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. 2024 Jun 10;2(7):422–423. doi: 10.1021/envhealth.4c00086

Development of “Threshold Microplastics Concentration” Concept and Framework in Drinking Water

Fengbang Wang †,‡, Maoyong Song †,‡,*
PMCID: PMC11504262  PMID: 39473889

Global attention on microplastics has increased because of their ubiquity in water, air, soil, and organisms.1−4 Knowledge of the sources, occurrences, behaviors, and health risks of microplastics is quite limited.5−7 Nevertheless, developing comprehensive health-based thresholds and regulatory guidelines for microplastics in water is impossible because of the limitations and complexity of determining their health effects. Writing in Environment & Health, Chowdhury et al. reported a “Threshold Microplastics Concentration” (TMC) concept and framework.8 The TMC framework was used to identify microplastic concentrations that may result in the intake of regulated contaminants on/in microplastics at levels of human health concern and treatment targets for managing those potential risks.8

A wide range of contaminants in drinking water are harmful to human health. Some health guidelines, including maximum contaminant levels (MCLs)9 or maximum acceptable concentrations (MACs),10 have been established to regulate a particular contaminant in drinking water and to monitor potential health risks of source and treated waters. Without comprehensive and conclusive toxicological data, the MCLs/MACs are unavailable for various microplastics.

Chowdhury et al. proposed a conservative approach using currently available data to identify the TMC. There are six primary types of microplastics detected in drinking water or wastewater, including Polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polypropylene (PP), and polycarbonate (PC).11−13 So they were selected in the TMC framework development.8 The authors chose six key data sets to evaluate the TMC: microplastic size, microplastic shape, microplastic polymer type, health guidelines defined by MCLs or MACs, contaminant adsorption capacity on plastics, and the extent of microplastic removal during drinking water treatment.8 The data were collected, summarized, and integrated.

TMC was calculated using three equations.8 In eq 1, TMCijpx is the concentration of microplastics with size i, shape j, and type p that can carry an amount of contaminant x equal to the MCL/MAC (Hx). ACSA, px is the converted adsorption capacity per unit surface area for a particular microplastic type and contaminant, and Aij is the surface area of the microplastic considered. The extent of treatment is represented by log-removal T, which corresponds to the removal of particles. In eq 2, AC*mass, px is the literature-based adsorption capacity per unit mass of microplastics type p and contaminant x based on the adsorption capacity experiment in the literature (*). SSA*p is the specific surface area defined as the total surface area per gram of microplastics used in the adsorption experiments, which is calculated by the area (A*) and volume (V*) of a single microplastic particle and the density (ρ) of microplastic type p.

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TMC was calculated for each of the 11 contaminants, seven shapes, and a range of longest dimensions.8 Chowdhury et al. showed that different factors can affect the TMC over a range of approximately 12 orders of magnitude.8 The size of microplastics is a major driver of TMC compared with microplastic shape. Based on the TMCs calculated, the types of contaminants sorbed on microplastics were a key driver in evaluating the potential health concerns from microplastics. Lower TMCs correspond to a higher potential risk because they indicate that fewer microplastic particles might lead to contaminant exposure at concentrations associated with health impacts.8

The TMCs were very difficult to evaluate because the microplastic concentrations and associated contaminant concentrations could not be accurately measured in drinking water. Despite the lack of ideal knowledge regarding microplastics and associated chemicals, the TMC framework can still be developed using available microplastics and contaminant data. Nevertheless, more experimental results will help improve the accuracy of TMC predictions. Unlike MCLs and MACs, which indicate the maximum acceptable concentration of a contaminant in drinking water, the TMC only indicates a microplastic concentration that may have a potential health risk if exceeded.8 For example, further water quality analysis is recommended if the microplastic concentration in the source water is higher than the TMC calculated using a reference size.

The future looks bright for using the TMC framework in various operational scenarios because it can be updated as more information on the adsorption of contaminants on microplastics and their toxicology becomes available. A prerequisite is to identify ways to obtain additional factors and meet environmental conditions. If this can be achieved generally, then the future deployment of TMC will be more effective in system-specific risk management.

Acknowledgments

This work was supported by the Stretegic Priority Research Program of the Chinese Academy of Sciences (XDB0750300) and the National Natural Science Foundation of China (grants 22241604 and 22125606).

There are no conflicts of interest to declare.

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