Evaluation of Potential Health Risks from Microplastics in Drinking Water

Microplastics have been detected, often abundantly, in freshwater environments over the past decade. While understanding of the ecological health implications of microplastics in aquatic environments has advanced considerably, the health risks of microplastics in drinking water are not well understood. Direct health impacts are attributed to the ingestion of microplastics materials themselves. In contrast, indirect health impacts are attributed to the chemical contaminants that sorb on and in microplastics in the aquatic environment and are concurrently ingested. While it is desirable to evaluate both types of health risks, there are currently no available and conclusive toxicological investigations of the health implications of microplastics ingestion by humans; current understanding is limited to microplastics impacts on small organisms or cell cultures. In contrast, considerable information regarding the health effects of some contaminants that sorb on or in microplastics is available. Although this information has not been integrated to inform health risks associated with microplastics ingestion via contaminated drinking water, this integration is pressingly needed to guide risk management. \nHere, the potential health risks attributable to chemical contaminants retained on or in microplastics in the aquatic environment and ingested via contaminated drinking water were assessed using a new concept developed in this research: the Threshold Microplastics Concentration (TMC). The TMC indicates the total number of microplastics particles per liter of water that, if ingested, constitutes exposure to potentially harmful concentrations of chemical contaminants retained on or in microplastics via sorption mechanisms. A TMC of 0.024 microplastics particles per liter was identified given currently available contaminant sorption data; this value increased to 2.550 microplastics particles per L in absence of antimony. Thus, these respective values indicate that source water concentrations of 24 or 2,550 microplastics particles per L or less should not pose health concerns attributable to sorbed chemical contaminants for well-operated conventional treatment systems in which a 3-log (i.e., 99.9%) reduction in microplastics concentration can be reasonably expected by physico-chemical filtration. Critically, a source water microplastics concentration that exceeds the TMC is not necessarily indicative of health risks from microplastics in drinking water; rather, it indicates that more detailed analysis may be warranted. For example, system specifics such as types of treatment implemented, sorbed contaminants present in the source water, size distribution of the microplastics, etc. affect the TMC. Notably, antimony was identified as a potential sentinel indicator of potential health risk from microplastics because it is especially toxic. Similarly, PVC was identified as a key microplastics type because of its contaminant sorption propensity. Only 11 contaminants and seven common microplastics materials were included in this analysis because of limited sorption and toxicity data for known chemical contaminants of human health concern; however, the “Microplastics Calculator” developed herein to calculate TMCs can be easily updated as chemical, plastics, and treatment data become available. \nMicroplastics are particles—in many ways they are not different than other particles removed during drinking water treatment. Their removal can therefore be explained by the physico-chemical processes that are involved in particle removal during filtration. Here, a synthesis of the current knowledge regarding the treatment of particulate contaminants including microplastics and a limited series of surface charge assessments and bench-scale coagulation and filtration experiments were conducted to confirm microplastics removal expectations during drinking water treatment. These experiments demonstrated the size dependency that would be expected by classical filtration theory: the order of particle removal efficiency by filtration was 45 μm > 10 μm > 1 μm. The surface charge of several common microplastics (polyethylene, polystyrene, acrylic, and polyetheretherketone) varied considerably and was impacted by the quality of the matrix in which they were suspended, as would be expected. Notably, however, coagulant addition at doses sufficient for achieving optimal particle destabilization in absence of the microplastics was also sufficient for destabilizing microplastics suspended at environmentally relevant concentrations in all matrices investigated (i.e., distilled deionized MilliQTM water; 100 mM KCl electrolyte solution; low turbidity, low dissolved organic carbon (DOC) Lake Ontario water; and moderate DOC, higher turbidity Grand River water). Overall, this analysis confirmed that the removal of microplastics particles by engineered physico-chemical filtration processes should be consistent with that which would be expected of other particles and particulate contaminants.