The U.S. Environmental Protection Agency’s (EPA) recent decision to include microplastics as a priority contaminant group on the draft Sixth Contaminant Candidate List (CCL6) reflects growing federal attention on microplastics. While awareness and discussion of microplastics are increasing across scientific, public health, and policy communities, this move raises important questions about whether the foundational science, tools, and infrastructure are ready to support such a significant step. 

Despite rapid growth in research funding and publications, several critical gaps remain. Moving forward without addressing them risks outpacing the science and undermining the credibility and effectiveness of the Safe Water Drinking Act (SDWA) framework. 

What Is the Contaminant Candidate List? 

The Contaminant Candidate List (CCL) is a screening and prioritization mechanism established under the SDWA to identify unregulated contaminants that are known or anticipated to occur in public water systems, and that may warrant additional evaluation.  

Every five years, EPA publishes a new CCL that prioritizes contaminants for research and data collection. EPA considers the CCL when determining which contaminants it will require public water systems to monitor, collect, and report. Under the Unregulated Contaminant Monitoring Rule (UCMR), data on nationally representative occurrence of contaminants are collected and used by EPA to develop a primary (enforceable) national drinking water standard for a contaminant, if warranted. 

Historically, this risk-based process has relied on well‑characterized substances, supported by standardized analytical methods, exposure data, and established toxicological datasets. The CCL is therefore intended to be a science‑driven prioritization tool, not a signal that regulation is inevitable or imminent. 

Why Microplastics are Difficult to Measure and Regulate 

No Harmonized Definition of Microplastics 

What are microplastics? Scientists still disagree. At present, there is no single, agreed‑upon definition of what constitutes “microplastics.” Regulators and researchers differ on particle-size ranges (including sub-micron thresholds), which materials are considered plastic, and how to treat variables such as shape, polymer type, additives, or degradation state. In many definitions, tire wear particles and synthetic textiles – previously reported¹ as significant sources of environmental microplastics – are explicitly excluded.  

Without a unified definition, it is difficult to compare studies, aggregate data, or interpret results across jurisdictions, making even basic questions about what is being measured hard to answer. 

Analytical Methods Are Not Yet Established for the Full Range of Microplastics 

Microplastics are not a single material. They represent a highly heterogeneous category of particulate materials that can differ significantly in polymer type, chemical composition and physical size and shape, making measurement a complex process. At present, no single method can measure microplastics across all sizes, shapes, and polymer types, so researchers must rely on multiple techniques that vary in sensitivity and scope. 

As a result, reported concentrations can differ by orders of magnitude depending on the method used—making it difficult to assess exposure, exposure trends, or potential risk through drinking water or other media. 

Microplastics Collection, Handling, Preparation, and Reporting Are Not Yet Standardized 

Beyond analytical methods, there is no harmonized approach for sampling, handling, processing, and reporting microplastics. This adds complexity not typically encountered for contaminants on the CCL; historically most contaminants have a single validated method that could be widely deployed. For microplastics, coordination across multiple methods, sampling and preparation protocols, and agreement on how to integrate results are still open challenges. 

These issues are especially important at the low concentrations relevant to drinking water, where background contamination, inconsistent quality controls, and false positive identification of non-plastic particles can make it difficult to distinguish true environmental signals from false signals introduced during sampling or analysis. 

EPA scientists have echoed these concerns. During a June 15^th meeting of the Science Advisory Board’s drinking water committee, one official in the Standards and Risk Management Division of EPA’s Office of Ground Water and Drinking Water stated²: 

“[T]o be able to consider microplastics for monitoring under the UCMR program, they need to have a validated analytical method. Because there’s no validated analytical method that’s approved by the agency at this point, there’s no way to monitor at the national scale for microplastics currently.” 

Capacity for National Drinking Water Testing Remains Unclear  

Microplastic analysis is technically demanding, costly, and currently limited to a small number of specialized laboratories. A national monitoring program depends on reliable, cost-effective access to commercial testing laboratories. Even if methods mature, it is unclear whether water providers—particularly smaller and rural systems—will have access to the necessary resources, expertise and capable laboratories.  

EPA Rightly Prioritizes Reliable, Actionable Monitoring 

EPA made the right decision to leave microplastics out of the proposed UCMR 6, recognizing testing limitations, insufficient laboratory capacity, and the consequences of diverting limited resources from monitoring other unregulated contaminants. As EPA explained³:  

“If microplastics were included on UCMR 6, the PWSs [Public Water Systems] subject to this rulemaking would be unable to successfully monitor for microplastics. Such monitoring is the central purpose of the UCMR as outlined by SDWA section 1445(a)(2). In addition, the agency and the public would lose an opportunity to gain occurrence information on other unregulated contaminants that have a drinking water analytical method available for UCMR 6.” 

