Toward an ecotoxicological risk assessment of microplastics: Comparison of available hazard and exposure data in freshwaters

Abstract

Microplastics have been detected in freshwaters all over the world in almost all samples, and ecotoxicological studies have shown adverse effects of microplastics on organisms. However, no risk assessment of microplastics has been performed specifically in freshwater so far. The aim of the present study was therefore to review all exposure and ecotoxicity data available for microplastics in freshwaters and to perform a preliminary probabilistic risk assessment. The exposure probability distribution was based on 391 concentrations measured in Asia, Europe, and North America. Because exposure data are mainly available in particle number-based metrics but results from hazard studies are mostly mass-based, the hazard results were converted into particle number concentrations. A statistical analysis of the hazard data showed that there was no significant influence of particle shape or type of polymer on the no-observed-effect concentration. The predicted-no-effect concentration (PNEC) was calculated as the fifth percentile of the probabilistic species sensitivity distribution, based on 53 values from 14 freshwater species, to have a mode of 7.4 × 10⁵ particles · m^-3 (25th and 75th quantiles of 6.1 × 10⁵ and 1.3 × 10⁶ particles · m^-3 , respectively). The exposure probability distribution was divided by the PNEC probability distribution to calculate risk characterization ratios (RCRs), with modes of 1.3 × 10^-6 in North America, 3.3 × 10^-6 in Europe, and 4.6 × 10^-3 in Asia. Probability distributions associated with the RCRs showed that ecological risks cannot be entirely excluded in Asia, where 0.4% of the RCR values were above 1. Environ Toxicol Chem 2019;38:436-447. © 2018 SETAC.

Keywords: Freshwater; Microplastics; Probabilistic risk assessment; Probabilistic species sensitivity distribution.

Figure 1

Cumulative probability curves of concentrations of microplastics measured in freshwater. Samples with no detectable microplastics have been converted to very low concentrations (10⁻⁵ particles · m⁻³) to show them on the logarithmic scale of the graph. Points represent means or medians (means preferred, according to availability) of concentrations measured in replicates. The areas visible on the figure are the result of the combination of 10 000 cumulative curves.

Figure 2

(A) Proportions of cutoff sizes used among studies to sample microplastics from freshwater. (B) Concentrations of microplastics measured in freshwater with different sampling cutoff sizes.

Figure 3

Probabilistic species sensitivity distributions and probability distribution of the predicted‐no‐effect concentration of microplastics in freshwater. The upper panel (A) shows mass‐based concentrations; the lower panel (B) shows particle number concentrations. NOEC = no‐observed‐effect concentration; PNEC = predicted‐no‐effect concentration; PSSD = probability species sensitivity distribution; Q = quantile (e.g., Q25 = 25th quantile).

Figure 4

Probability distributions of the measured environmental concentration of microplastics and their predicted‐no‐effect concentration in freshwater. The overlap of the 2 distributions is enhanced by the shaded area.

Figure 5

Probability distributions of risk characterization ratios in the world, Asia, Europe, and North America.

Figure 6

Cumulative curves of cutoff sizes used to recover microplastics from freshwater and mean sizes of microplastics tested for ecotoxicity.

Figure 7

Probabilistic species sensitivity distributions and probability distribution of the predicted‐no‐effect concentration of microplastics in freshwater, excluding the studies testing particles smaller than the smallest particle size cutoff used in exposure assessment. The upper panel (A) shows mass‐based concentrations; the lower panel (B) shows particle number concentrations. NOEC = no‐observed‐effect concentration; PNEC = predicted‐no‐effect concentration; PSSD = probabilistic species sensitivity distribution; Q = quantile (e.g., Q25 = 25th quantile).