including specific screening criteria, and threshold calculations were quickly performed.…”