Table 1 Summary of total assessment scores (TAS) and QA/QC criteria strengths and weaknesses of in vivo studies prioritized based on adequate evaluation of critical QA/QC identified as part of a Tier 1 screening assessment. Strengths and weaknesses listed reflect how the data support or limit, respectively, the interpretation of study results within a risk assessment context. Strengths and weaknesses are thus context-specific, and those listed as strengths here may be perceived as weaknesses under another context, and vice versa. Additional study information included in the Supplementary Information

Lim et al. (2021) [102]

Excellent reporting of study design characteristics; inclusion of both sub-organismal and organism level endpoints reported helping to inform potential mechanisms of toxicological action

Monodisperse polystyrene spherical particles – unclear how to extrapolate results to environmentally relevant exposures.

37

Ma-Hock et al. (2012) [97]

Excellent reporting of particle characteristics and exposure conditions, including an estimate relating external exposure to an internal dose; NOAEL reported based on no adverse effects up to the maximum concentration tested.

Single type of polymer tested at only two concentration doses, plus control – acrylic ester copolymer – unclear how to extrapolate results to environmentally relevant exposures.

37

Ong et al. (2020) [74]

Study conducted according to repeat oral dose OECD 408 guideline with excellent reporting for all three areas evaluated, including particle characteristics, study design and application for risk assessment – NOAEL reported.

Study targets the evaluation of adverse effects related to repeated dietary ingestion of fibrillated cellulose – unclear how to read-across to NMPs.

44

Amereh et al. (2019) [115]

Good characterization of particles, which include two different sizes in the sub-micron range; particles tested at both high and environmentally relevant concentrations, LOAEL can be extrapolated.

Single type of polymer tested – polystyrene spheres; particles dosed into drinking water, unclear actual delivery and/or homogeneity of exposure; adverse effects limited to sub-organismal level endpoints only.

34

Amereh et al. (2020) [114]

Good characterization of particles, which include two different sizes in the sub-micron range; particles tested at both high and environmentally relevant concentrations, LOAEL can be extrapolated.

Single type of polymer tested – polystyrene spheres; particles dosed into drinking water, unclear actual delivery and/or homogeneity of exposure; adverse effects limited to sub-organismal level endpoints only.

34

Deng et al. (2017) [116]

Inclusion of both sub-organismal and organism level endpoints reported helping to inform potential mechanisms of toxicological action, with exposure concentrations reported to represent environmentally relevant concentrations.

Two sizes of monodisperse polystyrene spheres; particles dosed into drinking water, unclear actual delivery and/or homogeneity of exposure; particles supplied as a dispersion in a solution containing 1:1 ethanol:water, unclear residual levels of ethanol in test system. Several comments published in the peer review literature raising concerns related to histopathological analysis and toxicokinetics.

34

Dolan et al. (2016) [112]

Study conducted according to repeat oral dose OECD 408 guideline with good reporting for study design and application for risk assessment – NOAEL reported.

Poor reporting of particle characteristics, where study reports adverse effects related to repeat dietary ingestion of pecan shell fiber, ground from pecan shells – unclear how to read-across to NMPs.

34

An et al. (2021) [96]

Lowest test concentration selected as being representative of concentrations reported for freshwater systems.

Monodisperse polystyrene spherical particles – unclear how to extrapolate results to environmentally relevant exposure; particles dosed into drinking water, unclear actual delivery and/or homogeneity of exposure; particles supplied as a dispersion in a solution containing 1:1 ethanol:water, unclear residual levels of ethanol in test system.

31

Kotkoskie et al. (1996) [113]

Concentration test range selected to be representative of concentrations in food product for human consumption, NOAEL reported.

Poor reporting of particle characteristics, where study reports adverse effects related to repeat dietary ingestion of cellulose fibers – unclear how to read-across to NMPs.

30

Park et al. (2020) [118]

Various organism and sub-organism level effects reported, including body weight, pathological effects in stomach epithelial cells, effects on reproduction and immune system, such as via Immunoglobins (Ig, IgA, total IgG, IgE, and IgM)

Monodisperse polyethylene particles – unclear how to extrapolate results to environmentally relevant exposure; particles dosed into drinking water, unclear actual delivery and/or homogeneity of exposure; unclear relevance of exposure concentration range used 3.75–60 mg/kg bw; questions related to the use of p < 0.5 as a measure of statistical significance.

30

Hou et al. (2020) [117]

Body weight and changes in organ coefficients, sperm damage analysis, including count, malformation, etc., inflammatory responses and apoptosis-related proteins and cells. A combination of both organism and sub-organism level endpoints. Concentrations of 0.1, 1, and 10 mg/L used in drinking water, with an estimate that mice drank 6–7 mL/d, resulting in a nominal derived concentration of 0.6–60 μg/d.

Poor particle characterization, with particle description limited to monodisperse polystyrene spherical particles – unclear how to extrapolate results to environmentally relevant exposure.

27

Li et al. (2020) [121]

Verification of particle size, shape and composition, while sufficiently reporting information for each of the critical criteria.

Monodisperse polystyrene spherical particles – unclear how to extrapolate results to environmentally relevant exposure; particles dosed into drinking water, unclear actual delivery and/or homogeneity of exposure; particles supplied as a dispersion in a solution containing 1:1 ethanol:water, unclear residual levels of ethanol in test system. Concentrations of > 7.18 × 10⁹ particles/L are perceived to be significantly greater than typical human exposure.

27

Wei et al. (2021) [119]

Verification of particle size, shape and composition, while sufficiently reporting information for each of the critical criteria.

Monodisperse polystyrene spherical particles – unclear how to extrapolate results to environmentally relevant exposure; particles dosed into drinking water, unclear actual delivery and/or homogeneity of exposure; particles supplied as a dispersion in a solution containing 1:1 ethanol:water, unclear residual levels of ethanol in test system.

27

Xie et al. (2020) [120]

Sufficient level of information reported for each of the critical criteria.

Monodisperse polystyrene spherical particles – unclear how to extrapolate results to environmentally relevant exposure.

27

Li et al. (2020) [104]

Sufficient level of information reported for each of the critical criteria.

Monodisperse polystyrene spherical particles – unclear how to extrapolate results to environmentally relevant exposure.

23