Developing validated methods and adequate laboratory capacity is the appropriate next step to ensure future microplastics monitoring produces reliable, useful data to inform public health decisions. 

Research Indicates Existing Water Treatment Is Effective at Removing Microplastics 

A growing body of literature indicates that conventional wastewater and drinking water treatment processes can substantially reduce microplastic particle concentrations before water is discharged or delivered to consumers.^4,5,6,7 Reviews of municipal wastewater treatment systems have consistently reported high particle capture rates, while studies evaluating drinking water treatment facilities have documented substantial reductions across a range of treatment configurations.  

Although these findings remain subject to important uncertainties, including differences in analytical methods, particle-size cutoffs, and reporting conventions, they nevertheless indicate that existing treatment infrastructure may already provide a significant barrier to microplastic exposure through drinking water. In this context, nationwide monitoring is unlikely to provide information that can be readily translated into risk-management decisions, particularly when key questions regarding exposure relevance, toxicological significance, and health-based occurrence thresholds remain unresolved. 

Given these realities, EPA should continue to support method development, harmonization, and targeted research while prioritizing UCMR monitoring for contaminants with clearer public health significance and more mature scientific foundations. Such an approach would preserve limited monitoring resources, generate more actionable data, and better align with the risk-based framework established under the Safe Drinking Water Act. 

Build the Scientific Foundation for Evidence-Based Decisions 

These limitations do not mean that microplastics are unimportant or undeserving of attention. Instead, EPA highlights the need to strengthen the scientific foundation before regulatory action to support defensible policy decisions⁸. 

“The agency acknowledges the interest in and concern for microplastics in drinking water, and believes that including these contaminants on the draft CCL 6 as a first step in responding to the petition will prioritize the research that is needed to define and better understand the characteristics of microplastics.” 

Industry, academic researchers, and federal agencies have already begun working toward these goals, including efforts to develop standardized materials and methods and to better understand environmental behavior and potential health impacts. Monitoring and research should continue, but they must be aligned with the realities of current capabilities. 

Legislation like the Plastics Health Research Act (PHRA) can provide a constructive path forward by establishing a federal framework to help harmonize definitions, methods, and data collection across agencies. By strengthening research and coordination, policymakers can help ensure that future drinking water decisions — whether through the CCL or other tools — are grounded in reliable evidence and aligned with the intent of the SDWA. 

References 

⁵ Balkenbusch, C., Glienke, J., Wu, Y. et al. Microplastic removal across ten drinking water treatment facilities and distribution systems. npj Clean Water 8, 103 (2025). https://doi.org/10.1038/s41545-025-00531-w.   

¹ Fauroux, M., Boucher, J., & Perreard, S. (2026). From Pack to Plate: A Global Assessment of Micro- and Nanoplastics Migrating from Food Packaging into Food. Earth Action. www.e-a.earth 

² Inside EPA. (2026, June 16). EPA, science advisory board doubt rapid SDWA action on microplastics. https://insideepa.com/daily-news/epa-science-advisory-board-doubt-rapid-sdwa-action-microplastics 

¹ Jiang, D., & Nowack, B. (2025). Reconciling plastic release: Comprehensive modeling of macro- and microplastic flows to the environment. Environmental Pollution, 383, Article 126800. https://doi.org/10.1016/j.envpol.2025.126800 

⁷ Pulido-Reyes et al. (2022). Nanoplastics removal during drinking water treatment: Laboratory- and pilot-scale experiments and modeling. J. of Hazardous Materials. 436: 129011. https://doi.org/10.1016/j.jhazmat.2022.129011. 

¹ UN Environment (2018). Mapping of global plastics value chain and plastics losses to the environment (with a particular focus on marine environment). Ryberg, M., Laurent, A., Hauschild, M. United Nations Environment Programme. Nairobi, Kenya. 

^{3 8} U.S. Environmental Protection Agency. (2026, July 1). Revisions to establish the sixth unregulated contaminant monitoring rule (UCMR 6) for public water systems (Proposed rule). Federal Register, 91, 39952. https://www.federalregister.gov/d/2026-13263 

⁴ World Health Organization (WHO). (2019). Microplastics in Drinking-water. Geneva: World Health Organization; Available at: https://www.who.int/publications/i/item/9789241516198. Last Accessed 06.01.2026.  

⁶ Zhang et al. (2020). Removal Efficiency of Micro- and Nanoplastics (180 nm – 125μm) During Drinking Water Treatment. Sci Total Environ. 2020 June 10; 720: 137383. doi:10.1016/j.scitotenv.2020.137383.