The environmental distribution and fate of microplastic in the marine environment represents a potential cause of concern. One aspect is the influence that microplastic may have on enhancing the transport and bioavailability of persistent, bioaccumulative, and toxic substances (PBT). In this study we assess these potential risks using a thermodynamic approach, aiming to prioritize the physicochemical properties of chemicals that are most likely absorbed by microplastic and therefore ingested by biota. Using a multimedia modeling approach, we define a chemical space aimed at improving our understanding of how chemicals partition in the marine environment with varying volume ratios of air/water/organic carbon/polyethylene, where polyethylene represents a main group of microplastic. Results suggest that chemicals with log KOW > 5 have the potential to partition >1% to polyethylene. Food-web model results suggest that reductions in body burden concentrations for nonpolar organic chemicals are likely to occur for chemicals with log KOW between 5.5 and 6.5. Thus the relative importance of microplastic as a vector of PBT substances to biological organisms is likely of limited importance, relative to other exposure pathways. Nevertheless, a number of data-gaps are identified, largely associated with improving our understanding of the physical fate of microplastic in the environment.

REACh introduces environmental release categories (ERCs) used to describe chemical emissions to the environment. These ERCs contain a number of default values that are combined with product usage data to estimate environmental exposure concentrations for use in risk assessment. The methodology presented in the present work constitutes a novel approach that allows for the coupling of population density and country-specific usage statistics for a range of home and personal care products. Spatially explicit usage estimates are presented for European Union (EU) hypothetical regions (200 × 200-km grid), as described in EU risk assessment frameworks. Recent sales and population density data are combined to assess the relevance of current default assumptions; that 10% of a product will be used in an EU hypothetical region that is inhabited by 20 million people. It is demonstrated that these assumptions are conservative and their relevance geographically limited. The highest usage estimates for hypothetical regions are located in southern England and a transboundary area in Europe consisting of Belgium, the Netherlands, and Germany. The maximum identified usage of 9 home and personal care product types, in an EU hypothetical region, ranges from 4.3% to 11.4%. The 99.5th percentile values, representative of an EU hypothetical region with a population of 20 million, range from 3.6% to 9.0%, which suggests that the current default parameterization of the ERCs may be overly protective for ingredients used in a number of home and personal care product types. Accordingly, it may be justified to use such an analysis to refine the default values in order to provide more realistic exposure estimates for use in REACh assessments.