Probabilistic environmental risk assessment of microplastics in soils

Abstract: Despite the convenience that plastics provide to society, there have long been concerns regarding the risks they present to the environment. Microplastics (MPs, plastic particles smaller than 5 mm) have been found in every environmental compartment—air, freshwater, oceans, but also soils—yet the emphasis to date has been on the risks they represent to aquatic environments. The present study, therefore, aimed to perform a comprehensive risk assessment of MPs in soils. A probabilistic approach was used to account for the variability and uncertainties in the available data. Measured exposure concentrations and ecotoxicity thresholds were extracted from pertinent peer-reviewed literature. Risk characterization ratios (RCRs) were then calculated for different land uses and geographical regions by dividing the predicted no-effect concentration (PNEC) distribution by the respective measured environmental concentration distribution. Using particle-number concentrations, the mean PNEC was calculated to be 82,000 part.kg⁻¹. The measured concentrations of MPs in soils ranged from 0 to 410,000 part.kg⁻¹ (median: 930 part.kg⁻¹). A human influence was clearly demonstrated, with higher concentrations (median: 3,600 part.kg⁻¹) in soils directly influenced by human activity than in natural or agricultural soils. 4.8 % of RCR calculations (median: 0.013) of the probability distribution were above 1, which is 40 and 240,000 times greater than that predicted for freshwater and marine habitats, respectively. Urban and industrial soils had the highest RCR, followed by agricultural and natural soils. The comparability of exposure and hazard datasets could be improved if the MPs tested for ecotoxicity were more representative of those found in environmental samples. There is a need for more ecotoxicity data on fibers, films, polyethylene, and weathered or aged MPs so that comparisons with real-world, observed exposure data can be built on more solid foundations.